Patent Publication Number: US-9404487-B2

Title: Apparatus and methods for evacuating air from a closed area

Description:
CROSS REFERENCE 
     This application contains references to U.S. Provisional Application Nos. 61/239,446, filed Sep. 3, 2009, 61/264,778, filed Nov. 27, 2009, 61/296,198, filed Jan. 19, 2010, and 61/448,620, filed Mar. 2, 2011, and PCT International Application No. US2010/002428, filed Sep. 3, 2010, the entire contents of which are hereby incorporated by reference herein. Priority is claimed to U.S. Provisional Application No. 61/448,620, filed Mar. 2, 2011. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to methods and apparatus that are used to generate a vacuum and, more particularly, is directed to an electronically controlled apparatus for evacuating air from a closed area. 
     BACKGROUND 
     Devices for the movement of gases are widely utilized. The very first aircraft engines were piston driven propellers. They worked by coupling a piston engine to a propeller. Their simplicity lead to widespread adoption until jet engines were invented. Turbojet engines work by the principle of coupling a turbine to a fuel combination system. Spinning of the turbine compresses a fuel-air mixture which, when burned, provides thrust and torque to rotate the turbine. The first turbojet engines derived their thrust from exhaust leaving the engines. Modern variants of the turbojet engines include turbo prop and turbofan engines, which use torque generated by the exhaust to drive a propeller or fan in addition to compressing the fuel-air mixture. Rocket engines are possibly one of the oldest mechanical propulsion systems, and have not changed much since their inception. A rocket comprises a tube or cone in which sits (or into which is fed) a fuel oxidizer mixture. Expanding gas from combustion of this mixture creates thrust. Rockets, while offering the highest fuel-thrust ratio of any existing propulsion systems, cannot easily vary the amount of thrust they generate. Even adding an ability to turn a rocket on or off significantly complicates its design. 
     Adhesion between two materials may be characterized into five types: mechanical, chemical, dispersive, electrostatic, and diffusive. Out of these five types, so far, only electrostatic and certain types of mechanical adhesion are easily reversible processes. Vacuum may be used to adhere surfaces and lift materials. However, such devices generally require separate mechanisms for generating a reduced pressure and applying the vacuum to a surface. A vacuum generating system will generally include a vacuum pump, control valve, air filter, vacuum gauge, vacuum reserve tank and power source. A benefit of using vacuum for adhesion, however, is that no residue is left. Typically, the other types of adhesion will usually leave behind a residue that is often undesired. 
     Generally, the conventional propulsion systems mentioned above can also be used to compress gas. It is also possible to compress gas via the ideal gas law, such as in piston or diaphragm pumps. Current devices generally require pumping apparatuses separate from a pressurized vessel. 
     The ability of temperature differential to drive gas flow at a surface has long been known. In 1873, Sir William Crookes developed a radiometer for measuring radiant energy of heat and light. Today, Crookes&#39;s radiometer is often sold as a novelty in museum stores. It consists of four vanes, each of which is blackened on one side and light on the other. These are attached to a rotor that can turn with very little friction. The mechanism is encased inside a clear glass bulb with most, but not all, of the air removed. When light falls on the vanes, the vanes turn with the black surfaces apparently being pushed by the light. 
     Crookes initially explained that light radiation caused a pressure on the black sides to turn the vanes. His paper was referred to by James Clerk Maxwell, who accepted the explanation as it seemed to agree with his theories of electromagnetism. However, light falling on the black side of the vanes is absorbed, while light falling on the silver side is reflected. This would put twice as much radiation pressure on the light side as on the black, meaning that the mill is turning the wrong way for Crookes&#39; initial explanation to be correct. Other incorrect explanations were subsequently proposed, some of which persist today. One suggestion was that the gas in the bulb would be heated more by radiation absorbed on the black side than the light side. The pressure of the warmer gas was proposed to push the dark side of the vanes. However, after a more thorough analysis Maxwell showed that there could be no net force from this effect, just a steady flow of heat across the vanes. Another incorrect explanation that is widely put forward even today is that the faster motion of hot molecules on the black side of the vane provide the push. 
     The correct explanation for the action of Crookes radiometer derives from work that Osborne Reynolds submitted to the Royal Society in early 1879. He described the flow of gas through porous plates caused by a temperature difference on opposing sides of the plates which he called “thermal transpiration.” Gas at uniform pressure flows through a porous plate from cold to hot. If the plates cannot move, equilibrium is reached when the ratio of pressures on either side is the square root of the ratio of absolute temperatures. Reynolds&#39; paper also discussed Crookes radiometer. Consider the edges of the radiometer vanes. The edge of the warmer side imparts a higher force to obliquely striking gas molecules than the cold edge. This effect causes gas to move across the temperature gradient at the edge surface. The vane moves away from the heated gas and towards the cooler gas, with the gas passing around the edge of the vanes in the opposite direction. Maxwell also referred to Reynolds&#39; paper, which prompted him to write his own paper, “On stresses in rarefied gases arising from inequalities of temperature.” Maxwell&#39;s paper, which both credited and criticized Reynolds, was published in the Philosophical Transactions of the Royal Society in late 1879, appearing prior to the publication of Reynolds&#39; paper. See, Philip Gibbs in “The Physics and Relativity FAQ,” 2006, at math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html. 
     Despite the descriptions by Reynolds and Maxwell of thermally driven gas flow on a surface dating from the late 19th century, the potential for movement of gases by interaction with hot and cold surfaces has not been fully realized. Operation of a Crookes radiometer requires rarefied gas (i.e., a gas whose pressure is much less than atmospheric pressure), and the flow of gas through porous plates does not yield usable thrust, partially due to the thickness and due to the random arrangement of pores in the porous plates. 
     SUMMARY 
     The present disclosure is directed to exemplary embodiments of apparatus and methods for generating a partial vacuum in a cavity. One exemplary method may include disposing a suction unit on a surface and powering-on the suction generating devices. The method may further include forming a cavity defined by at least the surface and the suction unit and evacuating gas from the cavity to generate the partial vacuum. The suction unit may include a plurality of suction generation devices embedded in the suction unit that suction gas therethrough. 
     Another exemplary method may include covering a cavity with a suction unit having a plurality of suction generation devices embedded in a resilient substrate and evacuating gas from the cavity by powering-on the embedded suction generation devices to generate at least a partial vacuum. 
     One exemplary apparatus may include a suction unit covering a cavity and having a plurality of suction generation devices embedded in a resilient substrate and a power supply for powering the suction generation devices for evacuating gas from the cavity to generate a partial vacuum. 
     The present disclosure is directed to other exemplary embodiments of apparatus and methods for generating a vacuum. The apparatus may include a container having an inlet for receiving a flowable substance and a closable opening, a cover for the closable opening that may be formed of a suction inducing material including gas flow generating devices and example of which is NMSET, and a control unit coupled to the suction inducting material for generating a command to the suction inducing material indicating that the vacuum is to be generated within the container. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present methods, devices and systems will now be described by way of exemplary embodiments to which the invention defined by the claims appended hereto are not limited. The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
         FIG. 1  shows a heat pump. This can be a Peltier slab, a slab driven by thermionic emission, or any other suitable means. 
         FIG. 2  shows gas flow patterns around the heat pump of  FIG. 1 . 
         FIG. 3  shows a gas confined in a square box with parallel hot walls and parallel cold walls. 
         FIG. 4  shows net forces on a stack of Nano Molecular Solid-state Electrodynamic Thrusters (NMSET) with sawtooth geometry. 
         FIG. 5  shows gas particle velocities around a stack of NMSET with sawtooth geometry. 
         FIG. 6  shows the thermo-tunneling enhanced Peltier effect. 
         FIG. 7  shows a stack of NMSET with a parabolic geometry. 
         FIG. 8  shows gas flow patterns around the stack of NMSET of  FIG. 7  and the momentum space of the gas. 
         FIG. 9  shows a stack of NMSET with a triangular geometry. 
         FIG. 10  shows the momentum space of the gas around the stack of NMSET with a triangular geometry. 
         FIG. 11  shows a stack of NMSET with a sawtooth geometry. 
         FIG. 12  shows the momentum space of the gas around the stack of NMSET with a sawtooth geometry. 
         FIG. 13  shows a cross sectional view of an NMSET with an internal arrangement of solid state heat pumps. These heat pumps can be driven by Peltier effect, thermionic emission, or any other suitable means. 
         FIG. 14  shows a perspective view of NMSET with an internal solid state heat pump arrangement on  FIG. 13 . 
         FIG. 15  shows a perspective view of an NMSET with an external solid state heat pump arrangement. 
         FIG. 16  shows a cross sectional view of NMSET with an external solid state heat pump arrangement of  FIG. 15 . 
         FIG. 17  shows a perspective view of NMSET with an external non-solid state heat pump arrangement. 
         FIG. 18  shows a cross sectional view of a staged NMSET arrangement. 
         FIG. 19  shows NMSET with a straight geometry. 
         FIG. 20  shows an exemplary method of manufacturing NMSET. 
         FIG. 21  shows another exemplary method of manufacturing NMSET. 
         FIG. 22  is a side cross-sectional view illustrating a thermal transpiration device. 
         FIG. 23  is a side cross-sectional view illustrating the operation of a thermal transpiration device. 
         FIG. 24  is a side cross-sectional view of a thermal transpiration device with one extended layer and angled walls. 
         FIG. 25  is a top cross-sectional view of the thermal transpiration device illustrated in  FIG. 24 . 
         FIG. 26  is a side cross-sectional view of a thermal transpiration device with one extended layer and wet or dry etched walls. 
         FIG. 27  is a top cross-sectional view of the thermal transpiration device illustrated in  FIG. 26 . 
         FIG. 28  is a side cross-sectional view of a thermal transpiration device with two extended layers and angled walls. 
         FIG. 29  is a cross-sectional view of a beginning construction of one embodiment of a thermal transpiration device. 
         FIG. 30  is a cross-sectional view of the continued construction of the thermal transpiration device shown in  FIG. 29 . 
         FIG. 31  is a side cross-sectional view of the continued construction of the thermal transpiration device shown in  FIG. 30 . 
         FIG. 32  is a cross-sectional view showing further construction of the thermal transpiration device shown in  FIG. 31 . 
         FIG. 33  is a cross-sectional view showing the islands formed in the construction of the thermal transpiration device. 
         FIG. 34  is a top view of an embodiment of a control system in accordance with the present disclosure. 
         FIG. 35  is a top view of the control system illustrated in  FIG. 34  showing the operation of supplying power to a set of connection paths. 
         FIG. 36  is a top view of the control system illustrated in  FIG. 34  showing the effects of a fault in one of the power lines. 
         FIG. 37  is a top view of an embodiment of a control system in accordance with the present disclosure that includes fault tolerance features. 
         FIG. 38  is a top view of another embodiment of the control system, designed to control larger arrays of distributed thrusters, than the control system in  FIG. 34 . 
         FIG. 39  is top view of another preferred embodiment of the control system, designed to control larger arrays of distributed thrusters than the control system in  FIG. 38 . 
         FIG. 40 a    is a top view of another embodiment of the present disclosure, showing primary and secondary affected areas when the control system activates the target area. 
         FIG. 40 b    is a cross sectional view of the embodiment shown in  FIG. 40 a   , with intersecting power lines located on the heated side of the device. 
         FIG. 40 c    is another cross sectional view of the embodiment shown in  FIG. 40 a   , with intersecting power lines on each side of the device. 
         FIG. 41 a    is a top view of another embodiment of the present disclosure with an electrical and or thermal insulator. 
         FIG. 41 b    is a cross sectional view of the embodiment shown in  FIG. 41 a   , with intersecting power lines located on the heated side of the device. 
         FIG. 42 a    is a top view of a grid structure for an array of distributed thrusters which includes a power supply line and a plurality of branch lines at the power line intersection point, to be used with the control system. 
         FIG. 42 b    a top view of a middle insulating layer placed on top of  FIG. 42   a.    
         FIG. 42 c    is a top view a grid structure of a power supply line and a plurality of branch lines that is placed on top of  FIG. 42 b    and creates a plurality of target points from a single power line intersection point to be used with the control system. 
         FIGS. 43 and 44  are schematic diagrams for creating a temperature gradient. 
         FIG. 45  is a plot showing a useful rise and fall in temperature in a device having a temperature gradient. 
         FIG. 46  is a top cross-sectional view of a plurality of thruster regions arranged in horizontal and vertical rows in accordance with the present disclosure. 
         FIG. 47  is a top cross-sectional view of a plurality thruster regions showing the heating effect of adjacent regions when a thruster region is activated. 
         FIG. 48  illustrates an activation sequence for temperature gradient devices among a plurality of temperature gradient devices in accordance with the present disclosure. 
         FIG. 49  shows a cross-sectional view of an exemplary apparatus for generating a vacuum. 
         FIG. 50  shows a top view of the exemplary apparatus of  FIG. 49 . 
         FIG. 51  shows a cross-sectional view of another exemplary apparatus. 
         FIG. 52  shows a cross-sectional view of a further exemplary apparatus. 
         FIG. 53  is a flow chart of an exemplary control method. 
         FIG. 54  is a flow chart of another exemplary control method. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     In preferred embodiments, one example of distributed thrusters, is an apparatus described herein that may be referred to as a Nano Molecular Solid-state Electrodynamic Thruster (NMSET). The basis of operation of NMSET makes it possible to apply NMSET in the fields of, for example, propulsion, adhesion, compression and refrigeration, depending on the manner in which an NMSET is employed. In preferred embodiments, NMSET and related distributed thrusters devices provide lightweight, compact, energy-efficient creation of a gas pressure differential with adjustable flow velocity. 
     Propulsion 
     In some embodiments, distributed thrusters such as NMSET can offer one or more of the following improvements in the field of gas propulsion: 
     1. Improved Resiliency: Damage to any area in a conventional gas propulsion system would probably lead to system-wide failure. Distributed thrusters provide enhanced redundancy and robustness. 
     2. Lightweight: Electrically driven distributed thrusters, may make use of photovoltaic thin films, in which case fuel load vanishes. Furthermore since each thruster in a distributed thrusters system creates a local gas pressure difference, this local effect may require fewer and or lighter apparatuses to maintain the structural integrity of such gas propulsion system, than what would be normally required in a non-distributed gas propulsion system that generates the same gas flow volume.
 
3. Scalability: Conventional gas propulsion systems cannot be easily scaled: optimal turbojets for small aircrafts are not scale reductions of optimal turbojets for large aircrafts. Distributed thrusters are easier to scale as scaling primarily changes the quantity of thrusters while leaving the individual thruster dimensions mostly intact.
 
4. Response Time: Less massive thrust producing devices spool up and down faster; as such, thrust from a distributed thruster gas propulsion system can be more easily adjusted in response to changes of need.
 
5. Power Independence: Most conventional propulsion systems require a specific type or class of fuels in order to operate, whereas some embodiments of distributed thrusters, such as NMSET only requires a source of temperature differential, which can generated by electricity.
 
6. Green Propulsion: Some embodiments of distributed thrusters, such as several embodiments of NMSET, expect an electrical input and as such, do not require fossil fuels to operate; therefore they do not produce polluting exhaust (e.g. carbon monoxide, nitrogen oxide) during ordinary operation when they use a non-polluting method of generating the required electrical currents.
 
     Adhesion 
     In some embodiments, distributed thrusters, such as NMSET, may be used as a lightweight mechanical adhesive that adheres to a surface through suction. The process can be reversible, as the only step required to reverse the adhesion is to cut power to the system, in some embodiments. Using such a system can provide further benefit over electrostatic adhesion in that such a system does not require a material to be adhered to be flat or conductive, and does not leave behind residue. Compared to other mechanical adhesion processes, using such a system may not require a surface being adhered to be pretreated. 
     Gas Compression 
     Because distributed thrusters, such as NMSET, can be arranged to drive gas flow through a surface, all or part of a pressurized vessel may function to provide gas compression. Thus, in some arrangements, separated pumping and pressurized containment may not be required. Moreover, because, the action of such a system generally occurs over a short distance, it is possible, in some embodiments, to use such a system as a highly compact compressor by stacking multiple stages of distributed thrusters. Conventional gas propulsion systems generally operate over length scales of centimeters and sometimes meters. Thus, stacking conventional propulsion systems tends to be a complex and expensive proposition. By contrast, distributed thrusters can be packaged to operate over smaller scales, down to, for example, micrometers. Furthermore, the versatility of such systems means that such a system can be readily adapted to function as a high-pressure pump, a standard atmospheric pump, or with a sufficient number of stages, as a high vacuum pump. 
     NMSET Design 
     In one aspect and embodiment, NMSET and some related devices described here may be thought of as functioning by reducing entropy in gas in contact with the system. Optionally, such device may add energy, in addition to the energy lost through inefficiencies in the system, e.g. thermal energy, to the gas. In another aspect and embodiment, the geometry of NMSET and some related devices can affect gas flow direction and convenience of use. Several embodiments of NMSET and some related devices may be further distinguished from previous thermal transpiration devices and the like by the combined application of scale parameters, materials having advantageous molecular reflection properties, geometries, design, construction and arrangement of elements that provide significant increase in efficiency, and or capabilities to operate at higher ambient pressures and/or produce higher flow rates. Described herein are various exemplary embodiments of NMSET with discussion of these and other parameters that, in preferred embodiments, can create a strong gas flow in a particular direction with minimal thermodynamic loss, and or operate at higher ambient pressures and or produce higher flow rates. 
     Reduction of entropy in a gas by NMSET may be represented by a transformation A in the momentum space k of the gas. A can be expressed in a matrix once a set of suitable bases is chosen for the momentum space k. If the expectation value of the transformed momentum space Ak is nonzero, the NMSET receives a net momentum in the opposite direction of the expectation value due to the conservation of momentum. 
     The geometry of NMSET may be optimized for more efficient functioning. The geometry of NMSET affects the transformation matrix A. A geometry that produces a matrix A essentially equal to an identity matrix I does not create a net momentum bias (i.e. will not make the transformed momentum space Ak have a nonzero expectation value). Rather, gas vortexes may be generated. Geometries that result in larger eigenvalues of A tend to imply a more efficient function, e.g., that more momentum is carried by gas particles moving in a particular direction. 
     As an example, consider a heat pump  100  immersed in a gas, shown in  FIG. 1 . The heat pump  100  comprises an upper layer  101  and a lower layer  102 . For simplicity, a Cartesian coordinate system can be referenced with a y-axis pointing from the lower layer  102  to the upper layer  101 . A temperature differential can be established by a Peltier device (not shown) between the layers or any suitable means such that the upper layer  101  is colder than the gas and the lower layer  102  is hotter than the gas. For sake of simplicity, one may assume that the heat pump  100  has 100% Carnot cycle efficiency. However, other efficiencies are contemplated. In this case, the heat pump  100  will not transfer net heat into the gas. Transformation caused by the heat pump  100  to the momentum space k of the gas can be expressed by a Hermetian matrix A. When a gas particle (molecule or atom) collides with the lower layer  102 , assuming the collision is diabatic, the gas particle rebounds off at a higher velocity than before the collision. When a gas particle collides with the upper layer  101 , assuming the collision is diabatic, the gas particle rebounds off the upper layer  101  at a reduced velocity than before the collision. The heat pump  100  feels a net force in the y direction. In other words, the lower layer  102  heats and thus increases pressure of the gas below the lower layer  102 , while the upper layer  101  cools and thus decreases the pressure of the gas above the upper layer  101 . The pressure difference exerts a force on the heat pump  100  in the y direction. In terms of transformation of the momentum space k of the gas, as gas particles rebounding from the upper layer  101  leave with less momentum than gas particles rebounding from the lower layer  102 , the transformed momentum space Ak becomes skewed preferentially in the −y direction, i.e., the expectation value p of the transformed momentum space Ak is nonzero and points to the −y direction. Assuming the gas and the heat pump  100  compose a closed system (i.e., no interaction with other objects), the heat pump  100  gains a momentum −p to conserve the total momentum of the closed system. 
     While the geometry of the heat pump  100  in  FIG. 1  does generate a direction force, in certain circumstances it may not be practical for the following reasons: 
     1. If the heat pump  100  is large, translational motion of the heat pump  100  along the y direction forces the gas to flow all the way around edges of the heat pump. 
     2. The vast majority of the heat is transferred from surfaces of the heat pump  100  via gas convection. 
     3. Gas near the surfaces has an insulating effect. Momentum transfer between the heat pump  100  and the gas is not efficient except in proximity of the edges of the slab, as shown in  FIG. 2 . 
     4. Surface area of the heat pump  100  is surface area of its convex hull. 
     These problems all relate to a single core issue, very little of the gas has any direct surface contact. Thus, a more complex geometry can be advantageous. Exemplary embodiments with three different geometries are described herein. 
     Principles of Operation 
     Although many different geometries of NMSET or related devices are possible, the principle of operation of NMSET remains the same. While not wanting to be limited to any particular theory, operation uses energy to reduce entropy on some device surfaces and transfer reduced entropy to a gas in contact with the surface. The device can optionally donate energy to the gas by raising the gas temperature. The function of NMSET may be therefore divided into three areas: the means by which entropy on surfaces of the device is reduced, the means by which the reduced entropy is transferred to the gas, and the optional means other than the inefficiency of the Carnot cycle of the heat pump by which the gas temperature is increased. 
     Temperature Differential 
     A temperature differential between layers of material or, more precisely, between two opposing surfaces is generally required for NMSET or related device to operate. In preferred embodiments described herein, a temperature differential can be established in a solid-state electrodynamic mechanism, i.e., the “SE” of NMSET. However, the devices and methods described here are not limited to electronic or purely solid state devices. For example, a temperature differential may be established by conduction of heat from combustion using a fluid coolant, exothermic chemical reaction, or other chemical source. A temperature differential may be established by simple resistive heating, by the Peltier effect, by thermionic emission, by the thermo-tunneling enhanced Peltier effect, or by any other suitable means, such as explained below. A means by which the temperature differential is established between two objects can be phenomenologically described by two characteristics: entropy-reduction (heat transfer between the two objects), and diabaticity (total heat transfer between environment and the two objects). 
     In one embodiment, the Peltier effect can be used to establish a temperature differential. The Peltier effect occurs when an electric current is applied through a loop composed of two materials with different Peltier coefficients joined at two junctions. Depending on the direction of the electric current, heat flows from one junction to the other, causing a temperature differential to be established between the junctions. The Peltier effect can be understood as follows: Heat capacity of charge carriers in a material is characterized by the Peltier coefficient Π, which is the amount of heat carried per unit charge carriers in the material. When an electric current/flows through a junction of material A with Peltier coefficients Π A  and material B with Peltier coefficient Π B , the amount heat carried by charge carriers to the junction in a unit time is I×(Π A −Π B ). 
     An ideal Peltier effect reduces entropy locally and is adiabatic. Assuming Joule heating and or Carnot cycle inefficiencies can be ignored, in the Peltier effect, heat is transferred from one junction to another, but no heat is added into the loop of the two materials. This entropy reduction can provide for advantages in the stackability of NMSET and related devices. Consequently, the Peltier effect lends itself particularly well to some embodiments. 
     In this embodiment, a power source drives an electric current between two surfaces. Charge carriers such as electrons and/or holes carry heat as they flow in the electric current, and thus create a temperature differential between the two surfaces. Entropy is reduced as the temperature differential is established. 
     Phonon flow reduces the temperature differential established by the Peltier effect. If phonons are permitted to flow freely (i.e., infinite thermal conductivity or zero heat capacity), their flow will cancel the temperature differential established by the Peltier effect. Efficiency of the Peltier effect can be increased by reducing electrical resistance and thermal conductance. 
     One way to reduce thermal conductance is to place a narrow vacuum gap in the path of the electric current. Phonons cannot easily pass the vacuum gap but charge carriers can do so under a voltage across the vacuum gap. This is called thermo-tunneling enhanced Peltier effect (or thermotunnel cooling).  FIG. 6  shows a diagram of the thermo-tunneling enhanced Peltier effect. Charge carriers  601  can tunnel through a vacuum gap  602 . 
     The thermo-tunneling enhanced Peltier effect is generally only significant at high temperatures or voltages, unless enhanced by choice of surface geometry and materials that can restrict behavior of charge carriers near the vacuum gap and increase tunneling probability. For example, suitable surface coatings and structures can function as a filter that do not allow low energy states of charge carriers but only high energy states of charge carriers near the vacuum gap. 
     In another embodiment, a temperature differential can be created and maintained by field-enhanced thermionic emission. Thermionic emission is a heat-induced flow of charge carriers over a potential-energy barrier. The charge carriers can be electrons or ions (i.e., thermions). In a simple approximation, the potential-energy barrier acts like a dam, in that it withholds carriers with thermal energy less than its height and allows carriers with thermal energy greater than its height to flow over. When the overflowing carriers pass the potential-energy barrier, heat is carried away with them. The carriers left behind the potential-energy barrier re-thermalize (redistribute in energy) to a lower temperature. Thermionic emission typically requires an operating temperature of several hundred degrees Celsius so that a non-negligible fraction of the carriers has thermal energies great enough to overcome the potential-energy barrier. An electrical field can assist thermionic emission by reducing the height of the potential-energy barrier and reducing the required operating temperature. 
     A temperature differential in NMSET or related device can also be established by using resistive heating (explained below) and/or by suitable chemical processes. In order to maintain the temperature differential without raising the overall temperature of the device, some cooling means can also be provided, such as a heat sink exposed to atmosphere. No matter what cooling means is used, the temperature differential is more pronounced if warmer surfaces of the device are not cooled as efficiently as cooler surfaces, which can be achieved, for example, by thermal insulation. 
     Force Generation 
     In one aspect, the production of net thrust may be thought of as the transfer of the reduced entropy from an established temperature differential to a gas. Without wishing to be bound by theory, consider a single device operating in a gas, as an adiabatic process. In this example, a temperature differential between a hot and a cold layer can be established by a suitable means such as the Peltier effect. For simplicity, assume no net heat transfer between the gas and the device. Particles of the gas will impact the hot and cold layers with equal probabilities, and their interaction with these layers will have consequences on local momentum space of the gas near surfaces of the hot and cold layers. The local momentum space of the gas very close to a surface of the hot and cold layers has nonzero expectation value when the gas and the surface have different temperatures. Assuming also that no gas particles penetrate the surface, the gas particles rebound from the surface with momenta different from their incident momenta, which skews the momentum space along the surface normal, and the magnitude of the skew is directly related to the temperature difference between the surface and the gas. 
     In an arrangement with random geometry (i.e. surface normals at different surface locations point to random directions), the weighted sum of expectation values of local momentum spaces of the gas is nearly zero, which results in almost no net thrust. In NMSET with an optimized geometry, however, the weighted sum of expectation values of local momentum spaces of the gas can be non-zero, which leads to a net thrust. 
     A trivial example of an arrangement that has non-zero net thrust is shown in  FIG. 1 , as described above. This geometry is not very efficient because macroscopic convective gas flows and vortex formation increase the entropy and limit the amount of useful work. Exemplary convective gas flows  120 ,  130  are shown in  FIG. 2 . Gas at ambient temperature  110  flows towards the cold layer  101  and gets cooled. Cooled gas flows  120  away from the cold layer  101  and around the edge of the heat pump  100  towards the hot layer  102 . Heated gas flows  130  away from the hot layer  102 . 
     To simplify the description, it may be helpful to think about the system in terms of Newton&#39;s second law and the kinetic theory of gases. Around the heat pump  100  in  FIGS. 1 and 2 , assuming that temperature of the gas is bracketed by the temperatures of the layers  101  and  102 , gas particles that collide with the layer  102  leave the layer  102  with greater momentum than before the collision. Similarly, gas particles that collide with the layer  101  leave the layer  101  with lesser momentum than before the collision. Since gas pressure is directly related to momenta of gas particles, gas near the layer  102  has higher pressure than gas near the layer  101 . This pressure bias pushes the entire heat pump  100  in the y direction. 
     In another embodiment, the heat pump  100  can have at least one through hole between the layer  101  and  102 . Gas spontaneously flows from the layer  101  to the layer  102  through the hole which enables higher heating rate of the gas. Such preferential flow of gas is referred to as thermal transpiration. Assuming gas near the layer  101  has temperature of T c  and pressure of P c , and gas near the layer  102  has temperature of T h  and pressure of P h , thermal transpiration causes the gas to flow from the layer  101  to the layer  102  through the hole, if the following equation is satisfied: 
     
       
         
           
             
               
                 
                   
                     
                       
                         T 
                         h 
                       
                       
                         T 
                         c 
                       
                     
                   
                   ≥ 
                   
                     
                       P 
                       h 
                     
                     
                       P 
                       c 
                     
                   
                 
               
               
                 
                   [ 
                   1 
                   ] 
                 
               
             
           
         
       
     
     In order to improve efficiency, it is helpful to understand where the classical limit exists within gas flows. Convective descriptions of gas flow break down at around length scales where the Knudsen number appears. As a result, in some aspects, the mean free path of a gas becomes a useful parameter in determining advantageous geometries of NMSET. 
     For instance, consider a gas at a particular pressure having a mean free path of 10 nm. If a cloud of such gas is trapped in a two dimensional square 20 nm by 20 nm box as shown in  FIG. 3 , a gas particle, within 10 nm of travel, will be approximately as likely to have struck another gas particle as it is to have struck the walls of the box. If the walls of the box are heated, then smaller boxes will reach thermodynamic equilibrium with gas therein faster than larger boxes, because gas particles in smaller boxes have more chances to collide with and exchange heat with the walls. Generally, when most of collisions in a gas are between gas particles and a surface, then thermodynamic equilibrium can be achieved approximately in the mean free time (the time it takes a gas particle to travel the mean free path). 
     For this reason, in some embodiments, the characteristic scale of individual features of NMSET and related devices may be nanoscale, i.e., the “NM” of NMSET. However, it must be understood that the methods and devices described here are not limited to nanoscale embodiments. The mean free path parameter is dependent on gas density so that in some embodiments and uses, larger scale features may be employed. Furthermore, as described herein, pluralities of NMSET and related device elements can be combined to provide action over a large surface. For example, distributed thrusters such as NMSET may advantageously be arranged in arrays and or arrays of arrays to provide directed movement of gas over across large surfaces, for example, as illustrated in  FIGS. 15, 16 and 17 . Distributed thrusters such as NMSET can also be arranged in one or more stages to achieve a greater pressure differential, for example as illustrated in  FIGS. 18A-18D .  FIG. 18A  illustrates a cross sectional view of an array of staged distributed thrusters such as NMSET arrangements  1800 . Each staged arrangement  1800  is composed of stages  1810 ,  1820 ,  1830  of in the form of concentric half-spheres containing arrays of distributed thrusters such as NMSET  1840 ,  1850 ,  1860  illustrated in blow-up in  FIGS. 18B-18D . Individual distributed thruster apertures  1845 ,  1855 ,  1865  in each stage increase in optimal size and thickness in accordance with the decreasing ambient pressure that would be experienced at each previous stage in operation. 
     Surface Interaction 
     Interaction between surfaces can affect the momentum space transformation matrix A. If nearby surfaces can easily exchange phonons via gas particles, then the entropy at these surfaces will locally increase at a higher rate than surfaces which cannot easily exchange phonons via development of vortexes. This will generally reduce the efficiency of a system. 
     One method by which phonon exchange may be reduced is to limit or eliminate any shared bases between surfaces. For instance, consider gas particles in the box  300  in  FIG. 3 . Box  300  comprises two planar hot walls  302  parallel to each other, and two planar cold walls  301  parallel to each other and perpendicular to the hot walls  302 . If the box  300  is comparable in size to the mean free path of the gas particles therein and the walls  301  and  302  are perfectly specular, the gas particles can reach thermal equilibrium with the cold walls  301  and the hot walls  302  independently. This is because surface normals of the walls are only shared between the two cold walls  301  or between the two hot walls  302 , but not between a cold wall  301  and a hot wall  302 . Consequently, little to no momentum can be exchanged between the hot walls  302  and the cold walls  301  by the gas particles. This is because interaction between the gas particles and the cold walls  301  only affect momenta in the x direction but not momenta in the y direction; and interaction between the gas particles and the hot walls  302  only affect momenta in the y direction but not momenta in the x direction and the fact that momenta in the x direction are orthogonal with momenta in the y direction, assuming no collisions between gas particles. After thermal equilibrium is reached between the gas particles and the walls, the gas particles move faster in the y direction than in the x direction. 
     As a practical matter, surfaces are usually not perfectly specular. However, specular surface properties exist very strongly in some materials so that there are angles for which convective flows in corners may be reduced. This effect is generally observed when the Knudsen numbers are large, which is a preferred condition for NMSET and related devices, particularly in nanoscale embodiments. The Knudsen number (Kn), named after Danish physicist Martin Knudsen (1871-1949), is a dimensionless number defined as the ratio of the molecular mean free path to a representative physical length scale. In NMSET or the related devices discussed here, the representative physical length scale is taken to be the order of magnitude of the aperture diameter of the device, i.e., the representative physical scale length is, for example, a nanometer if the aperture is measured in nanometers, and a micrometer if the aperture is measured in micrometers. In preferred methods of using the devices disclosed herein the Knudsen number is preferably greater than 0.1, or greater than 1, or greater than 10. 
     Methods of Optimizing NMSET and Related Devices 
     Modeling 
     Performance of NMSET with a specific geometry can be simulated by a Monte-Carlo method for optimization. Specifically, a simulation for NMSET or related device with any given geometry starts with a group of gas particles with random initial positions and momenta around the device. Positions and momenta of these particles after a small time interval are calculated from the initial positions and momenta, using known physical laws, parameters such as temperature, pressure, chemical identity, geometry of the device, interaction between surfaces of the device and the gas particles. The simulation is run through a chosen number of iterations and simulation results are analyzed. The geometry of the device can be optimized using simulation results. In preferred embodiments, a device is constructed using the results of the simulation analysis. 
     In a preferred embodiment, a simulation can be represented in the following table: 
     
       
         
           
               
             
               
                   
               
               
                 Algorithm 1 EVOLVE MODEL(M, P, k) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 M←0 
               
               
                   
                 P←a set of search parameters 
               
               
                   
                 k←number of iterations 
               
               
                   
                 for i = 1 to k do 
               
               
                   
                 V←An instance of P 
               
               
                   
                 V←V perturbed by M 
               
               
                   
                 E←MONTE CARLO(V) 
               
               
                   
                 Update M using E 
               
               
                   
                 end for 
               
               
                   
                   
               
            
           
         
       
     
     A perturbation model M is evolved through a number (k) of iterations. First, M is initialized to an empty set, indicating no solution knowledge. Then, a loop is started in which the search parameters generate an arbitrary element from the definite search space P and the prior learned knowledge M is used to perturb P. The specific algorithm used to perturb as an implementation detail. 
     If run in a grid computing environment, M should ideally be identical among all nodes, but this is not necessary due to the inherently stochastic nature of the process. The step of EVOLVE_MODEL which actually runs the Monte-Carlo simulation is the most computationally expensive of all by far and offers a lot of time to synchronize M. 
     Specific parameters depend on the environment. The parameters that the user can specify include the following: 
     1. Molecular diagrams, in some embodiments containing up to three atoms, such as CO9 2  or H 2 O. 
     2. Partial concentrations for constituent molecules. 
     3. Initial temperature and pressure of the entire gas. 
     In a stationary simulation, the Monte-Carlo simulation can be run with periodic bounds in all axes. In the y axis, however, particles encountering the periodic bound are stochastically thermostatted according to temperature and pressure settings in order to simulate ambient conditions. In the x axis, particle velocities are unmodified in order to simulate a periodic ensemble of identical device assemblies along that direction. The simulation may be run in two dimensions to reduce the computational complexity of the simulation. A three dimensional simulation should give similar results where the modeled device has cylindrical symmetry. Note that in general, a simulator does not have to use the periodicity as indicated here and may not specify any boundaries at all; they are only defined as a computational convenience. 
     In preferred embodiments, potential device geometries can be evaluated in consideration of the conditions under which a device will be used and known surface reflection properties of the material from which it will be constructed. Geometrical parameters can be optimized by analyzing results from simulation before the geometry is actually used in manufacture of NMSET and related devices. 
     Example Geometries 
     Four embodiments with different geometries are particularly discussed below. These four geometries will be referred to as Straight, Parabolic, Triangular, and Sawtooth. It must be noted that the geometries of the NMSET and related devices described here can vary considerably and these examples should be taken only as illustrations for the purpose of discussing the effects of certain design choices on system efficiencies. 
     Straight 
       FIG. 19  shows an embodiment of NMSET or related device  1900  with a straight geometry. In this embodiment, the device  1900  comprises a hot layer  1902  and a cold layer  1901 . The terms “hot layer” and “cold layer” mean that these layers have a temperature difference therebetween, not that the “hot layer” is necessarily hotter or the “cold layer” is necessarily colder, than a gas that the NMSET or related device is immersed in. At least one straight through hole  1910  extends through all layers of the device  1900  and preferably has a similar cross-sectional shape and size for each set of layers. The straight through hole  1910  can have any cross-sectional shape such as circular, slit, and comb. 
     Preferably, a total length  1910 L (i.e. a distance from one entrance to the other entrance) of the straight through hole  1910  is up to 10 times, up to 5 times or up to 2 times of the mean free path of a gas in which the device  1900  is immersed. The mean free path of air at the standard atmosphere pressure is about 55 nm. At higher altitude, the mean free path of air increases. For atmospheric applications, the total length  1910 L is preferably not greater than 1500 nm, and depending on application more preferably not greater than 550 nm, not greater than 275 nm or not greater than 110 nm. A temperature differential between the hot layer  1902  and the cold layer  1901  is preferably at least 0.5° C., more preferably at least 30° C., more preferably at least 50° C., and most preferably at least 100° C. 
     The hot layer  1902  and the cold layer  1901  may be separated by a gap therebetween for thermal isolation. The gap preferably is a vacuum gap and/or contains a thermal insulator. In one example, the gap contains a plurality of thin pillars made of a good thermal insulator such as silicon dioxide. 
     The device  1900  has preferably at least 10 straight through holes per square centimeter. A total perimeter length of all the straight through holes of the device  1900  per square centimeter is preferably at least two centimeters. 
     Parabolic 
       FIG. 7  shows an embodiment of an NMSET or related device  700  with a parabolic geometry. In this embodiment, alternating hot layers  702  and cold layers  701  are stacked. In the illustration, each hot layer  702  and cold layer  701  has a straight through hole. All the holes are aligned. The hole in each hot layer  702  has the similar size as the hole in the cold layer  701  immediately above, and is smaller than the hole in the cold layer  701  immediately below. Each cold layer  701  is colder than its immediate adjacent hot layers  702  and each hot layer  702  is hotter than its immediate adjacent cold layers  701 . A surface  702   a  of each hot layer  702 , which has a surface normal in the −y direction, is exposed. All the holes collectively form a nozzle with a contour of a parabolic surface. This geometry minimizes shared bases between the hot and cold layers. However, because NMSET or related device may not substantially increase the energy of the gas, the increasing hole diameter may result in a drop in gas pressure at the edges. This can create strong vortexes near the lower aperture, which reduce total efficiency. NMSET with the parabolic geometry can be adiabatic or isobaric, but not both. An approximation of gas flow in NMSET or related device with the parabolic geometry is shown in  FIG. 8 . The momentum space of the gas is skewed such that the expectation value of the momentum points to the −y direction. 
     Although the parabolic geometry is effective in NMSET or related device, a drop in gas pressure puts an upper bound on the size of the lower aperture. In general, any adiabatic device in which the gas being moved undergoes a change in volume will suffer in its efficiency. 
     If the temperature differential in a device with the parabolic geometry is established by a diabatic means (i.e. the device raises the overall temperature of the gas), then the NMSET with the parabolic geometry may not suffer in its efficiency from the gas undergoing a change in volume, as long as the amount of heat added to the gas is sufficient to prevent the formation of vortexes. However, such a device suffers in its efficiency from higher total entropy, i.e., the eigenvectors of the momentum space of the gas are not as far apart if the gas has to expand, but supplying heat at small scales is typically easier than carrying it away. 
     Triangular 
     The triangular geometry detailed in  FIG. 9  is a partial optimization of the parabolic geometry for adiabatic flows. In this case, the gas is not permitted to experience a sufficient expansion to trigger large-scale vortex generation. Furthermore, because the apertures do not change size, a triangular arrangement such as this one may be easily stacked. 
     The momentum space of this triangular geometry is more efficiently biased, as is illustrated in  FIG. 10 . As in the parabolic arrangement, the exposed hot and cold surfaces meet at preferably a 90-degree angle; however, a source of inefficiency arises when particles carry heat back and forth between surfaces across the center gap. 
       FIG. 9  shows a stack  900  of NMSET or related device with the triangular geometry. Each device in the stack  900  comprises a hot layer  902  and a cold layer  901  of equal thickness. The temperature differential between the cold and hot layers  901  and  902  can be established by any suitable means such as the Peltier effect or any other heat pump. Each device has a through hole  903 . Each though hole  903  has approximately a 45° chamfer ( 9031  and  9032 ) on each entrance. The surface of the chamfers  9031  and  9032  is, for example, from 1.40 to 1.42 times of the thickness of the cold and hot layers  901  and  902 , not including modifications to the acute angles for structural considerations. The through holes  903  in all layers in the stack  900  are aligned. In general, the temperatures of the hot layers  902  in a device in the stack  900  do not increase monotonically from one side of the stack to the other side. In general, the temperatures of the cold layers  901  in a device in the stack  900  do not decrease monotonically from one side of the stack to the other side. Preferably, each cold layer  901  is colder than its immediate adjacent hot layers  902  and each hot layer  902  is hotter than its immediate adjacent cold layers  901 . For engineering reasons, the hot and cold surfaces of the triangular arrangement may not come to a fine point. 
     Sawtooth 
       FIG. 11  shows a stack  1100  of NMSET or related device with a sawtooth geometry. Each device in the stack  1100  comprises a hot layer  1102  with a thickness of t h  and a cold layer  1101  with a thickness t c . The temperature differential between the cold and hot layers  1101  and  1102  can be established by any suitable means such as the Peltier effect or any other heat pump. Each device has a through hole  1103 . In the illustrated device, each through hole  1103  has a chamfer  11031  at the entrance on the side of the cold layer  1101 , and a chamfer  11032  at the entrance on the side of the hot layer  1102 . An angle between the chamfer  11031  and a center axis of the through hole  1103  is θ 1 ; an angle between the chamfer  11032  and a center axis of the through hole  1103  is θ 2 . The sum of θ 1  and θ 2  is preferably from 75° to 105°, more preferably from 85° to 95, and more preferably from 88° to 92°. The ratio of t c  to t h  is substantially equal to the ratio of cotangent of θ 1  to cotangent of θ 2 . θ 2  is preferably from 70° to 85°. 
     The relationships of the chamfer angles described here are preferred limitations, not hard boundaries. In general for materials exhibit perfectly specular molecular reflection properties, the relationships of the chamfer angles can be slightly relaxed. For materials exhibit less than perfectly specular molecular reflection properties, the relationships shall be stringent. The chamfer geometries are preferably arranged so as to minimize shared bases. The surface normals of the specularly reflecting chamfer surfaces can thus preferably be orthogonal. Deviations from orthogonality can incur a penalty in efficiency as a cosine function. For engineering reasons, the hot and cold surfaces of the sawtooth arrangement may not come to a fine point. 
     In the illustrated device, the through holes  1103  in all layers in the stack  1100  are aligned. Temperatures of the hot layers  1102  in each device in the stack  1100  do not increase monotonically from one side of the stack to the other side. Temperatures of the cold layers  1101  in each device in the stack  1100  do not decrease monotonically from one side of the stack  1100  to the other side. Each cold layer  1101  is colder than its immediate adjacent hot layers  1102  and each hot layer  1102  is hotter than its immediate adjacent cold layers  1101 . 
     The sawtooth geometry shown in  FIG. 11  offers an improvement over the triangular geometry in that all hot layers  1102  are preferably oriented in nearly the same direction (i.e., θ 2  is preferably nearly 90°). This reduces direct interaction between hot and cold layers  1102  and  1101  across the through hole  1103 , and improves overall efficiency. 
     Furthermore, because the hot layers  1102  have a lower exposed surface area than the cold layers  1101 , and because the cold layers  1101  are preferably oriented at a shallower angle relative to the center axis of the through hole  1103  than in the triangular geometry, the sawtooth geometry is capable of reducing the entropy in the gas (and thereby causing it to do more work) more efficiently than the triangular geometry. The momentum space of this sawtooth geometry is more efficiently biased than the momentum space of the triangular geometry, as is illustrated in  FIG. 12 . 
     In the triangular configuration, device slices on opposite sides of a cross section have a magnitude of 1/√{square root over (2)} in the y axis because their separation angle 90 degrees. This limits the efficiency of entropy reduction, as some of the entropy is going to be neutralized in direct inter-surface interaction. 
     In the sawtooth configuration, however, the hot layers  1102  not only share no basis with the adjacent cold layers  1101 , but also share very little basis with hot and cold layers across the through hole  1103 . This combined property makes the sawtooth geometry more efficient than the triangular geometry. 
     After NMSET or related device is powered (i.e. temperature differential is established), gas particles rebounding from cold layers have a reduced net velocity, while gas particles rebounding from hot layers have higher net velocity.  FIG. 4  shows net forces the layers of the stack  1100  (sawtooth geometry) experience. In a stable state, low pressure is generated at the entrance aperture (upper aperture in  FIG. 4 ) which in turn generates a corresponding low-pressure region above the stack  1100 , and a high-pressure region below the stack  1100 . Gas particle velocities of the stack  1100  resulting from the gas particle collisions are shown in  FIG. 5 . 
     Means for Establishing Temperature Differential 
     Internal Peltier 
     According to one embodiment, each element in the device geometry acts both as a particle director and as the entropy reducer. In a Peltier device, the hot and cold plates are made of materials with different Peltier coefficients. Electrical current is made to flow between the cold and hot plates. This flow of current carries with it Peltier heat, establishing the temperature differential necessary to operate the device. In some embodiments, piezoelectric spacers can be disposed between device elements to maintain the separation gaps therebetween. 
     A cross section of NMSET or related device according to an embodiment with an internal Peltier arrangement is detailed in  FIGS. 13 and 14 . All hot layers  1302  are connected. All cold layers  1301  are connected. Electric current flows through a Peltier device interposed between the cold and hot layers to establish a temperature differential. The thinner the layers are, the higher the electric current is necessary. 
     NMSET or related device with the internal Peltier arrangement can make it easier to reduce the size of the device. A single stack such as the one shown in  FIG. 14  can be fully functional to generate thrust. NMSET or related device with the internal heat pump are further suitable for use in microelectromechanical systems (MEMs) that emphasize the highest degree of granularity. 
     Field-Enhanced Thermionic Emission 
     In another embodiment, the temperature differential can be generated by field-enhanced thermionic emission. As shown in  FIG. 19 , an electrical field can be established between the layers  1901  and  1902  such that charge carriers thermally emitted from the cold layer  1901  carry heat from the cold layer  1901  to the hot layer  1902 . 
     External Peltier 
     In another embodiment, the temperature differential can be generated by a heat pump, such as a Peltier device external to NMSET or related device. This Peltier device arranged in a checker board fashion is thermally coupled to NMSET or related device stack  1500  via interface layers  1510  and  1520  as detailed in  FIGS. 15 and 16 . 
     A device with an external Peltier device has the benefit of separating the materials used to generate gas flow from the materials used to generate the temperature differential. From an engineering standpoint this may be desirable, as the materials suitable for a heat pump may not be suitable for microstructures, or vice versa. In addition, an external heat pump can be made larger and more efficient, and may require less current to establish a sufficient temperature differential. 
     Piezoelectric spacers can be used between layers. Materials suitable for use in NMSET preferably are strong enough to mechanically withstand thermal expansion and contraction, and/or preferably have very small expansion coefficients. Otherwise, holes in the layers could become misaligned, which could reduce efficiency. 
     External Non-Peltier 
     According to yet another embodiment, a temperature differential is established by any suitable heat source and/or heat sinks. For example, the heat sources might be field-enhanced thermionic emission, resistive heaters, chemical reaction, combustion, and/or direct illumination of bright light or other forms of radiation. An illustration of such an embodiment is shown in  FIG. 17 . In the example shown, a heating surface  1702  can be resistive heating material, or a material that can efficiently receive radiative heating. The external non-Peltier heat pump is convenient because it does not require a built in heat pump such as a Peltier device. For some applications, it may be convenient to direct the heating surface towards a source of radiation, such as the sun, rather than first converting radiation into electricity and drive a heat pump. Alternatively, a source of radiation may be directed toward a heat absorbing surface in thermal communication with the hot layer of NMSET or related device. In an external non-Peltier heat pump, however, more care is preferably taken to ensure that the NMSET or related device does not overheat. 
     The capillaries  1750  illustrated in  FIG. 17  provide an exemplary mechanism by which a heat sink could be provided; however, it is also possible for the heat sink to simply be a series of vanes, or any other suitable heat sinks. Alternatively, the external non-Peltier heat pump in  FIG. 17  could be configured to provide a heat source through the capillaries  1750 . The heat source can be an exothermic chemical reaction, preferably one that does not generate too much pressure. 
     Materials 
     NMSET and related devices may be constructed of a wide range of materials. In various aspects, properties of materials may be exploited in combination with desirable geometries. 
     Specular reflection of gas molecules is a preferred property of the materials which form the gas-exposed surfaces of NMSET or related device, e.g. the heated and cooled surfaces which are in contact with flowing gas. Specular reflection is the mirror-like reflection of light, or in this case gas particles, from a surface. On a specular surface, incoming gas particles at a single incident angle are reflected from the surface into a single outgoing angle. If the incoming gas particles and the surface have the same temperature, the incident angle and the outgoing angle with respect to the surface normal are the same. That is, the angle of incidence equals the angle of reflection. A second defining characteristic of specular reflection is that incident, normal, and reflected directions are coplanar. If the incoming gas particles and the surface are not at the same temperature and the reflection is diabatic (i.e. with heat exchange between the gas particles and the surface), the angle of reflection is a function of heat transferred between the surface and the gas particles. 
     The degree of specularity of a material may be represented by a reflection kernel (such as the Cercignani-Lampis kernel) which is defined as the probability density function of reflected state of the gas particles per unit volume of the phase space. Details of the reflection kernel are disclosed in “Numerical Analysis of Gas-Surface Scattering Effect on Thermal Transpiration in the Free Molecular Regime”,  Vacuum , Vol. 82, Page 20-29, 2009, and references cited therein, all of which are hereby incorporated by reference. 
     Individual hot and cold layers may also be constructed of one or more structural elements which can comprise structural materials, e.g. a means for conferring rigidity, thermal conductive material, e.g., a means for heat transfer to and from a temperature differential generating means, and atomic reflection material, e.g. means for providing a desirable reflection kernel properties. In some embodiment, individual hot and cold layers may be constructed of layered composites of such materials. 
     Thus, the choice of materials is and composition is widely variable. In some embodiments, materials suitable for construction of NMSET or related device can include titanium, silicon, steel, and/or iron. Titanium is light weight and possesses a hexagonal crystalline structure. Interfaces of titanium may be created at orthogonal angles without crystalline warping and therefore no stress limit. Material costs of titanium are high. Silicon is inexpensive and has well understood properties and processes for machining. The crystalline structure of silicon is diamond cubic. Steel is cheaper than titanium, possesses a cubic crystalline structure, and is highly resistant to gaseous intrusion. Iron is cheaper than steel and has a crystalline form which makes it suitable for application in NMSET and related devices. 
     Exemplary Methods of Manufacturing NMSET or Related Device 
     According to one embodiment as shown in  FIG. 20 , a method of manufacturing an NMSET or related device comprises: (a) providing a suitable substrate  2001  such as, for example, amorphous silicon, crystalline silicon, ceramic, etc., the substrate preferably having a thickness of 500 to 1500 microns; however thinner and thicker substrates are possible; (b) depositing a first layer  2002 , a mostly sacrificial layer, preferably an electrical insulator, such as, for example, silicon dioxide, the first layer  2002  preferably having a thickness of 200 nm to 50 microns, however thinner and thicker layers are possible. Furthermore, depending on the area of the substrate window  2001   a , it is advantageous for this layer to have a tunable stress level. For example, for a 1 cm 2  substrate window  2001   a , successful results have been achieved with SiO x N y  at 60 MPa tensile strength; (c) forming a pattern of discrete islands in any suitable shape such as, for example, strip, square, circle from the first layer  2002  by photolithography and etching the first layer  2002 ; (d) depositing a second layer  2003  over the discrete islands, the second layer  2003  being an electrical conductor such as, for example, Al, Nb or Zn, preferably having a thickness of 5 to 200 nm, however other thicknesses are contemplated; (e) depositing a third layer  2004  over the second layer  2003 , the third layer  2004  being an electrical insulator such as, for example, silicon dioxide or the same material used in layer  2002 , preferably having the same thickness as the first layer  2002 , however other thicknesses are contemplated; (f) partially removing the third layer  2004  and the second layer  2003  until the first layer  2002  is exposed; (g) depositing a fourth layer  2005 , the fourth layer  2005  being an electrical insulator such as, for example, silicon dioxide preferably of the same material as  2003 , the fourth layer  2005  preferably having a thickness of 3 to 15 nm, thinner is better as long as there are few or no gaps in coverage; (h) depositing a fifth layer  2006 , the fifth layer being an electrical conductor such as, for example, Pt, Ni or Cu, and preferably having a thickness of 5 to 200 nm, however other thicknesses are contemplated; (i) depositing a sixth layer  2007 , such layer being formed to protect the front side of the substrate while the backside is being worked on. Such layer can be made of, for example, wax, photoresist, or silicon dioxide substrate attached to the fifth layer  2006  via thermal release tape, the sixth layer  2007  preferably having a thickness of 500 to 1500 microns, however other thicknesses are contemplated; (j) forming through holes  2001   a  in the substrate  2001  by photolithography and etching the substrate  2001 , such that at least one discrete island of the first layer  2002  is exposed therein, the through holes  2001   a  having any suitable shape such as, for example, hexagons, squares and circles, the through holes  2001  being arranged in any suitable pattern such as, for example, a hexagonal grid, square grid and a polar grid; (k) removing exposed discrete islands by etching until portions of the fourth layer  2005  there above are exposed; (l) removing exposed portions of the fourth layer  2005  by etching until portions of the fifth layer  2006  there above are exposed; (m) removing exposed portions of the fifth layer  2006  by etching; (n) partially removing the fourth layer  2005  by etching laterally such that the second layer  2003  and the fifth layer  2006  overhang the fourth layer  2005  by 2-10 nm; (o) completely removing the sixth layer  2007  by thermal release, dissolving or etching. The second layer  2003  and the fifth layer  2006  preferably have a difference of at least 0.1 eV, at least 1 eV, at least 2 eV or at least 3 eV in their work-functions. 
     According to another embodiment as shown in  FIG. 21 , a method of manufacturing an NMSET or related device comprises: (a) providing a suitable substrate  2101  such as, for example, amorphous silicon, crystalline silicon, ceramic, etc., the substrate preferably having a thickness of 500 to 1500 microns; however thinner and thicker substrates are possible; (b) depositing a first layer  2102 , a mostly sacrificial layer, preferably an electrical insulator such as, for example, silicon dioxide, the first layer  2102  preferably having a thickness of 50 nm to 1000 nm; however thinner and thicker layers are possible. Furthermore, depending on the area of the substrate window  2101   a , it is advantageous for this layer to have a tunable stress level. For example, for a 1 cm 2  substrate window  2101   a , successful results have been achieved with SiO x N y  at 60 MPa tensile strength; (c) depositing a second layer  2103  over the first layer  2102 , the second layer  2103  being an electrical conductor such as, for example, Al, Nb or Zn and preferably having a thickness of 5 to 150 nm, however other thicknesses are contemplated; (d) depositing a third layer  2104  over the second layer  2103 , the third layer  2104  being an electrical insulator such as, for example, silicon dioxide and preferably having a thickness of 5 to 100 nm, however other thicknesses are contemplated, and preferably the same material as  2102 ; (e) depositing a fourth layer  2105  over the third layer  2104 , the fourth layer  2105  being an electrical conductor such as, for example, Pt, Ni or Cu and preferably having a thickness of 5-150 nm, however other thicknesses are contemplated; (f) forming holes through the second layer  2103 , the third layer  2104  and the fourth layer  2105  by photolithography and etching, the holes having any suitable shape such as, for example, strips, squares, circles; (g) partially removing the third layer  2104  by etching laterally such that the second layer  2103  and the fourth layer  2105  overhang the third layer  2104 ; (h) forming through holes  2101   a  in the substrate  2101  by photolithography and etching the substrate  2101 , such that at least one hole through the second layer  2103 , the third layer  2104  and the fourth layer  2105  overlaps with one through hole  2101   a , the through holes  2101   a  having any suitable shape such as, for example, hexagons, squares and circles, the through holes  2101  being arranged in any suitable pattern such as, for example, a hexagonal grid, square grid and a polar grid; (i) removing portions of the first layer  2102  exposed in the through holes  2101   a . The second layer  2103  and the fourth layer  2105  preferably have a difference of at least 0.1 eV, at least 1 eV, at least 2 eV or at least 3 eV in their work-functions. 
     Exemplary Thermal Transpiration Devices with Vacuum Layer 
     Though somewhat redundant,  FIG. 22  is a side cross-sectional view illustrating a thermal transpiration device, such as NMSET or related device, shown generally at  2204 . The thermal transpiration device includes a cold side membrane  2202  and a hot side membrane  2201 , with a thermal insulator  2200  provided in between. The thermal insulator  2200  may be formed of a vacuum, which can be achieved, for example, via the Venturi effect. The thermal transpiration device  2204  includes a thickness  2203  defined by the cold side membrane  2202 , the thermal insulator  2200  and the hot side membrane  2201 . 
       FIG. 23  is a side cross-sectional view illustrating the operation of a thermal transpiration device, shown generally at  2309 . The device  2309  includes a hotter layer  2301 , a colder layer  2302 , with a thermal insulator  2300  provided there between. Apertures  2308  are formed in the device  2309  in a manner as previously described. The thermal transpiration device  2309  includes a thickness  2303  defined by the colder layer  2302 , the thermal insulator  2300  and the hotter layer  2301 . The thermal insulator  2300  can be formed of a vacuum, which can be achieved, for example, via the Venturi effect. 
     Colder gas particles  2304 , which have a mean free path (average distance traveled before hitting another particle) shown by radius  2305 , enter the aperture  2308 , or the edge thereof, and collide with other particles, thus exchanging energy. Hotter gas particles  2306 , which have a mean free path shown by radius  2307 , collide into the hotter layer  2301 , thus gaining energy in the process and imparting a positive momentum force. The colder gas particles  2304  reduce the temperature of the hotter gas particles  2306 , which collide back into the hotter layer  2301 , thus gaining energy and imparting a positive momentum force and increased pressure on the hot layer  2301 . 
       FIGS. 24 and 25  are respective side and top cross-sectional views of a thermal transpiration device, shown generally at  2414 , with one extended layer having angled walls. The device  2414  includes a hotter layer  2401  and a colder layer  2402 , with a thermal insulator  2400  provided there between. The thermal insulator  2400  can be formed as a vacuum, which can be achieved, for example, via the Venturi effect. The total thickness of the device  2414  is indicated by reference number  2403 , and is defined by the colder layer  2402 , the thermal insulator  2400  and the hotter layer  2401 . 
     Apertures  2408  are provided in the device  2414 , forming angled walls  2415  in the hotter layer  2401 , in a manner as previously described. The apertures  2408 , and/or edges thereof, aid in defining a hotter surface  2409 , a colder surface  2410 , an active area  2411  generally where thermal transpiration occurs, and a support area  2412 . As shown in  FIG. 24 , the angle  2413  of the hotter surface  2409  is less than 90-degrees in order to form the angled walls  2415 . 
     While  FIGS. 24 and 25  illustrate the extended layer having angled walls as being the hotter layer  2401 , one skilled in the art will appreciate that the colder layer  2402  could be implemented as the extended layer having angled walls as an acceptable alternative. 
       FIGS. 26 and 27  are respective side and top cross-sectional views of a thermal transpiration device, shown generally at  2615 , with one extended layer having wet or dry etched walls. The device  2615  includes a hotter layer  2601 , a colder layer  2602 , with a thermal insulator  2600  provided there between. The thermal insulator  2600  can be formed as a vacuum, which can be achieved, for example, via the Venturi effect. The total thickness of the device  2615  is indicated by reference number  2603 , and is defined by the colder layer  2602 , the thermal insulator  2600  and the hotter layer  2601 . 
     Apertures  2608  are provided in the device  2615 , and forming wet or dry etched walls  2614  in the hotter layer  2601  having a generally parabolic shape, in a manner as previously described. The apertures  2608 , and/or edges thereof, aid in defining a hotter surface  2609 , a colder surface  2610 , an active area  2611  generally where thermal transpiration occurs, a support area  2612  and wet or dry etched surfaces  2614 . 
     Reference number  2605  indicates the mean free path radius of colder gas particles  2604 . Reference number  2607  indicates the mean free path radius (the average distance traveled before hitting other particles) of hotter gas particles  2606 . The colder gas particles  2604 , enter the aperture  2608 , or the edge thereof, and collide with other particles, thus exchanging energy. The hotter gas particles  2606  collide into the hotter layer  2601  at the outer edge thereof or at the wet-etched surface  2614 , thus gaining energy in the process and imparting a positive momentum force. The colder gas particles  2604  reduce the temperature of the hotter gas particles  2606 , which collide back into the hotter layer  2601  thus gaining energy and imparting a positive momentum force and increased pressure on the hot layer  2601 . 
     While  FIGS. 26 and 27  illustrate the extended layer having wet-etched walls as being the hotter layer  2601 ; one skilled in the art will appreciate that the colder layer  2602  could be implemented as the extended layer having angled walls as an acceptable alternative. 
       FIG. 28  is a side cross-sectional view of a thermal transpiration device, shown generally at  2816 , with two extended layers having angled walls. The device  2816  includes a hotter layer  2801  and a colder layer  2802 , with a thermal insulator  2800  provided there between. The thermal insulator  2800  can be formed as a vacuum, which can be achieved, for example, via the Venturi effect. The device  2816  has a total thickness  2803 , defined by the colder layer  2802 , the thermal insulator  2800  and the hotter layer  2801 . 
     Apertures  2808  are provided in the device  2816 , forming angled walls  2817  and  2818  in the hotter  2801  and colder  2802  layers, respectively, in a manner as previously described. The apertures  2808 , and/or edges thereof, aid in defining a hotter surface  2809 , a colder surface  2810 , an active area  2811  generally where thermal transpiration occurs, a support area  2812  for the hotter layer  2801 , and a support area  2815  for the colder layer  2802 . As shown in  FIG. 28 , the angle  2819  of both the hotter  2809  and colder  2810  surfaces are less than 90-degrees in order to form the angled walls  2818  and  2817 , respectively. While the angles  2819  of the hotter  2809  and colder  2810  are shown in  FIG. 28  as being approximately the same angle, the hotter  2809  and colder  2810  surfaces may be angled at different angles as an acceptable alternative depending on the embodiment. 
     In an ideal thermal transpiration device, the total thickness of the active area of the device designed to operate in atmosphere should be less than 500 nm. For optimization purposes, the thickness between the hot and cold surfaces should be no greater than 100 nm. Such small thicknesses make the device extremely fragile and difficult to work with. If, for example, the device layers, or membranes, are made thicker in order to provide the required thickness for the stability and strength of the device, its overall thickness would increase to a point that it exceeds the ideal thickness, as discussed above. 
       FIG. 29  is a cross-sectional view of the beginning construction of one embodiment of a thermal transpiration device, shown generally at in accordance with the present disclosure, which allows the thickness of a thermal transpiration device to be made thicker to enhance it durability and strength, while at the same time maintaining the thickness of the device in its critical area to within an ideal thickness range. 
     As shown in  FIG. 29 , the construction of the device is as follows. First, a silicon substrate layer  2916  is provided. A first metal layer  2917  of, for example, approximately 40 nm of aluminum, is deposited on the substrate  2916 . The deposition process may be evaporation, but other deposition methods may be used, such as, for example, sputtering, metal organic vapor deposition, etc. Hence, the first metal layer  2917  may be 40 nm of evaporated aluminum. 
     A dielectric layer  2918  is deposited on top of the first metal layer  2917 . The dielectric layer  2918  must be low stress and may be formed of a plastic or inorganic non-electrically conducting film material. The film (i.e., dielectric layer  2918 ) may be, in particular, low-stress (e.g., 60 MPa) plasma enhanced chemical vapor deposition oxynitride that is 2 microns thick. Other thicknesses are also contemplated. 
     An adhesion promoter layer  2919  may be deposited on dielectric layer  2918  to promote adhesion to the dielectric and or to act as an enhanced masking layer. Such material may be a chemical monolayer, such as HMDS, a thin film of organic resist, or a metal, in particular, 6 nm of chromium. The adhesion promoter layer  2919  may not be necessary on certain combinations of thin films and etching methods or etching chemicals. 
     The device is then etched, as is conventionally known, using a mask  2920  of approximately 1.3 microns SPR-3012, for example, with an unmasked area  2921 . Etching may be achieved by depositing the photoresist layer, or mask,  2920  over the adhesion promoter layer  2919 , as is known to do by one of ordinary skill in the art. Such a photoresist is preferably Shipley SPR-3012; however, other photoresists may be utilized. The photoresist layer  2920  may then be exposed through a conventional mask to develop unmasked areas  2921 . Exposure can be made, for example, using an appropriate wavelength of light. Contact lithography may also be used as would be understood by one of ordinary skill in the art. Once exposed, the photoresist layer  2920  may be developed in a solution appropriate for that purpose to form the unmasked areas  2921 . Such a solution may be, for example, 0.26M tetramethylamonium hydroxide for SPR-3012 for approximately 60 seconds. 
     As shown in  FIG. 30 , the device is etched at the unmasked areas  2921  (see  FIG. 29 ) to form etched areas  3022 . The etched areas  3022  are formed by etching into the adhesion promoter layer  2919  and the dielectric layer  2918  until portions of the first metal layer  2917  are exposed. The photoresist layer  2920  (see  FIG. 29 ) is then removed. The adhesion promoter layer  2919  may be etched using a wet etch, such as, for example, a chromium etch  1020  from Transene, until the silicon substrate  2916  is exposed. The dielectric layer  2918  may be etched, for example, with a chemical that will not etch the first metal layer  2917 . In the case of oxynitride on aluminum, the aqueous acid solution Silox Vapox II from Transene can be used. Other wet chemistries may also be used, or a dry plasma etch. 
       FIG. 31  is a side cross-sectional view showing further etching of the thermal transpiration device shown in  FIGS. 29-30 . Reference number  2916  is the silicon substrate, reference number  2917  is the first metal layer, reference number  2918  is the dielectric layer, and reference number  2919  is the adhesive promoter layer. In  FIG. 31 , the device, namely, the etched area  3022  (see  FIG. 30 ), has been further etched to provide an etched area  3122  and under cut area  3123 . To form the etched area  3122 , the first metal later  2917  is etched, and then portions of the substrate  2916  are etched. One method of forming the undercut areas  3123 , portions of the substrate  2916  which are underneath the first metal layer  2917  are isotropically etched. 
     The first metal layer  2917  may be etched with either wet or dry etching. In the case of aluminum, for example, an aluminum etch in a reactive ion etcher with chlorine and argon at low pressure may be used to etch the first metal layer  2917 . An example of an etch for 40 nm of aluminum is 50 sccm BCl3, 20 sccm Cl2, 10 mTorr, with 300 W RF power. 
     A wet or vapor etch can be used to etch the substrate  2916 , as long as the chemistry does not etch the first metal layer  2917 , the dielectric layer  2918  or the second metal layer  2919 . In the case of a silicon substrate with aluminum and oxynitride, the silicon may be etched, for example, with the gas XeF2. The substrate  2916  may also be treated to remove boron. One exemplary method of such a treatment is to use a fluorine based reactive ion plasma under the conditions of 35 sccm CF4, 20 mTorr, and 300 W RF power. 
       FIG. 32  is a cross-sectional view showing further formation of the thermal transpiration device shown in  FIGS. 29-31 . A thin layer  3224  of silicon dioxide or another electrical insulator is provided over the device. The silicon dioxide layer  3224  can be, for example, approximately 2-10 nm thick. Thinner is generally better as long as there are few or no gaps in coverage, especially near the first metal layer  2917 . The layer  3224  of silicon dioxide is provided to control tunneling thickness. The layer  3224  can be added by evaporation, or other known techniques. For example, other methods, such as sputtering, plasma enhanced chemical vapor deposition, atomic layer deposition, etc., may be used as well, along with other materials. A second metal layer  3225  is provided over the silicon dioxide layer  3224 . The second metal layer  3225  may be a layer of metal, such as nickel or copper, and may be approximately 40 nm thick. The second metal layer  3225  may be formed by evaporation, but other methods may be used as well, such as, for example, sputtering or ion assisted deposition. 
     The substrate  2916  is then mounted to a carrier substrate (not shown) with the thin film stack facing the carrier. The mount material could be, for example, a double-sided tape, such as Revalpha thermal release tape. However, other tapes and materials, such as, for example, wax or photoresist, may be used as well. 
     The remaining silicon substrate  2916  is then removed with, for example, an XeF2 vapor etch. The small portions of the silicon dioxide layer  3224  and the second metal layer  3225  formed in the etched portion of the substrate  2916  are removed with the substrate  2916 . Wet chemistry may also be used to remove the substrate  2916 , as long as it does not etch the first and second metal layers  2917  and  3225 . What is left, as shown in  FIG. 33 , are the islands  3127  formed by the first metal layer  2917 , the dielectric layer  2918 , the adhesion promoter layer  2919 , the thermo tunneling layer  3224 , and the second metal layer  3225 . The device is then sonicated to remove any Nickel plugs. In the case of Revalpha thermal release tape, for example, the carrier substrate can be placed on a hotplate of sufficient temperature to aid in removing the device. 
     Fault Tolerant Control System for Distributed Micro-Thrusters 
     In order to drive an object using distributed thrusters in a particular direction and or at a desired speed, a control system is needed. The control system is used to selectively activate and or adjust power levels to a distributed thruster or plurality of distributed thrusters to provide the desired force in the desired direction. 
     In accordance with the present control system, a control system for controlling the operation of distributed thrusters may be constructed as a grid of elements (each containing one or more thrusters) fed by at least a redundant two dimensional network of power distribution wiring. The distribution network is constructed as a plurality of loops comprised of horizontal and vertical lines or wires that are coupled to a plurality of horizontal rows and vertical columns of thrusters. 
     According to one embodiment of the present control system, each row and column loop meet or intersect in at least four locations, but alternating topologies may be designed to balance redundancy, number of loops, and the granularity of addressing. Alternate topologies may have a different number of crossings. 
     At least one power source may be supplied for each element in the grid or for a plurality of elements. One element may contain a plurality of thrusters. One terminal of the power source is connected to a horizontal line, and the other terminal of the power source is connected to a vertical line. This connection permits an element or group of elements to be addressed by connecting the terminals of a power source to the appropriate row and column. 
     In accordance with the general operation of the distributed thrusters such as NMSET, an electrical circuit is used to activate distributed thrusters by supplying and or regulating the amount of heat to the distributed thruster. An electrical circuit is formed by a loop comprised of the horizontal and vertical lines. Both ends of a given loop are driven at the same electrical potential. This means that a single cut anywhere in a given loop (as a result, for example, from damage to the array surface) will minimize a cascading loss of functionality. The heating or cooling caused by electrical circuit may be implemented by way of a heat pump, such as one driven by the Peltier effect using a Peltier slab. In this instance, the wiring are on either side of the distributed thrusters, and in a resistance embodiment explained below, they may be only on the hot side. In further embodiments of distributed thrusters, other methods of powering the distributed thrusters can be used. 
       FIG. 34  is a top view of one embodiment of a control system  3400  for an array  3401  of distributed thrusters  3402  in accordance with the control system. As can be seen in  FIG. 34 , in the array  3401 , a plurality of distributed thrusters  3402  are arranged in a grid-like manner in parallel horizontal rows and parallel vertical columns. 
     At least one power supply  3406  provides power to selected distributed thrusters  3402  using a first plurality of power lines  3404  and a second plurality of powers lines  3405  which are coupled to the distributed thrusters in each of the horizontal rows and in each of the vertical columns, respectively. When one of the power lines  3404  is selected along with one of the power lines  3405 , an electrical circuit is completed and at least one of the distributed thrusters is activated by the methods the distributed thrusters convert energy into thrust. A control unit  3403  controls the activation and or power levels of the selected power lines  3404  and  3405  for the desired thruster or group of thrusters. 
     As used in the present control system, the power supply  3406  may be a battery and the control unit  3403  may be a central processing unit. Further, thruster  3402  may comprise a plurality of thruster devices. 
     A NMSET device may comprise an apparatus operable to propel a gas where the apparatus comprises at least a first layer and a second layer arranged in a stack and means for heating and/or cooling the first and second layers to form a hot layer and a cold layer wherein the cold layer has a lower temperature than the hot layer, and at least one through hole in the stack. A surface of each hot layer is exposed in an interior of the through hole, a surface of each cold layer is exposed in the interior of the through hole, and an entire length of the active area of the through hole is up to 10 times of a mean free path of a gas in which the apparatus is immersed and/or is not greater than 1500 nm, as explained above. 
     In a given NMSET device at least one through hole may have a straight geometry, a sawtooth geometry, a triangular geometry, a parabolic geometry, or any geometry that may be determined to be beneficial for the NMSET device, as explained above. 
       FIG. 35  illustrates power lines  3504  and  3505  that meet at area  3506  to activate the distributed thruster&#39;s adjacent area  3506 . The control unit  3503  activates the distributed thruster&#39;s adjacent area  3506  by causing the power supply  3506  to provide electricity to power lines  3504  and  3505 . 
       FIG. 36  illustrates a fault condition where there is an open circuit in power line  3605 . As shown in  FIG. 36 , power line  3605  is associated with a vertical column of thrusters that are associated with the area around points  3608 . Because there is an open circuit  3607  in power line  3605 , the thrusters associated with the area around points  3608  cannot be activated due to this fault condition. 
     In one embodiment of the control system, in order to achieve redundancy and avoid system failure when a fault condition occurs in a power line, redundant path connections are provided as illustrated in  FIG. 37 . Power lines  3701  are coupled to the horizontal rows of thrusters and that power lines  3702  are coupled to the vertical columns of thrusters. Thus, a redundant path is provided to point  3706  in the event that a fault  3707  occurs in line  3705  as shown in  FIG. 37 . Redundancy is provided by power lines  3705  and  3704  wherein the control unit  3700  reroutes electricity from the first to the second connection point of power line  3707  or the power line is internally looped to activate the thrusters near point  3706 . In another embodiment of the present control system, a fault detection device  3708  is provided to detect a fault condition in any one of the power lines as shown in  FIG. 37 . The fault detection device  3708  is coupled to the power supply  3703  and control unit  3700  and which controls activation of an appropriate power line to compensate for, reroute, report and or replace the power line in which the fault condition is present. 
     A capacitor bank voltage sensing technique may be used to detect a fault. By designing the capacitor bank to not discharge completely in a single pulse, and measuring the voltage charge before and after a power pulse has been sent to a thruster element or a group of thruster elements, it is possible to determine the power consumed by the thruster or group of thrusters and compare this to the expected power. If the drop is significantly smaller than expected, this is a sign of an open circuit; a significantly large drop indicates a short. 
     In-line current sensing may also be used to detect a fault. A shunt resistor may be placed in series with the power distribution lines in order to measure the instantaneous current being drawn by the array. If the current is usually low, some cells may be open. If the current is excessively high, there is a short. The primary disadvantage of this method is that it increases the series resistance between the power supply and the thrusters by a small (but nonzero) amount. 
     The significant advantage of this method over sensing the capacitor voltage after a pulse is that it is possible to design a system fast enough (most likely at a few MHz level sampling rate) to respond in real time to a short circuit and abort the pulse before enough energy has been released to cause serious damage to adjacent thrusters from arcing, or to the power supply from rapid discharge and consequently overheating. This system may also be applied to a distributed thrusters operated in the continuous-duty mode. 
     Once a portion of the distributed thrusters has been declared faulty by any of the above methods, or another method as recognized by one of ordinary skill in the art, corrective action must be taken to minimize loss of thrust and or prevent cascading failures. 
     When performing timing analysis of pulsed distributed thrusters during the design phase, it is prudent to allow more than the minimum required cool-down time between successive pulses to any section of thrusters. If this is done, the overall thrust may be maintained by removing the damaged thrusters or section of thrusters from the firing sequence and operating the remaining undamaged thrusters or sections at a slightly increased duty cycle. 
     An increase in duty cycle can only compensate for a maximum amount of damage to the system. If this threshold is exceeded, a reduction in available thrust is unavoidable; an array&#39;s control system can be designed to compensate for loss of thrust capacity on one side of a craft or other application using the distributed thruster by slightly reducing the thrust on the corresponding opposite panel to maintain a level trim. 
       FIG. 38  is a top view of another embodiment of an exemplary control system, particularly useful for larger distributed thrusters systems and/or applications that can use a less granular control. This embodiment may be advantageous due to the decrease in the number of power and/or control lines, and/or the decrease in the required computing power, than what would normally be required for more granular control. The exemplary control system is showing an array of power supply lines  3801  and  3802  and sub power line  3810  that are used to activate sections of a group of distributed thrusters  3803 ,  3804 ,  3805 , and  3806  (shown in dotted line and which may further include a plurality of individual thrusters at the power line intersections). For example, the control unit  3800  may connect the power supply  3803  to an appropriate power line of power supply lines  3801  and an appropriate power line of power supply lines  3802  in order to cause electricity to flow in the corresponding sub power lines  3810  at and around the corresponding thruster or regions of thrusters in  3803 ,  3804 ,  3805 ,  3806 . Additionally, the control unit  3800  can be designed to activate thrusters region  3803  and thrusters region  3805  or  3804  or  3806  simultaneously, sequentially, or in a desired pattern or for a desired effect by causing electricity to flow in the appropriate power lines of power supply lines  3801 ,  3802  and sub power lines  3810  and through the inclusion of an additional microprocessor at the intersections of sub power lines with the power lines, and using a digital signal to communicate with those microprocessors. 
       FIG. 39  is a top view of a further embodiment similar to that shown in  FIG. 38  of an exemplary control system.  FIG. 39  illustrates a plurality of power supply lines  3901  and  3902  and a plurality of sub-power supply lines  3910  that form a grid structure as shown. The control unit  3900  may connect the power supply  3911  to an appropriate power line of power supply lines  3901  and an appropriate power line of power supply lines  3902  in order to cause electricity to flow in the sub-power lines  3910  at and around the corresponding thruster or regions of thrusters  3903 . Furthermore, as discussed with  FIG. 38 , the control unit  3900  may activate any of distributed thrusters  3903 ,  3904 ,  3905 ,  3906 ,  3907 ,  3908 , and  3909  in a group or individually. 
       FIGS. 40 a , 40 b  and 40 c    shows an enlarged illustration of the embodiment of the control system shown in  FIG. 34 . Power lines  4001  and  4002  are used to address the thruster regions that operate on temperature gradients  4006  around addressed point  4003 . When the thrusters around point  4003  are addressed due to the flow of electrical current through power lines  4001  and  4002 , point  4003  heats up with area  4004  being the primary area affected and area  4005  being the secondary area affected. 
     Because it may be undesirable for the heating of one point to cause heating of adjacent points, another exemplary embodiment is illustrated in  FIGS. 41 a  and 41 b   , which shows the inclusion of a heat barrier  4117 , which may be in the form of a conductive pad, an insulator, a gap, or any other form of heat barrier as recognized by one of ordinary skill in the art. The heat barrier  4117  has the effect of changing the heat conductivity and isolating the conductive areas. The heat barrier  4117  is shown as a perimeter around the thruster regions  4108  that are adjacent a junction  4106  of power lines  4105  and  4104 , however the heat barrier  4117  may be configured differently based on a different desired effect. By energizing power lines  4105  and  4104  the thruster regions  4108  adjacent junction  4106  are activated and the heat barrier  4117  prevents other thruster regions outside of the shaded box area shown as  4119  from being inadvertently activated. 
       FIGS. 42 a , 42 b  and 42 c    show the power lines or conductive structures of another embodiment of the control system.  FIG. 42 a    shows a top layer grid structure  4202  of conductive lines to be used for activating thruster regions, where power supply line  4200  is designed to be connected to a power supply and a plurality of branch lines  4201  are designed to be positioned in proximity to a plurality of thruster regions. 
       FIG. 42 b    illustrates an optimized middle layer showing insulators  4202  and resistors, temperature gradient generating device or other means of activating thruster regions  4203  to be used in between the grid structure shown in  FIG. 42 a    and further grid structure that will intersect the branch lines  4201  in  FIG. 42 a    at the thruster regions  4203 . 
       FIG. 42 c    illustrates the combination of  FIGS. 42 a  and 42 b   , where the top layer of  FIG. 42 a    is placed over the middle layer of  FIG. 42 b   .  FIG. 42 c    shows power supply line  4200  and branch lines  4201  overlaid onto the thermo-resistive heating junctions formed by resistors  4203  or temperature gradient generating device or other means of activating thruster regions and insulators  4202  as in an embodiment of the control system, to control a plurality of target points from a single power line intersection point. 
     Exemplary Resistive Temperature Gradient Formation 
     Reference is made to the section entitled “Principles of Operation” and subsection “Temperature Differential”, above, incorporated here by reference.  FIG. 43  is schematic diagram of a device that can be used to create a temperature gradient in accordance with the present disclosure. In this section, the heat pump or thermal gradient device may be, but is not limited to driving a NMSET device. The device includes a colder layer  4301  of an electrically conductive material having a top surface  4302  and a bottom surface  4305 . A top surface  4306  of a hotter layer  4304  is closely proximate to and may be attached to bottom surface  4305  of colder layer  4301  directly or through a thermally and/or electrically insulating intermediate material depending on implementation. 
     One terminal of power supply  4307  is connected to top surface  4302  of the colder layer  4301  and the other terminal of power supply  4307  is connected to one side of switch  4308 . The other side of switch  4308  is connected to bottom surface  4303  of the hotter layer  4304 . The hotter layer  4304  is made of or is a structure with sub-layers that include a layer of a resistive material that heats up through resistive or Joule heating when electrical current passes through it. In embodiments with sub-layers, one might be an insulating material with reduced thickness near the locations a thermal gradient is to be produced, and a metallization layer that is configured to heat at a greater rate at the thermal gradient locations. 
     The colder layer  4301  might be of a material less subject to Joule heating in the operative locations. The difference in resistive, Joule heating characteristics can be accomplished through selection of materials, configuration (e.g., the hotter layer being thinner at the sites where heat is to be generated when compared to an opposing location of the colder layer so that the electron density in the hotter layer promotes Joule heating at a greater extent that the colder layer) or other factors that permit one layer to heat up to a greater extent or faster than an adjacent layer, or combinations thereof of these characteristics, depending on a particular embodiment. For instance, the hotter layer can be made up of surface wires that thin or become more narrow or otherwise have smaller in cross-section at sites where heating is desired, e.g., at a NMSET structure or groups of NMSET structures, such that the charge carrier density/resistance is greater at those sites, and Joule heating is more apparent. The colder layer can be a thicker, less resistive material having a broader area (e.g., cover the entire surface of the hotter layer) to reduce carrier density. Whatever the mechanism, the current in one layer promotes Joule heating, and in the other layer does not, at least not to the same extent of Joule heating in the one layer. 
     Further, the mechanism for passing current from one layer to the other can follow any suitable method or mechanism, such as quantum tunneling, semiconductor conduction were the colder and hotter layers are P-type and N-type semiconductors forming a PN junction, with electrode formed thereon on opposing surfaces, transistors connected to address line, similar to the read/write and address lines of memory devices, that permit an adjacent electrode to heat on one surface, with the switch being much like the structure of an addressable memory site or pixel, but with the memory site or pixel structure being replaced with an electrode that thermally heats, or nearly any other type of structure that will selectively address thermal gradient devices or clusters of such devices. 
     Alternatively or additionally, the hotter layer can have an input side and an output side in the same layer, wherein current passes through from one side to the other, resistively heating the hotter layer. This embodiment can produce heat at selected sites, and less so elsewhere, when the hotter surface is not entirely covered by an electrically conductive material, but rather has conductive lines, wherein the lines have characteristics that permit heating at selected sites, such as NMSET structures of groupings. That is, the lines can be large enough is cross-section to not heat, but at selected sites have a reduced cross-section to selectively heat upon application of current. 
     In the embodiment of  FIG. 43 , electrical current passes from the top layer  4301  to the bottom layer  4304 . As shown in  FIG. 43 , switch  4308  is in an open condition. Thus, no current flows through layers  4301  and  4304 . Accordingly, there is no temperature gradient or difference in temperature between surface  4302  and surface  4303 . 
       FIG. 44  showing the state where switch  4406  is closed. Thus, current from power supply  4407  flows through layers  4401  and  4402 . As result of the current flow, layer  4402  begins to heat because of its resistive characteristics, thus causing layer  4401  to heat as well. The heating of layer  4402  causes a temperature gradient  4405  to be created between top layer  4403  and bottom layer  4404 . When switch  4406  is opened, current no longer flows through layers  4401  and  4402 . Thus, temperature gradient  4405  begins to diminish eventually to zero difference in temperature between surfaces  4403  and  4404 . 
       FIG. 45  plots of the temperature increase of surface  4404  as current begins to flow when switch  4406  is closed. Temperature is plotted along the y-axis and time is plotted along the x-axis. Note that the temperature of surface  4404  in  FIG. 45  rapidly rises as indicated by plot  4501  to an equilibrium temperature  4504 . The switch  4406  in  FIG. 44  is then opened and current no longer flows, the temperature will begin to drop. 
     The temperature of surface  4403  when switch  4406  is closed follows a similar but delayed pattern  4507  as the heat from layer  4402  begins to migrate toward surface  4403  through layer  4401  as indicated by plot  4502 . The temperature of surface  4403  continues to rise even slightly after the switch  4406  in  FIG. 44  has been opened, to its equilibrium temperature  4505 . Reference number  4506  in  FIG. 45  indicates the length of time that switch  4406  remained closed. If the length of time  4506  the switch  4406  remains closed exceeds the time it takes to reach equilibrium temperature  4504 , than the temperature of surface  4403  will continue to rise, until the temperature gradient  4503  vanishes. 
     Thus, the temperature gradient between temperature  4504  of surface  4404  and the temperature  4505  of surface  4403  at a given time is represented in  FIG. 45  as temperature gradient  4503 . 
     As  FIG. 45  illustrates, it takes a finite amount of time for the temperatures of surfaces  4403  and  4404  to return to their ambient state after current stops flowing through layers  4301  and  4304 . The residual heat can cause problems if adjacent temperature gradient devices are in close proximity. 
       FIG. 46  is a top cross-sectional view of a plurality of distributed thruster devices such as one operated by temperature gradient devices  4603  arranged in horizontal and vertical rows in accordance with another exemplary embodiment. Current flow is supplied to each device by a plurality of power lines  4601  and  4602  from a power and control unit  4300  in a matrix type manner. The control unit may be formed of a processor, particularly a programmable processor that can selectively actuate particular sites, as explained above with respect to control electronics at the active sites when the power lines operate like read/write and address lines to control adjacent control electronics at the active site, or simply by adding current to horizontal and vertical power lines such that at cross points enough current is present to create a temperature gradient. A source of electrical energy may be formed of a battery, or any other source of carriers, whether AC or DC, depending on implementation. Also, the section entitled “Fault Tolerant Control System”, above, is incorporated herein. 
     With reference again to  FIG. 46 , if an adjacent temperature gradient device  4603  is activated before the first temperature gradient device  4603  is allowed to fully cool, the temperature gradient of the newly activated device may not be the expected gradient. Depending on the application, this may not be optimal. Such a condition is illustrated in  FIG. 47  (and similarly in  FIGS. 40 a , 40 b  and 40 c   , for instance) where a temperature gradient device  4703  is activated by power lines  4704  and  4705 . As shown in  FIG. 47 , the generated heat radiates to a primary area  4701  and further to a secondary area  4702 . Note that the radiated area encroaches upon other adjacent temperature gradient devices and could cause those devices not to produce the proper temperature gradient when they are activated. This potential problem can be mitigated or resolved by the selective activation of thermal gradient devices. 
     For example, the control unit  4600  shown in  FIG. 46  avoids activation of those temperature gradient devices that are adjacent previously activated temperature gradient devices for a predetermined period of time. Doing so allows the previously activated temperature gradient device to fully cool, or at least cool to a satisfactory temperature, so that no residual heat interferes with the operation of adjacent temperature gradient devices. Further, the temperature gradient devices can be selectively addressed, either individually or in clusters, by read and address lines in a manner similar to the manner in which pixels on a digital display or memory sites in a memory array are addressed and controlled. 
       FIG. 48  illustrates one embodiment of an activation sequence of temperature gradient devices in an array of temperature gradient devices in accordance with this embodiment. Reference number  4801  represents a temperature gradient device in an array of such devices as illustrated in  FIG. 46 . Reference number  4802  represents an adjacent temperature gradient device, or an adjacent array of such devices. The pattern repeats as indicated by reference numbers  4803 - 4816  for a total of 16 temperature gradient devices, or arrays of such devices as illustrated, though of course in most embodiments involving NMSET devices there would be more. 
     Using  FIG. 48 , one of ordinary skill in the art will readily understand that an activation sequence for individual or sets of temperature gradient devices can be determined which avoids or mitigates thermal interference from a previously activated adjacent device. This is so because enough time has elapsed for the previously activated adjacent device to sufficiently cool. For example, temperature gradient device pairs ( 4801 ,  4809 ), ( 4803 ,  4811 ), ( 4805 ,  4813 ) and ( 4807 ,  4815 ) may be activated followed by pairs ( 4802 ,  4810 ), ( 4804 ,  4812 ), ( 4806 ,  4814 ) and ( 4808 ,  4816 ) without significant causing thermal interference to any previously activated adjacent devices. Other activation sequences will become known to those skilled in the art from a review of  FIG. 48 . 
     As can be seen, the disclosed embodiments can have many applications for creating and maintaining thermal gradients. In particular, though not limited thereto, the thermal gradient structures can be in heat pumps to drive distributed thrusters, and even more particularly distributed thrusters driven by NMSET of many forms and variations disclosed elsewhere herein. 
     Vacuum Generation Using Gas Flow Generating Devices 
       FIG. 49  and  FIG. 50  show side and top views of an exemplary apparatus  4950  for generating a vacuum or a partial vacuum. 
     Referring to  FIGS. 49 and 50 , the apparatus  4950  may include a suction unit (e.g., a structure)  4901  that is generally suction cup-shaped or dome-shaped. The structure  4901  may include a central region  4902  including one or more suction generation devices (or suction inducing material)  4910  that may be embedded in the central region (or suction inducing portion)  4902  for generating suction and a peripheral region  4920  circumferentially surrounding the central region  4902  for supporting the structure  4901 . The structure  4901  may define a center cavity  4904  when placed on a surface  4960  (e.g., a non-porous surface). The one or more suction generation devices  4910  embedded in the central region  4902  may generate a vacuum or a partial vacuum in the center cavity  4904  responsive to the one or more suction generation devices  4910  being powered-on by a power supply  4940 . The structure  4901  may include gas flow generating devices such as, for example, NMSET as described in more detail below. 
     In certain exemplary embodiments, the central region  4902  may be structured to enable gas flow generating devices such as, for example, NMSET to be embedded in and surrounded by resilient material such a rubber or other elastomeric material. It is contemplated that the material may be selected for temperature performance or compatibility with specific fluids or gases. Suitable materials may include, but are not limited to, (1) natural rubber; (2) thermoplastic and/or (3) copolymer rubbers, among others. The structure may be formed using molding techniques. As used herein, the term resilient generally means exhibiting rubber like qualities such as compliancy, resiliency, compression deflection, flexibility, and/or high deformation recovery. 
     The outer edge  4945  of the peripheral region  4920  may be adopted to form a seal with, for example, the surface  4960  of substrate  4900  when a partial vacuum is formed between the structure  4901  and the surface  4960 . One of skill in the art understands that as gas is withdrawn from the center cavity  4904 , as illustrated by arrows  4903 , atmospheric pressure, as indicated by arrows  4905 , may push down on the structure  4901  causing the structure to adhere (e.g., seal), for example, to surface  4960  (e.g., a non-porous surface) as a result of the vacuum or partial vacuum generated (e.g., created) within the center cavity  4904 . 
     A center portion  4906  of the central region  4902  may include the power supply  4940  (e.g., a battery, other energy storage device and/or electric generation device, among others), power and control circuits  4970  that may, for example, control via command signals the power supply  4940  and a switch  4980  configured as a two-pole device to switch-on or switch-off the suction generation devices  4910 . 
     In certain exemplary embodiments, the switch  4980  may include another pole (e.g., may be a three-pole device) to: (1) switch-on in a first position when gas flow is desired to be in a first direction; (2) switch-off in a second, middle position when the suction generation devices  4910  are off; or (3) switch-on in a third position when gas flow is desired to be in a reverse direction. 
     In certain exemplary embodiments, an array or a plurality of suction generation devices  4910  may be disposed at a plurality of adjacent portions  4911 - 4913  within the central region  4902 . The plurality of adjacent portions  4911 - 4913  may be symmetrical with respect to a center of the central portion  4906 . 
     Although three adjacent portions (regions)  4911 - 4913  are shown surrounding the center portion  4906 , it is contemplated that more or less of these individual regions of suction generation devices  4910  are possible. 
     Although the plurality of adjacent portions  4911 - 4913  are shown to be symmetrical with respect to the center of the central portion  4906 , other configurations are contemplated including configurations in which the central portion  4906  and its corresponding electronics may be replaced by separate sets of corresponding electronics for each adjacent portion or other configurations in which the different portions  4911 - 4913  may be non-symmetrical with respect to the center of the central portion  4906 . 
     Responsive to the suction generation devices  4910  being powered-on and the switch being positioned in the first position, voltage from the power supply  4940  (e.g., battery) may be supplied to the array of suction generation devices  4910  of each adjacent portion  4911 - 4913  via conductors (not shown) in a first voltage configuration to evacuate gas (e.g., air) from inside the center cavity  4904 . Responsive to the suction generation devices  4910  being powered-off (e.g., the switch being positioned in the middle position), voltage from the power supply  4940  (e.g., battery) may be blocked (disconnected) from being supplied to the array of suction generation devices of each adjacent portion  4911 - 4913  and the gas (e.g., air) from inside the center cavity  4904  may not be evacuated. Responsive to the suction generation devices  4910  being powered-on and the switch being positioned in the third position, voltage from the power supply  4940  (e.g., battery) may be supplied to the array of suction generation devices  4910  of each adjacent portion  4911 - 4913  via the conductors in a second, reverse voltage configuration to draw gas (e.g., air) into the center cavity  4904  to build up pressure therein. 
       FIG. 51  shows a cross-sectional view of another exemplary apparatus. 
     Referring to  FIG. 51 , the apparatus  5150  may include a membrane  5101  (e.g., a flexible and/or resilient membrane) configured to couple to (e.g., seal to) a surface  5100 . The surface  5120  of substrate  5100  may be uniform (or regular in profile) or non-uniform (or irregular in profile). The membrane  5101  may include a plurality of regions  5110   a ,  5110   b  . . .  5110   n . Each region  5110   a ,  5110   b  . . .  5110   n  may include one or more suction generating devices (e.g., NMSet elements or suction inducing materials)  5102   a ,  5102   b  . . .  5102   n  and a cavity  5103   a ,  5103   b  . . .  5103   n , respectively. The membrane may be structured such that the bottom surface  5115   a ,  5115   b  . . .  5115   n , and  5115   n +1 may adhere (e.g., seal) to the surface  5120  when gas is evacuated from one or more of the cavities  5103   a ,  5103   b  . . .  5103   n . The gas may be drawn individually and independently from each cavity  5103   a ,  5103   b  . . .  5103   n  and may form individual vacuums or partial vacuums. 
     The membrane  5101  may define each cavity  5103   a ,  5103   b  . . .  5103   n  which may have a cup shape or dome shape. Each region  5110   a ,  5110   b  . . .  5110   n  may include a central portion in which the one or more suction generation devices  5102   a ,  5102   b  . . .  5102   n  are respectively embedded (or disposed) for generating suction for its corresponding cavity  5103   a ,  5103   b  . . .  5103   n . Each region  5110   a ,  5110   b  . . .  5110   n  may include a peripheral portion circumferentially surrounding the one or more suction generation devices  5102   a ,  5102   b  . . .  5102   n  to provide structural support for the membrane and to provide sealing surfaces  5115   a ,  5115   b ,  5115   n  . . .  5115   n +1 to seal to the surface  5120 . 
     The power supply and control circuitry are omitted to simplify  FIG. 51 . The power supply and control circuitry may be implemented similarly to that of  FIG. 49  with a portion of the membrane including the power supply and control circuitry with electrical connection to each suction generation device  5102   a ,  5102   b  . . .  5102   n . It is also contemplated that each region may include an individual power supply adjacent to the respective region. 
     In certain exemplary embodiments, the flexible membrane  5101  may be formed from gas flow generating devices such as, for example, NMSET embedded in and surrounded by resilient material such a rubber or other elastomeric material. The bottom surfaces  5115   a ,  5115   b  . . .  5115   n  and  5115   n +1 of the membrane  5101  may be adopted to form individual seals with, for example, the surface  5120  when partial vacuums are formed in individual cavities  5103   a ,  5103   b  . . .  5103   n.    
     Responsive to the suction generation devices  5102   a ,  5102   a  . . .  5102   n  being powered-on and, for example, a control switch (not shown) being positioned in the first position, voltage from a power supply (e.g., battery) may be supplied to each array or individual groups of suction generation devices  5102   a ,  5102   b  . . .  5102   n  via conductors in a first voltage configuration to evacuate gas (e.g., air) from inside each cavity  5103   a ,  5103   b  . . .  5103   n . Responsive to the suction generation devices  5102   a ,  5102   b  . . .  5102   n  being powered-off, voltage from the power supply may be disconnected from the array or individual groups of suction generation devices  5102   a ,  5102   b  . . .  5102   n  and the gas (e.g., air) from inside the center cavities  5103   a ,  5103   b  . . .  5103   n  may stop being evacuated. Responsive to the suction generation devices  5102   a ,  5102   b  . . .  5102   n  being powered-on and the control switch being positioned in a reverse power position, voltage from the power supply may be supplied to each array or individual groups of suction generation devices  5102   a ,  5102   b  . . .  5102   n  in a second, reverse voltage configuration to draw gas (e.g., air) into the cavities  5103   a ,  5103   b  . . .  5103   n  to buildup pressure therein. 
     In certain exemplary embodiments, the pressure change generated by the suction generation devices  5102   a ,  5102   b  . . .  5102   n  may be maintained without power by including a thermal layer that flows over the suction generation devices  5102   a ,  5102   b  . . .  5102   n  after some or all of the cavities  5103   a ,  5103   b  . . .  5103   n  have achieved an evacuated state. In other exemplary embodiments, the pressure change generated by the suction generation devices  5102   a ,  5102   b  . . .  5102   n  may be maintained without power by attaching or applying a cover layer to cover over the suction generation devices  5102   a ,  5102   b  . . .  5102   n  after some or all of the cavities  5103   a ,  5103   b  . . .  5103   n  have achieved the evacuated state. 
       FIG. 52  shows a top view of a further exemplary apparatus  5250 . 
     Referring to  FIG. 52 , the apparatus  5250  may include a container  5200  (e.g., a jar or a bottle, among others) configured to draw a flowable substance  5207  (e.g., a gas or fluid) into the container  5200 . 
     The container  5200  may include; (1) a lower portion  5210  (e.g. that may be non-compressible and may have a cavity therein); (2) an inlet nozzle  5201  for drawing (e.g., sucking) the flowable substance  5207  into the container  5200 ; (3) an upper portion  5202  (e.g., a non-compressible cap attachable to the lower portion  5210 ) to close the lower portion  5210 ; (4) power and control circuitry  5204  for powering and controlling the amount of suction generated; (5) and the suction generation device  5203  for generating the suction. 
     The cap  5202  may be a screw-type or pressure-type cap such that when fit to the lower portion  5210 , the cavity may be sealed at the border of the upper and lower portions  5202  and  5210  of the container  5200 . By drawing a vacuum by suctioning gas  5206  from the container  5200 , for example, at a top portion of the cap  5202  using the suction generation device  5203  disposed at the top portion of the cap  5202 , the pressure inside the container may be lowered and may cause a flowable material  5207  positioned at an inlet  5220  of the inlet nozzle  5201  to be drawn into the container  5200 . The inlet nozzle  5201  may include the inlet  5220  and an outlet  5230 , and the outlet  5230  may be positioned above the inlet  5220  to enable the flowable material  5207  to be drawn though the inlet nozzle  5201  and to drop into the container  5200  as shown by the dotted arrow  5205 . 
     In certain exemplary embodiments, the outlet  5230  may be positioned in proximity to the cap  5202  to increase the capacity of the container  5200 . 
     Although power and control circuitry  5204  and suction generation device  5203  are shown on the top surface of the cap  5202 , it is contemplated that they may be located on any portion of the container and may be positioned above the outlet  5230  to ensure that the flowable material cannot exit via the opening associated with the suction generation device  5203 . 
     In certain exemplary embodiments, suction generation device  5203  may be formed of gas flow generating devices such as, for example, NMSET, the power and control circuitry  5204  may include one or more batteries and a control unit to switch on and/or off power from the batteries to the suction generation device  5203 . The power and control circuitry  5204  may be disposed adjacent to the suction generation device  5203 . 
     In certain exemplary embodiments, the suction generation device  5203  may be switched on or off manually using a control switch (not shown). In other exemplary embodiments, the suction generation device  5203  may be switched on or off automatically based on a detection signal from a sensor  5260 . The sensor  5260  may detect the fluid level and/or gas concentration level and may generate the detection signal, for example a binary logic signal, of a first logic state responsive to the detected level exceeding a predetermined threshold or user specified threshold to automatically turn-off the suction generation device  5203  and may generate the detection signal of a second logic state responsive to the detected level being below the predetermined threshold or the user specified threshold to automatically turn-on the suction generation device  5203 . For example, when commanded by the control circuitry  5204 , the suction generation device  5203  may draw air from an interior of container  5200  causing the flowable substance  5207  to be drawn into container  5200  as a result of the vacuum created within container  5200 . 
       FIG. 53  is a flow chart of an exemplary control method. 
     Referring to  FIG. 53 , the method may generate a partial vacuum in a cavity. At block  5310 , a suction unit  4901  may be disposing on a surface  4960 . The suction unit  4901  may include a plurality of suction generation devices  4910  embedded in the suction unit  4901  that suction gas therethrough. At block  5320 , the power supply  4940  may power the suction generating devices  4910 . At block  5330 , a cavity  4904  may be formed that is defined by at least the surface  4960  and the suction unit  4901 . At block  5340 , gas may be evacuated from the cavity  4904  to generate the partial vacuum. 
     In certain exemplary embodiments, the plurality of the suction generating devices  4910  may be disposed in a substrate which is configured to seal to the surface  4960 . The suction unit  4901  may be formed by molding techniques, for example, standard or injection molding techniques, among others. 
     In certain exemplary embodiments, during the evacuation of gas from the cavity  4904 , the substrate may flex to form a seal at the outer edge  4945  of the peripheral portion between the surface  4960  and the substrate. 
     In certain exemplary embodiments, the suction unit  4901  may be formed by embedding (e.g., via molding) a plurality of gas flow generating devices such as, for example, NMSET in the substrate. 
     In certain exemplary embodiments, the evacuation of the gas from the formed cavity  4904  does not use any moving parts. 
     In certain exemplary embodiments, the surface  5120  may be a non-uniform surface, and the suction unit  5100  may includes a plurality of regions  5110   a ,  5110   b  . . .  5110   n  configured to individually seal a portion of the non-uniform surface  5120 . The plurality of regions  5110   a ,  5110   b  . . .  5110   n  may form a plurality of cavities  5103   a ,  5103   b  . . .  5103   n , responsive to the powering-on of the suction generating devices  5102   a ,  5102   b  . . .  5102   n  to generate for respective ones of the cavities  5103   a ,  5103   b  . . .  5103   n  corresponding vacuums. 
     In certain exemplary embodiments, in response to evacuating the gas from the cavity, the suction inlet  5201  may suction the flowable material  5207 . 
     In certain exemplary embodiments, the cavity formed by the surface and the suction unit may include a lower portion and the flowable material may be collected in the lower portion of the cavity for storage. 
       FIG. 54  is a flow chart of another exemplary control method. 
     Referring to  FIG. 54 , the method may generate a partial vacuum in a cavity  4904 . At block  5410 , a suction unit  4901  having a plurality of suction generation devices  4910  embedded in a resilient substrate  4901  may cover the cavity  4904 . At block  5420 , gas from the cavity  4904  may be evacuated by powering-on the suction generation devices  4910  to generate at least the partial vacuum. 
     In certain exemplary embodiments, the suction unit  5101  may include a plurality of regions  5110   a ,  5110   b  . . .  5110   n  configured to individually seal against respective portions of the surface by forming a plurality of cavities  5103   a ,  5103   b  . . .  5103   n  and generating one or more partial vacuums. 
     As described herein, for example, the invention may be embodied in software (e.g., a plug-in or standalone software), in a machine (e.g., a computer system, a microprocessor-based appliance, etc.) that includes software in memory, or in a non-transitory computer-readable storage medium configured to carry out the control schemes (e.g., in a selfcontained silicon device, a solid state memory, an optical disc, or a magnetic disc, among others). 
     While the foregoing specification teaches the principles of the present invention, with examples provided for the purpose of illustration only, it will be appreciated by one skilled in the art from reading this disclosure that various changes and modifications in form and detail can be made, and equivalents employed, without departing from scope of the appended claims, which are to be given their full breadth.