Abstract:
A number of thermal elements are used in a microfluidic device to move or manipulate nano-liter and pico-liter amounts of adsorbed fluid analytes and reagents on the device surface. All of the basic microfluidic operations of transport, merge, subdivide, separate, sort, remove, and capture are provided. A typical device embodiment has a flat or curved surface with the thermal elements located at or near the surface and arranged in any of a number of patterns that make possible specific manipulations of the adsorbed fluids on the surface. The thermal elements may be electrical resistive heaters or Peltier Effect junctions, and are activated by a series of electrical pulses from a control means. The heated or cooled thermal elements produce localized thermal gradients in the surface which in turn induce a surface tension gradient between the adsorbed fluid and the surface, making possible a variety of fluid manipulations on the surface.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [[0001]]     The United States Government has rights in this invention pursuant to contract no. DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to microfluidic devices capable of manipulating fluid analytes and reagents adsorbed onto the device surface. The device provides the basic microfluidic operations of transport, merge, subdivide, separate, sort, remove, and capture. These operations are made possible by controlling the generation and placement of localized thermal gradients that induce localized surface tension gradients in the fluids on the surface.  
       BACKGROUND OF THE INVENTION  
       [0003]     The need for a cost-effective and flexible microfluidic device that can readily manipulate nano-liter and pico-liter amounts of fluids is increasingly important as many fields of science explore the nanometer regime. Popular methods for handling microfluids use a physical flow path such as micro-channels or hydrophilic/hydrophobic patterns. All physical paths have the drawback of a static channel network, limiting the fluid to a predefined route.  
         [0004]     Often in microfluidic systems, flow actuation is accomplished by non-mechanical means such as dielectrophoretic forces and surface tension. In the presence of a surface tension gradient it is well known that fluids adsorbed onto a surface can be laterally transported. Adsorbed fluids move from a high temperature region to a lower temperature region. This surface-tension-driven fluid motion is called the Marangoni effect (1, 2).  
         [0005]     A surface tension gradient can be produced by several approaches: chemical, composition, thermal, electrochemical, and photochemical. Chemical and composition gradients usually result in static surface tension heterogeneity. The latter three approaches lend the possibility of a dynamically applied surface tension gradient at one or more specified locations, of which thermal is the most versatile since it does not require special reactant chemicals. In addition, all analytes have characteristic thermophysical properties that will respond differently to a surface tension gradient, making possible the selective transport of analytes based on species. Since a thermal gradient causes a surface tension gradient, which in turn causes adsorbate motion, the terms thermal gradient and surface tension gradient will be used interchangeably. Also, the terms analyte, reagent, adsorbed mass, molecules adsorbed onto a surface, fluid adsorbed onto a surface, and fluid will be used interchangeably.  
         [0006]     Our device utilizes a controllable array of micro-scale surface or sub-surface thermal elements that can be made to produce dynamic, micro-scale, overlapping surface tension gradients on demand. The result is the precise production and placement of locally confined surface tension gradients that make possible the basic microfluidic operations of transport, merge, subdivide, separate, sort, remove (desorb), and capture (adsorb).  
         [0007]     Transport occurs when a thermal gradient is produced directly under the analyte, causing the analyte to move in one direction. Merging occurs when one or more fluids are transported to the same location, causing the analytes to collide into one adsorbate mass. Subdivision occurs when the source of heat, either a dot or line, is directly underneath the analyte and a thermal gradient radiates in all directions from that source, causing the adsorbate mass to split into two or more smaller adsorbate masses. Separation occurs when a thermal gradient of a particular temperature distribution causes only one type of analyte to be transported. Sort occurs when separated analytes are ordered through transport. Removal occurs when the temperature of the surface directly under the analyte is above its vaporization point, causing the analyte to evaporate or sublimate off the surface. Capture occurs when the temperature of the surface is cooled, causing fluid to be adsorbed onto the surface.  
         [0008]     This versatile microfluidic device has many applications, including “laboratories on a chip” (lab-on-a-chip) and pre-concentration. Lab-on-a-chip technologies offer disposable, fast, and inexpensive chemical experiments. By spatially controlling molecules adsorbed onto a surface, the device permits micro-scale studies of chemistry, biology, and physics. For example, fundamental studies in surface tension and interface phenomena can be explored with the operations of transport, merge, subdivide, separate, sort, remove, and capture. The device allows micro-chemical analysis of complex fluids. Analytes, cells, proteins, and DNA may be transported, separated, sorted, and merged. Micro-scale reactions may be executed by merging individual reactants in an ordered sequence.  
         [0009]     Another application of this microfluidic device is a preconcentrator to increase detection sensitivity of analytical instruments such as gas chromatographs, chemiluminescence detectors or thermal energy analyzers, ion mobility spectrometers, mass spectrometers, micro-electro-mechanical-system (MEMS) sensors, and other sensor/detector devices. Most preconcentrators are cumbersome instruments that draw a large volume of air, collect organic compounds from the surroundings onto a chemical filter, and vaporize the organics into the analytical instrument. Our microfluidic device can perform the same function in an economical, compact manner.  
         [0010]     A particularly valuable application of our invention is a preconcentrator to a MEMS sensor. Because of their small mass, MEMS-based sensors offer a number of unique and distinct advantages. However for a MEMS sensor, a Faustian bargain exists between sensitivity and probability. For example, one type of MEMS sensor is the microcantilever (3), where single molecules adsorbed on the cantilever surface can be detected but whose surface area is only about 10 −4  cm 2 . The small surface area means that the probability of a particle interacting with the sensor area is extremely low, resulting in lower sensitivity for a given analyte concentration. However, a microfluidic manipulator adsorbing particles onto an area of about 1 cm 2 , concentrating the particles to a smaller area, and delivering the particles to the microcantilever through vaporization, would effectively increase the probability of capturing a particle by a factor of 10 4 . Prior to our invention, none of the currently available technologies have been able to offer a clear path to the development of such an extremely sensitive, hand held, MEMS-based sensor.  
         [0011]     Thus, we provide a multipurpose microfluidic device that spatially controls adsorbed molecules on a surface by providing the basic microfluidic operations of transport, merge, subdivide, separate, sort, remove, and capture. Further and other aspects of the present invention will become apparent from the description contained herein.  
       REFERENCES  
       [0012]    
       
          1. Y-T Tseng et. al., “Experimental and Numerical Studies on Micro-Droplet Movement Driven by Marangoni Effect”, IEEE 12th Int. Conf. on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 8-12, 2003, pp. 1879-1882.  
          2. N. Gamier, et. al., “Optical Manipulation of Microscale Fluid Flow”, Phys. Rev. Lett., Vol. 91.054501, pp. 1-4 (2003).  
          3. U.S. Pat. No. 5,719,324, issued Feb. 17, 1998, “Microcantilever Sensor”, T. G. Thundat, et. al.  
       
     
       SUMMARY OF THE INVENTION  
       [0015]     In one embodiment, the invention is a microfluidic manipulator for an adsorbed fluid, comprising a material having a surface for adsorbing fluids, the material provided with a plurality of individually controllable thermal elements that produce thermal gradients on the surface that produce surface tension gradients at the interface between the adsorbed fluid and the surface sufficient to cause the adsorbed fluid to move on the surface; wherein one or more of the thermal elements are controlled to transport adsorbed fluids on the surface.  
         [0016]     In another embodiment, the invention is a microfluidic manipulator for an adsorbed fluid, comprising a material having a surface for adsorbing fluids, the material provided with a plurality of individually controllable thermal elements that produce thermal gradients on the surface that produce surface tension gradients at the interface between the adsorbed fluid and the surface sufficient to cause the adsorbed fluid to move on the surface; wherein one or more of the thermal elements are controlled to merge adsorbed fluids on the surface.  
         [0017]     In a further embodiment, the invention is a microfluidic manipulator for an adsorbed fluid, comprising a material having a surface for adsorbing fluids, the material provided with a plurality of individually controllable thermal elements that produce thermal gradients on the surface that produce surface tension gradients at the interface between the adsorbed fluid and the surface sufficient to cause the adsorbed fluid to move on the surface; wherein one or more of the thermal elements are controlled to subdivide adsorbed fluids on the surface.  
         [0018]     In a still further embodiment, the invention is a microfluidic manipulator for an adsorbed fluid, comprising a material having a surface for adsorbing fluids, the material provided with a plurality of individually controllable thermal elements that produce thermal gradients on the surface that produce surface tension gradients at the interface between the adsorbed fluid and the surface sufficient to cause the adsorbed fluid to move on the surface; wherein one or more of the thermal elements are controlled to separate adsorbed fluids on the surface.  
         [0019]     In yet another embodiment, the invention is a microfluidic manipulator for an adsorbed fluid, comprising a material having a surface for adsorbing fluids, the material provided with a plurality of individually controllable thermal elements that produce thermal gradients on the surface that produce surface tension gradients at the interface between the adsorbed fluid and the surface sufficient to cause the adsorbed fluid to move on the surface; wherein one or more of the thermal elements are controlled to sort adsorbed fluids on the surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  illustrates an embodiment of the invention that features thermal elements in the form of non-intersecting lines.  
         [0021]      FIG. 2  illustrates an embodiment of the invention that features thermal elements in the form of an X-Y orthogonal system of lines.  
         [0022]      FIG. 3  illustrates an embodiment of the invention that features thermal elements in the form of non-intersecting closed lines.  
         [0023]      FIG. 4  illustrates an embodiment of the invention that features thermal elements in the form of an R-θ system of orthogonal lines.  
         [0024]      FIG. 5  illustrates an embodiment of the invention that features thermal elements in the form of a combination of patterned lines.  
         [0025]      FIG. 6  illustrates an embodiment of the invention that features thermal elements and a micro-electro-mechanical-system (MEMS) sensor/detector.  
         [0026]      FIG. 7  illustrates an embodiment of the invention that features collectively controlled thermal elements.  
         [0027]      FIG. 8  illustrates an embodiment of the invention that features thermal elements in the form of an array of dots.  
         [0028]      FIG. 9  illustrates an embodiment of the invention that features thermal elements in the form of a stochastic system of dots of various sizes.  
         [0029]      FIG. 10  illustrates an embodiment of the invention that features thermal elements in the form of a combination of lines and dots.  
         [0030]      FIGS. 11 and 12  illustrate the transport operation of the invention using the embodiment of  FIG. 2 .  
         [0031]      FIGS. 13 and 14  illustrate the subdivide operation of the invention using the embodiment of  FIG. 2 .  
         [0032]      FIGS. 15 and 16  illustrate the subdivide operation of the invention using the embodiment of  FIG. 8 .  
         [0033]      FIGS. 17 and 18  illustrate the merge operation of the invention using the embodiment of  FIG. 2 .  
         [0034]      FIGS. 19 through 21  illustrate the separate operation of the invention using the embodiment of  FIG. 2 .  
         [0035]      FIGS. 22 and 23  illustrate the sort operation of the invention using the embodiment of  FIG. 2 .  
         [0036]      FIGS. 24 through 26  illustrate the desorb operation of the invention using the mbodiment of  FIG. 8 .  
         [0037]      FIGS. 27 and 28  illustrate the adsorb operation of the invention using the embodiment of  FIG. 8 .  
         [0038]      FIG. 29  illustrates the  FIG. 2  embodiment of the invention in more detail, and also illustrates a control system that may be used with all the embodiments of the invention.  
         [0039]      FIG. 30  illustrates the embodiment of  FIG. 29  in further detail.  
         [0040]      FIG. 31  illustrates the embodiment of  FIG. 29  in still further detail.  
         [0041]      FIG. 32  illustrates the transport operation of the embodiment of  FIG. 29 .  
         [0042]      FIG. 33  also illustrates the transport operation of the embodiment of  FIG. 29   
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]     The microfluidic manipulator is illustrated in ten embodiments in  FIGS. 1-10 . In all of these embodiments, not drawn to scale, the microfluidic manipulator has a surface upon which the analyte vapors are allowed to adsorb. The manipulator is provided with individually controllable thermal elements that produce thermal gradients on the surface and control the temperature on the surface. The thermal elements may take the form of non-intersecting lines in  FIG. 1 , an X-Y orthogonal system of lines in  FIG. 2 , non-intersecting closed lines in  FIG. 3 , an R-θ system of orthogonal lines in  FIG. 4 , a combination of patterned lines in  FIG. 5 , a combination of thermal elements and a micro-electro-mechanical-system (MEMS) sensor/detector as in  FIG. 6 , collectively controlled thermal elements as in  FIG. 7 , an array of dots in  FIG. 8 , a stochastic system of dots of various sizes as in  FIG. 9 , and a combination of line and dots as in  FIG. 10 . Fluids are adsorbed and desorbed at selected locations on the surface by controlling the localized surface temperature by the thermal elements. The adsorbed fluids are preferentially manipulated by localized thermal gradients caused by the thermal elements.  
         [0044]     In the device embodiments shown in  FIGS. 1-10  the microfluidic manipulators  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800 ,  900 ,  1000  with surfaces  101 ,  201 ,  301 ,  401 ,  501 ,  601 ,  701 ,  801 ,  901 ,  1001  for fluid adsorption may be fabricated from any suitable material that will electrically isolate and sufficiently thermally isolate the thermal elements  102 ,  202 ,  302 ,  402 ,  502 ,  503 ,  602 ,  702 ,  703 ,  802 ,  902 ,  1002 ,  1003 . The device can be fabricated from a semiconducting material such as silicon, gallium arsenide, germanium, etc. The device can also be fabricated from insulating materials such as mica, glass, silicon dioxide, silicon nitride, silicon carbide, sapphire, diamond, fused silica, fused quartz, etc. The device may be a polymer such as silicone rubber or polyimide. The material may be rigid or flexible.  
         [0045]     The thermal elements  102 ,  202 ,  302 ,  402 ,  502 ,  503 ,  602 ,  702 ,  703 ,  802 ,  902 ,  1002 ,  1003  can be resistive heaters that heat the surface in order to produce a thermal gradient when electrical current is applied. The thermal elements  802 ,  902 ,  1002  can also be Peltier Effect junctions that heat or cool the surface in order to produce a thermal gradient, depending on the direction of the applied electrical current. The methods used to fabricate the thermal elements  102 ,  202 ,  302 ,  402 ,  502 ,  503 ,  602 ,  702 ,  703 ,  802 ,  902 ,  1002 ,  1003  include conducting thin films and ion implantation. Conducting or metal thin films may include gold, platinum, palladium, aluminum, nickel, copper, chrome, etc. Compound thin films may include hafnium diboride (HfB 2 ), titanium-tungsten nitride (TiWN), cobalt silicide (CoSi 2 ), titanium silicide (TiSi 2 ) or other silicides (molybdenum, tungsten, magnesium), etc.  
         [0046]     In the embodiments of  FIGS. 1 and 3 , the thermal elements  102 ,  302  take the form of non-intersecting lines that produce thermal gradients in one direction on the surface  101 ,  301 . In  FIG. 1 , the thermal elements  102  extending in the Y direction will produce thermal gradients in the X direction. Likewise in  FIG. 3 , the thermal elements  302  extending in the θ direction will produce thermal gradients in the r direction.  
         [0047]     In the embodiments of  FIGS. 2 and 4 , the thermal lines  202 ,  402  are disposed orthogonally to be capable of producing thermal gradients in two directions. When a current is passed through individually selected lines  202 ,  402 , the result is two-dimensional control of the thermal gradient in either the X-Y or r-θ direction on the surface  201 ,  401 .  
         [0048]     In the embodiment of  FIG. 5 , the thermal lines  502 ,  503  take the form of a combination of different line shapes, each operated for a particular fluid manipulation operation. For example, the curved thermal elements  503  can be individually controlled to transport adsorbed fluid onto the alternatingly patterned thermal element  502 , after which the thermal element  502  is heated to desorb the fluid off the surface  501 . This embodiment would be useful as a preconcentrator for a nearby detector device, for example.  
         [0049]     In the embodiment of  FIG. 6 , the microfluidic manipulator  600  is integrated with a sensor/detector device. A MEMS sensor/detector in the form of a microcantilever  603  is attached to, or made integral with, the surface  601 . The thermal elements  602  are controlled in a manner to transport adsorbed fluids from the larger surface  601  onto the much smaller microcantilever  603 .  
         [0050]     In the embodiment of  FIG. 7 , two or more thermal elements  702 ,  703  may be electrically connected to efficiently control the thermal gradient for a specific application. For example, the two sets of thermal lines  702 ,  703  may be operated consecutively for accelerated transport in the Y direction.  
         [0051]     In the embodiments of  FIGS. 8 and 9 , the thermal elements  802 ,  902  take the form of dot heaters. These may be resistive heaters or Peltier Effect junctions capable of producing thermal gradients at a single spot on the surface  801 ,  901  by either heating or cooling the surface. Each element  802 ,  902  produces a spatially localized thermal gradient on the surface  801 ,  901  radially direction from that element. The thermal elements  802 ,  902  in the form of dots can be individually controlled for the microfluidic manipulations of transport, merge, subdivide, separate, and sort. In addition, each thermal element  802 ,  902  controls the surface temperature at a specific location. Adsorbed fluid may be desorbed, that is, removed from a specific location by heating that location. If the thermal elements  802 ,  902  are Peltier Effect junctions, a greater adsorption will occur at a specific location on the surface  801 ,  901  by cooling that location.  
         [0052]     In the embodiment of  FIG. 10 , the thermal elements  1002 ,  1003  take the form of dots  1002  and lines  1003 . The thermal dots  1002  may be Peltier Effect junctions that can both heat and cool while the thermal lines  1003  may be resistive heaters.  FIG. 10  thus illustrates the use of both resistive heaters and Peltier Effect junctions.  
         [0053]     All of the embodiments of the microfluidic manipulator shown in  FIGS. 1-10  may be operated to transport, subdivide, merge, separate, sort, remove, and capture fluids adsorbed onto the surface.  
         [0054]     The transporting of adsorbed fluids is illustrated in  FIGS. 11 and 12 . The device  1100  has a surface  1101  provided with a plurality of mutually orthogonal thermal elements  1102 ,  1103 . Adsorbed fluids  1104 ,  1105  are present on the surface  1101 . The heating elements  1102 ,  1103  are heated to produce thermal gradients in the Y and X directions, respectively. When the thermal element  1102  is heated, the adsorbed fluids  1104 ,  1105  are close enough to the thermal element  1102  to be affected by the surface tension gradient, and consequently move in the Y direction away from the higher temperature. This is shown in  FIG. 12 . Similarly, when the thermal element  1103  is heated, the adsorbed fluid  1105  moves in the X direction away from the higher temperature, also shown in  FIG. 12 . The adsorbed fluids  1104  are too far away from thermal element  1103 , and thus are not moved in the X direction by the surface tension gradient from the thermal element  1103 . It is readily seen that the thermal elements  1102 ,  1103  may be heated consecutively or simultaneously. Thus, by proper design and control of the many thermal elements capable of producing the X and Y thermal gradients, it is possible to efficiently transport adsorbed fluids over the surface  1101 . In one example, the transport operation may move adsorbed fluids scattered over a large surface area to one localized area on the surface, thereby concentrating the adsorbed fluids. This embodiment of the invention, then, provides a novel chemical pre-concentrator that could be used, for example, as the front-end to an analytical instrument.  
         [0055]     The subdividing of adsorbed fluids is illustrated in the two embodiments shown in  FIGS. 13, 14  and  15 ,  16  respectively. In  FIG. 13 , the device  1200  has a surface  1201  provided with a plurality of mutually orthogonal thermal elements  1202  on which adsorbed fluids  1203  are present. The heating elements  1202  are heated to produce thermal gradients in the X and Y directions directly under the adsorbed fluid  1203 . As a result, the adsorbed fluid  1203  is subdivided into small volumes  1204  on the surface  1201 , as shown in  FIG. 14 .  
         [0056]     In the other embodiment shown in  FIGS. 15, 16 , the device  1300  has a surface  1301  provided with a plurality of Peltier Effect heating elements  1302 , on which an adsorbed fluid (or fluids)  1303  is present. The Peltier junction  1302  located directly under the adsorbed fluid  1303  is heated to produce a thermal gradient that is radially directed. As a result, the adsorbed fluid  1303  is subdivided into a number of smaller volumes  1304  of varying sizes, as shown in  FIG. 16 .  
         [0057]     The merging of adsorbed fluids is illustrated in  FIGS. 17 and 18 . The device  1400  has a surface  1401  provided with a plurality of X-direction and Y-direction thermal elements on which adsorbed fluids  1403  are present. The Y-direction heating elements  1402  are heated to produce thermal gradients in the X direction. As the adsorbed fluids  1403  move away from the regions of higher temperature produced by the thermal elements  1402 , the fluids merge to form a larger volume  1404  due to nucleation, as shown in  FIG. 18 . One application of this embodiment of the invention would be as a surface for merging several different adsorbed species in an ordered sequence for micro-scale reactions.  
         [0058]     The separating of adsorbed fluids is illustrated in  FIGS. 19, 20 , and  21 . The device  1500  has a surface  1501  provided with thermal elements  1502 - 1507 , on which adsorbed fluids  1508  are present. The adsorbed fluid  1508  is comprised of two dissimilar species  1509 ,  1510 . The thermal elements  1503  and  1506  located directly under the adsorbed fluid volume  1508  are heated to produce thermal gradients in the X and Y directions. As a result of the thermal gradients, the adsorbed fluid  1508  is subdivided into small volumes  1511  on the surface  1501 , as illustrated in  FIG. 20 . The thermal elements  1502 ,  1504 ,  1505 ,  1507  are then heated to produce thermal gradients in the X and Y directions which further subdivide and separate the fluid into smaller volumes of like species, illustrated at  1509 ,  1510  in  FIG. 21 . The separation occurs because different species have different surface tension, mass, and mobility, thus the different species will be transported different distances under the influence of the same thermal gradient. This embodiment of the invention can be the basis for a novel way of obtaining chemical selectivity.  
         [0059]     The sorting of absorbed fluids is illustrated in  FIGS. 22 and 23 . The device  1600  has a surface  1601  provided with thermal elements  1602 , on which two dissimilar adsorbed fluids  1603 ,  1604  are present. The thermal elements  1602  are heated to produce thermal gradients in the Y direction. Because different species have different surface tension, mass, and mobility, they will be transported different distances under the influence of the same thermal gradient. As a result, the two species  1603 ,  1604  may be sorted to different locations on the surface  1601 , as illustrated in  FIG. 23 .  
         [0060]     The removal, or desorption, of absorbed fluids is illustrated in  FIGS. 24, 25 , and  26 . The device  1700  has a surface  1701  provided with a plurality of Peltier Effect junctions  1702 , on which two dissimilar adsorbed fluids  1703 ,  1704  are present. The Peltier heating elements  1702  are heated to selectively or collectively produce a surface temperature sufficient to desorb some of the adsorbed fluid from the surface. Because the two dissimilar adsorbed fluids  1703 ,  1704  will desorb at different surface temperatures, the surface temperature is controlled to affect one species of adsorbed fluid  1703 , but not the other  1704 , or vice versa.  FIG. 25  illustrates, for example, that when the single Peltier heating element  1702  is heated sufficiently, the adsorbed fluid  1704  (shown in  FIG. 24 ) directly over that heating element is removed from the surface  1701 . In addition,  FIG. 26  shows that when many or all of the Peltier Effect junctions  1702  are heated to precisely control the temperature of the surface  1701 , one adsorbed fluid species ( 1704  in  FIG. 23 ) may be entirely desorbed while the other species  1703  remains on the surface  1701 .  
         [0061]     The capturing, or adsorbing, of fluids is illustrated in  FIGS. 27 and 28 . In  FIG. 27 , the device  1800  has a surface  1801  provided with Peltier heating elements  1802 . The Peltier elements  1802  are cooled in order to produce a low surface temperature at a specific location on the surface  1801 . As a result, fluids  1803  from the surroundings will preferentially adsorb at that location, as shown in  FIG. 28 .  
         [0062]     One example of a microfluidic manipulator is illustrated in  FIGS. 29-33 . In  FIG. 29 , the microfluidic manipulator  1900  has a surface  1901  provided with thermal elements  1902 ,  1903  arranged in both the X and Y directions for two-dimensional manipulation of adsorbed fluids. The surface area  1901  for adsorption in this example is about one cm 2 , but can be made any desired area. The thermal elements  1902 ,  1903  are 10 μm wide, 500 nm thick, 1 cm long, and spaced at a 30 μm pitch. The resistivity of each thermal element is about 100 Ω. The thermal elements  1902 ,  1903  have pads  1904 - 1907  at their ends for making external electrical connections. In this example, the pads  1905 ,  1907  on one side of the thermal elements  1902 ,  1903  are grounded while the pads  1904 ,  1906  on the other side of the thermal elements  1902 ,  1903  are connected with wires  1914  which carry electrical signals that activate the thermal elements  1902 ,  1903 . For example, the electrical signals required to transport an adsorbed fluid may be a pulse of 20 V, 300 mA amplitude, 10 ms width, and 100 ms period with a repetition rate of 20. Such an electrical signal may be generated with a control system that includes a transistor-transistor logic (TTL) controlled switching system  1910 , a TTL output module  1911 , a programmable DC source  1912 , and a computer  1913 . The DC source  1912  provides the required voltage and current (20 V-300 mA) to the switching system  1910  with electrical connections  1917 . The DC source may be a power supply, batteries, analog or digital output modules, a pulse generator, etc. In this example, all thermal elements operated simultaneously would receive the same voltage and current. However, each thermal element may also be provided with independent power sources. The TTL output module  1911  selects which thermal elements are to be activated by connecting lines  1916  to the TTL control of each switch  1915 . In addition, the TTL output module  1911  determines the pulse width (10 ms), period (100 ms), and repetition (20). A separate switch  1915  is provided for each thermal element  1902 ,  1903  that is individually controlled. The switches  1915  may be relays, monolithic ICs, multiplexers, data acquisition (DAC) modules, field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), etc. The computer  1913  controls the TTL output module  1911  and the programmable DC power supply  1912  through control lines  1918 ,  1919 .  
         [0063]     The construction of the microfluidic manipulator  1900  is illustrated in  FIGS. 30 and 31 . The surface  1901  is depicted as smooth and flat, although any surface topography can be used. A cross-section along a thermal element  1903  in the Y direction is shown in  FIG. 30  and a cross-section along a thermal element  1902  in the X direction is shown in  FIG. 31 , both figures not to scale. A support  1908  serves as a platform on which the thermal elements  1902   1903  are placed. The support  1908  may be made of insulative or semiconducting materials. Insulative materials include silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), silicon carbide (SiC), diamond (C), sapphire, ceramic, silica glass, fused silica, fused quartz and mica. Flexible polymeric insulative materials include silicone rubber, and polyimide. Semiconducting materials include silicon, gallium arsenide, and germanium. The support  1908  may be flexible or rigid and its thickness may vary. For example, a 500-micrometer thick fused quartz wafer may serve as the support  1908 .  
         [0064]     In  FIGS. 30 and 31 , the thermal elements  1903  in the Y direction are located beneath the surface  1901  while their pads  1904 ,  1905  are exposed to the surface  1901  for electrical connections. The thermal elements  1902  in the X direction are buried about 50 nm beneath the thermal elements  1903  in the Y direction while their pads  1906 ,  1907  are exposed to the surface  1901  for electrical connections. The types of thermal elements  1902 ,  1903  include electrical resistive heaters and Peltier Effect junctions. The methods used to fabricate thermal elements  1902 ,  1903  include conducting thin films and ion implantation. Conducting thin films may be gold, platinum, palladium, aluminum, nickel, copper, and chrome. Compound thin films may be HfB 2 , TiWN, CoSi 2 , TiSi 2  or other silicides (molybdenum, tungsten, magnesium). The pads  1904 - 1907  are made of a conducting material that may be the same as or similar to the thermal elements  1902 ,  1903 . The thermal elements  1902 ,  1903  are electrically isolated from each other by means of a surrounding insulative or semiconducting material  1909  similar to the support  1908 . These materials provide electrical isolation for the thermal elements  1902 ,  1903  as well as thermal isolation for spatially localized thermal gradients and heating.  
         [0065]     An example of the operation of the microfluidic manipulator  1900  is shown in  FIGS. 32 and 33 . In  FIG. 32 , an adsorbed fluid  1916  on the surface  1901  is located to the right of a thermal element  1903 . The thermal element  1903  is given one or a series of electrical pulses such that a surface tension gradient (not shown) is produced between the adsorbed fluid  1916  and the surface  1901  in the X direction. The surface tension gradient is such that the adsorbed fluid  1916  is transported in the X direction past the adjacent thermal element  1914 , as shown in  FIG. 33 . Since the transported adsorbed fluid ( 1916  in  FIG. 33 ) stops to the right of the adjacent thermal element  1914 , the thermal element  1914  may in turn be activated so that the adsorbed fluid  1916  continues to be transported to the right in the X direction. Only the number of thermal elements available limits the distance transported. If (in  FIG. 32 ) the surface tension gradient is not capable of transporting the adsorbed fluid  1916  beyond the adjacent thermal element  1914 , then the adsorbed fluid will remain between the two thermal elements  1903 ,  1914 . If the thermal elements  1903 ,  1914  are Peltier Effect devices, then a steeper thermal gradient is created by heating one thermal element  1903  while cooling the adjacent thermal element  1914 .  
         [0066]     While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be prepared therein without departing from the scope of the invention defined by the appended claims.