Abstract:
A method of actuating, comprising: filling at least a portion of a tube ( 21 ) with a liquid ( 19 ) containing electrolytes, the tube ( 21 ) having an inner surface that is electrically chargeable when in contact with the liquid ( 19 ); positioning an object ( 28 ) in fluid communication with the liquid in the tube; and applying an electrical field ( 46 ) along a lengthwise axis across the tube at said portion for producing a pressure in the liquid. The pressure in the liquid exerts a force on the object so as to actuate the object ( 28, 30 ). An actuator ( 20 ) is also disclosed.

Description:
CROSS-REFERENCE TO OTHER APPLICATIONS 
     This is a National Phase of International Application No. PCT/SG2004/000371, filed on Nov. 17, 2004, which claims priority from U.S. Provisional Patent Application No. 60/520,643, filed on Nov. 18, 2003. 
    
    
     FIELD OF INVENTION 
     This invention relates to a method of actuating and an actuator, and refers particularly, though not exclusively, to an electrokinetic actuator and method for fluids. The use of such a method and actuator is particularly relevant, though not exclusively so, for compressing gases or vapour, for transporting gases and vapors, for delivering non-conducting, non-polar liquids in micro-scaled channels, and for enhancing mixing in microfluidics. 
     BACKGROUND 
     Electroosmosis is an electrokinetic phenomenon that occurs when an electrolyte fluid interacts with solid surfaces causing a charged layer to form at the interface between the solid and the liquid. Immobilized electric charges develop at the surface of the solid surface in contact with the electrolyte fluid due to electro-chemical phenomena. The surface charge leads to the formation of an electric double layer (“EDL”) by influencing the distribution of counter-ions and co-ions in the electrolyte fluid. In a diffuse layer of the EDL, the counter-ions predominate over the co-ions to neutralize the surface charge. As such, the local net charge density is not zero. A Columbic force is exerted on the ions within the EDL when an electric field is applied tangentially along the charged surface. Consequently, an electroosmotic flow (EOF) results whereby the migration of mobile ions will carry the adjacent and bulk liquid phase by viscosity. 
     The build-up of pressure as a result of electroosmosis facilitates the transport and manipulation of liquids in microfluidic devices for biomedical applications. These principles have been applied in the operation of many electroosmotic pumps. Such electroosmotic pumps work without movable mechanical parts, consequently improving durability and minimizing difficulties in production. Such electroosmotic pumps are essential for biochemical analyses as they enable the pumping of liquids over a wide range of fluid conductivities. 
     Given that electroosmosis is essentially a surface dominated phenomenon, the use of a porous structure with a high surface area-to-volume ratio can enhance the pressure-building capacity. Paul et al. [1998 Electrokinetic generation of high pressures using porous microstructures in: Proceedings of the Micro Total Analysis Systems &#39;98 Workshop, Banff, Canada] proposed a method to generate high pressure using DC electroosmosis through a microchannel packed with microparticles. The pressure of 10 atm at 1.5 kV applied voltage has been achieved using fused-silica capillaries packed with charged 1.5 μm silica beads. S. Zeng et al, [Fabrication and Characterization of Electroosmotic Micropupms, Sensors and Actuators B 2001, 79, 107-114] fabricated an electroosmotic pump that can generate maximum pressures in excess of 20 atm or maximum flow rates of 3.6 μl/min by applying a 2 kV electric voltage over 5.4 cm long, 500-700 μm in diameter fused-silica capillaries packed with 3.5 μm silica particles. S. Yao et al, [A Large Flowrate Electroosmotic Pump with Micro Pores, Proceedings of IMECE, ASME, 2001, New York, N.Y.] developed an electroosmotic pump for a large flowrate with micro pores which can generate a maximum flowrate of 7 ml/min and a maximum pressure of 2.5 atm for 200V applied potential. In a recent development, L. Chen et al, [Generating High-Pressure Sub-Microliter Flow Rate in Packed Microchannel by Electroosmotic Force: Potential Application in Microfluidic Systems, Sensors and Actuators B 2003 88 260-265] developed a pump made of microchannels packed with porous fine dielectric material, which can generate a maximum pressure of 15 MPa. 
     The aforementioned sampling of documents show that the use of electroosmotic principles are commonly employed in micro-fluid pumping. Thus far, there has been no disclosure of the application of electroosmotic principles for actuation. 
     SUMMARY 
     According to a first preferred aspect there is provided a method of actuating, comprising: filling at least a portion of a tube with a liquid containing electrolytes, the tube having an inner surface that is electrically chargeable when in contact with the liquid; positioning an object in fluid communication with the liquid in the tube; and applying an electrical field along a lengthwise axis across the tube at said portion for producing a pressure in the liquid. The inner surface is advantageously electrically chargeable due to electrochemical phenomena. The pressure in the liquid exerts a force on the object so as to actuate the object. The tube may preferably be selected from a capillary tube or a micro-capillary tube. It is preferable that the tube has an open end and the object is in fluid communication with the liquid in the tube through the open end. 
     Preferably, there is an additional plurality of tubes each at least partially filled with a liquid containing electrolytes in fluid communication with the object, The plurality of tubes may be formed in a porous material. It is preferable that the porous material may be made from electrically non-conducting material such as silica or ceramic. The porous material may advantageously have one or more material properties such as a porous structure, micro capillaries, small particles, electrically non-conductive, and hydrophilic. 
     The electric field may be generated from AC or DC power supplies. It is advantageous that the DC power supply is linked to an on-off frequency controller. Advantageously, the pressure in the liquid is caused by electroosmotic flow. 
     A higher force on the object may be generated by preferably adopting techniques like using porous material with small pore sizes, using porous material with large cross-sectional areas, using a lower concentration of the liquid containing electrolytes, or by generating a stronger electric field. 
     There is also provided an actuator comprising: a tube with an inner surface and at least partially filled with a liquid containing an electrolyte, the inner surface being electrically chargeable when in contact with the liquid; an electric field generator for generating a field along a lengthwise axis of the tube for inducing a pressure in the tube; and an object in fluid communication with the liquid such that the pressure in the liquid exerts a force on the object for actuating the object. The inner surface may be electrically chargeable due to electrochemical phenomena and the pressure in the liquid exerts a force on the object so as to move the object. The tube may preferably be selected from either a capillary tube or a micro-capillary tube. Advantageously, the tube has an open end and the object is in fluid communication with the liquid in the tube through the open end. There may be an additional plurality of tubes each at least partially filled with a liquid, the liquid containing electrolytes in fluid communication with the object. The plurality of tubes may be formed in a porous material. The porous material may preferably be made from electrically non-conducting material selected from silica or ceramic. Advantageously, the porous material may have material properties such as hydrophilic, electrically non-conductive, porous structure, micro capillaries, and small particles. 
     The electric field generator may generate either AC or DC power. Preferably, the DC power supply is linked to an on-off frequency controller. In addition, The pressure in the liquid is preferably caused by electroosmotic flow. 
     A higher force on the object may be generated by adopting techniques such as using porous material with small pore sizes, using porous material with large cross-sectional areas, using a lower concentration of the liquid containing electrolytes, or by generating a stronger electric field. 
     Preferably, a housing defines a chamber containing the tube, and a cylinder in fluid communication with the chamber. The tube may be in the cylinder and the object may be a piston slideably mounted in the cylinder. It is preferable that the piston is biased to resist a force exerted thereon from the tube. The actuator may further comprising a displacement amplifier operatively connected to the piston. 
     Preferably, the piston has silicone seals. The actuator may further include a compensating piston to prevent a drop of pressure in the porous material. The actuator may advantageously further include a vent in the housing for allowing the exchange of air within the chamber. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       In order that the invention may be better understood and readily put into practical effect, there shall now be described by way of non-limitative example only preferred embodiments of the present invention, the description being in reference to the accompanying illustrative drawings in which: 
         FIG. 1  is a schematic diagram of a preferred embodiment; 
         FIG. 2  is a graph of pressure gradient against time in a micro-capillary of a preferred embodiment when a DC electric field is applied; 
         FIG. 3  is a graph of pressure gradient against ratio of the hydraulic diameter of a micro-capillary to a reference diameter with a geometric size of 40×100 μm when a DC electric field is applied; 
         FIG. 4  is a graph of pressure gradient against characteristic moments in a micro-capillary of a preferred embodiment when an AC electric field is applied; and 
         FIG. 5  is a graph of pressure gradient against ratio of applied frequency to characteristic frequency f* in a micro-capillary of a preferred embodiment when an AC electric field is applied. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , there is provided a schematic diagram of an electrokinetic actuator  20 . The actuator  20  may have a housing  22 . The housing  22  may be of different cross-sectional shapes, such as, for example, circular, rectangular, approximating a square, polygonal, and the like. The housing  22  has a circular cross section forming a cylinder. 
     The housing  22  defines a chamber  21  that encases hydraulic fluid  19 , a primary cylinder  24  and a secondary cylinder  26 . The primary cylinder  24  and the secondary cylinder  26  have different bore diameters. The hydraulic fluid  19  acts as a damping medium. The primary cylinder  24  has a larger bore than the secondary cylinder  26 . Correspondingly, a primary piston  28  in the primary cylinder  24  has a larger diameter than a secondary piston  30 . There is also a primary retaining spring  32  and a secondary retaining spring  34  in the primary  24  and secondary  26  cylinders respectively. The primary retaining spring  32  prevents the primary piston  28  from contacting a top end  40  of the primary cylinder  24  and to return the primary piston  28  to a resting position. Similarly, the secondary retaining spring  34  prevents the secondary piston  30  from contacting a top end  48  of the secondary cylinder  26  and to return the secondary piston  30  to a resting position. 
     The primary  24  and secondary  26  cylinders may be connected as shown in  FIG. 1 . The top end  40  of the primary cylinder  24  may have an opening (not necessarily central) joining to a bottom end  42  of the secondary cylinder  26 . The primary  24  and secondary  26  cylinders may be filled with hydraulic fluid that flows within the chambers of both the primary  24  and secondary  26  cylinders. The hydraulic fluid may also line the walls of the chambers of both the primary  24  and secondary  26  cylinders as a lubricating film. The hydraulic fluid may be contained primarily towards the top end  40  of the primary cylinder  24  and the bottom end  42  of the secondary cylinder  26 . The primary piston  28  and the secondary piston  30  may include at least one circumferential-seal  44  to prevent hydraulic fluid from leaking to other portions of the chambers of the primary  24  and secondary  26  cylinders. The circumferential seal may be made of silicone or other suitable materials. 
     Electroosmosis is ordinarily associated with DC (direct current) electric field, which is used to generate a steady-state mono-directional electroosmotic flow. An AC (alternating current) electric field may also be applied to induce electroosmosis. However, the electroosmotic flow then becomes time and frequency dependent, with an oscillating flow direction. A frequency-dependent excitation electric field may be applied across a non-conductive capillary with single sealed ends to induce electroosmosis. Each capillary may be filled with an aqueous liquid. Since each capillary is sealed at one end, there is no net flow of ions and this builds up the pressure in each capillary. This pressure can be converted into an actuating force. 
     In a preferred embodiment of the present invention, the primary cylinder  24  may contain hundreds or thousands of microcapillaries bundled together in a lower end  36  of the primary cylinder  24 . For ease of fabrication, a porous material  38  (as shown) filled/soaked with an electrically conducting or an aqueous solution may be used in place of the microcapillary bundles. The porous material  38  may be made of electrically non-conducting materials. 
     Mathematical models have been developed to analyze the frequency-dependent electroosmotic flow in empty or packed microcapillaries. The corresponding Navier-Stokes equation is solved using the Green&#39;s function method and the complete Poisson-Boltzmann equation governing the EDL potential field is solved under an analytical scheme for arbitrary zeta potentials. When both capillary ends are closed, the oscillating flow independently generated by the AC electroosmosis is balanced by the oscillating counter-flow. 
     While operating the electrokinetic actuator  20 , an AC power supply  46  (or any sinusoidal power supply) with a switch  47  may be applied across the porous material  38  which is filled/soaked with an electrically conducting, any aqueous liquid, or any ionic solution such as, for example, demonized water. With the AC power supply  46  turned on, an oscillating electrical field generated causes the ions in porous channels of the porous material  38  to flow in an oscillating manner. As the porous channels of the porous material  38  simulates closed channels, the oscillating electrical field causes the liquid flow at the central part of these porous channels in the porous medium  38  to change direction often, creating a high pressure gradient in the porous channels in the porous medium  38 . This high pressure gradient generates a high back pressure in the porous channels in the porous medium  38 . The back pressure in the porous channels in the porous medium  38  may be used to push/move the primary piston  28  in the primary cylinder  24 . The primary piston  28  compresses the hydraulic fluid in the top end  40  of the primary cylinder  24  and the bottom end  42  of the secondary cylinder  26 . The compressed hydraulic fluid then pushes/moves the secondary piston  30  of the secondary cylinder  26 . 
     The secondary piston  30  may be connected to an actuation cylinder  50  that provides linear actuation to external applications/devices. The difference in the bore sizes of the primary cylinder  24  and the secondary cylinder  26  creates a displacement amplifier effect because of the non-compressitivity of the hydraulic fluid. The volume of hydraulic fluid forced out of the primary cylinder  24  by the movement of the primary piston  28  will be the same volume of hydraulic fluid forced in the secondary cylinder  26 . The smaller bore size of the secondary cylinder  26  compared to the bore size of the primary cylinder  24  allows the actuator  20  to be used in situations where large amplitudes of actuation are required. Based on the principle of conservation of fluid, the amplitude of actuation of the actuating cylinder  50  is the amplitude of motion of the primary piston  28  multiplied by the square of the diameter ratio of the primary piston  28  and the secondary piston  30 . Hence, it can be seen that a slight movement by the primary piston  28  would induce a significant movement of the actuation piston  50 , especially if the diameter ratio of the primary piston  28  and the secondary piston  30  is large. The converse is true when the secondary piston  30  falls from its highest position. 
     In an alternative embodiment, a DC power supply instead of an AC power supply may be used to work the actuator  20 . The DC power supply may be linked to an on-off frequency controller that provides necessary on-off modulation for the system to simulate the sinusoidal oscillating patterns of an AC power supply. The DC power supply should preferably be of high voltage because of the high electric field required. The actuation frequency may be specified using the on-off frequency controller. 
     The lower end  36  of the primary cylinder  24  may include a compensating piston  52 . The compensating piston  52  may be used to prevent the drop of pressure in the porous channels of the porous material  38  during the operation of the actuator  20  due to the displacement of the primary piston  28 . 
     A vent  54  may be incorporated at the lower end  36  of the primary cylinder  24 . The vent  54  may be used to facilitate the movement of the compensating piston  52 . The vent  54  allows the compensating piston  52  to return to its rest position when the primary piston  28  returns to its own rest position by exposing the void in the housing  22  created by the movement of the compensating piston  52  to atmospheric pressure. 
     Simulations have been carried out to determine the characteristics of different parameters in a sealed micro-capillary during frequency-dependent electroosmotic flow. The simulations and calculations were carried out based on a set of fixed parameters. The working fluid was NaCl (sodium chloride) with valence of 1 at 293 K and with density, viscosity and dielectric constant of 10000 kg/m 3 , 9×10 −4  Ns/m 2  and 80, respectively. The reference velocity was set at 1 mm/s and the electric field for both DC and AC were applied with a field strength of 1000V/m. The characteristic hydraulic diameter of the micro-capillary was set at 57 μm. Other important parameters were a characteristic time t*=155.3 μs with a corresponding eigen-frequency of f*=6.44 kHz. 
       FIG. 2  shows that in the instance of a DC electric field applied to a micro-capillary, the highest magnitude of the pressure gradient ( 80 ) is generated during time first instant immediately after the application of an electric field. It can be seen from  FIG. 2  that the pressure gradient gradually decreases thereafter and eventually attains a steady state of a low pressure gradient. 
       FIG. 3  shows a graph of pressure gradient against a ratio of a hydraulic diameter of a micro-capillary to a reference diameter with a geometric size of 40×100 μm when a DC electric field is applied. The negative gradient of  FIG. 3  suggests that as the size of the micro-capillary increases, the magnitude of the pressure generated in the chamber decreases. 
       FIGS. 4 and 5  show the behaviour in the micro-capillary that occurs when an AC supply electric field is applied to a micro-capillary.  FIG. 4  shows a graph of pressure gradient against characteristic moments in a micro-capillary of a preferred embodiment of the present invention when an AC electric field is applied.  FIG. 4  shows that at higher excitation frequencies, for example, when f=10f*, the larger the oscillation in values of the pressure gradient. 
       FIG. 5  shows a graph of pressure gradient against a ratio of applied frequency to characteristic frequency f* in a micro-capillary of a preferred embodiment of the present invention when an AC electric field is applied. As denoted in  FIG. 5 , the supply frequency should exceed the characteristic frequency f* in order to attain a high pressure gradient within a micro-capillary. It can also be seen that when the ratio of applied frequency to characteristic frequency f* (dimensionless excitation frequency) exceeds the value of 100, the magnitude of the pressure gradient increases linearly with the magnitude of the dimensionless excitation frequency, and correspondingly, the supply frequency. 
     The force required for the actuation of the primary piston  28  may depend on various factors. Higher forces may be attained by utilizing a porous medium  38  with small pores sizes. It should be noted that the overlapping of EDLs should be avoided to maximize the amount of force generated. Other approaches to attaining greater forces include using a lower concentration of the electrolyte solution, using a larger cross-sectional area of a porous column, and using a stronger electric field. Forces in excess of 2 KN may be obtainable from the use of a 0.1 m diameter porous column. 
     The actuator  20  is a simple design with few moving parts that is not complicated in construction. Each actuator  20  requires minimal maintenance, if any. The materials used to manufacture the actuator  20  may include metals, silica, ceramic and plastics. The materials used and the corresponding material costs would be determined by the environment that the actuator  20  is employed at The cost of manufacture would be low as the actuator  20  may be manufactured using existing manufacturing techniques and technology. 
     The actuator  20  is an energy-efficient device. Although the applied voltage to generate an electric field is relatively high, the current drawn is low and correspondingly, power consumption is low as well. 
     The actuator  20  may be used in a myriad of different applications ranging from situations that require precision micro actuation, to situations that require large displacement linear actuation such as in a linear motor. It may be unnecessary to use a displacement amplifier in the actuator  20  for precision micro actuation. The actuation may be obtained directly from the primary piston  28 . 
     The actuator  20  may be used in applications involving linear actuations. The actuation may be in nano, micro or macro scales. The actuator  20  may also be used for positioning in relevant fields of applications. 
     The actuator  20  may be used as a precision actuator as its actuation can correspondingly be controlled precisely since the displacement of the actuator  20  is proportional to the applied electric field strength. As such, the actuator  20  may be employed to replace piezo-actuators. The actuator  20  may also be employed in a hard disk drive to position the head arms to different tracks on the surface of the platter during the writing/retrieval of data. Similarly, the actuator  20  may be used as a precision micro-actuator in cameras, microscopes or any applications that require precision linear or possibly even rotational actuation. For larger amplitudes, of actuation, a linear displacement amplifier (as described earlier) may be used to amplify the amplitude of displacement. 
     In an alternative application, the actuator  20  may be employed as a linear motor. It may be possible to generate larger linear displacements from the actuator with the aid of a mechanical displacement amplifier. The actuator  20  may be used in various scales as a linear motor in delivering linear actuations such as, for example, driving linear air compressors or refrigeration compressors for air-conditioners or refrigerators or in a miniature refrigeration system for CPU cooling. The fabrication of the miniature refrigeration compressor may be possible because of the miniaturisation of the actuator  20 . The actuator  20  may also be used for hydraulic or pneumatic actuations. 
     The actuator  20  may also be used for precision positioning applications. As the pressure generated by such actuators  20  may attain very high levels (in excess of 100 atmospheric pressures), it may thus be suitable for applications in static and dynamic nano, micro and macro positioning, depending on the design parameters of the actuator  20 . 
     Whilst there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications may be made to details of design or construction without departing from the present invention. 
     The present invention extends to all features disclosed either individually, or in all possible permutations and combinations.