Abstract:
A micropump utilizes electrically controlled interfacial tension between a mercury column and an electrolyte solution in a capillary tube, and also uses the gas pressure in a confined chamber connected to the capillary tube as the restoring pressure. Accordingly, the pump operates without generating external jitter or noise, in vertical, horizontal, or in any orientation against the gravitational force. In this manner, the flow rate of the pumped fluid can be widely and conveniently controlled. Further, the micropump is small in size and simple in construction, and needs extremely small power consumption.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a micropump for delivering fluid in small amounts; and, more particularly, to a micropump taking advantages of the change in surface tensions at the mercury/aqueous electrolyte interfaces caused by periodically changing potentials between two preset values. 
       BACKGROUND OF THE INVENTION 
       [0002]    Reliable and reproducible micropumps have been in demands for continuous delivery of drugs or other biologically active substances, continuous operation of micro total analysis systems (pTAS) such as the lab-on-a-chip as well as other microanalysis apparatuses, continuous injection of reactants in a reaction vessel such as in miniaturized fuel cell systems, printer heads, and active cooling of microelectronics. 
         [0003]    Technologies including piezoelectric devices and those utilizing electrocapillary effects and reversible electrochemical gas evolution-dissolution reactions have been employed to construct micropumps. However, no satisfactory micropumps have been constructed thus far, which meet the technical specifications necessary for operation of the above demands. 
         [0004]    Of these, the micropump employing electrocapillary effects takes advantages of the changes in surface tensions of the mercury/electrolyte interfaces. Micropumps constructed and patented thus far, however, used the perpendicular movement of mercury column by the electrocapillary effect, which returns back to its original position by the gravitational force. For this reason, the mercury column had to be oriented perpendicular to the surface of the earth; relatively voluminous uses of mercury may result in its spill causing environmental problems. 
       SUMMARY OF THE INVENTION 
       [0005]    It is, therefore, an object of the present invention to provide a micropump to pump fluids, such as liquids or gases, by taking advantage of the electrocapillary effect due to the changes in surface tension at the mercury/electrolyte solution interface and by arranging the component tubes appropriately. Hence the pump can be operated independent of its spatial orientation without having to worry about the gravity effect. 
         [0006]    In accordance with the present invention, there is provided a micropump for a controlled flow of a fluid in a designated spatial orientation. The micropump includes: a capillary tube for holding a liquid column and an electrolyte solution, the electrolyte solution forming an interfacial boundary with the liquid column; an electrode installed in the electrolyte solution; a metal pin connected to the liquid column; a voltage source connected to the electrode and the metal pin, to thereby periodically change an interfacial tension between the liquid column and the electrolyte solution, resulting in bidirectional movement of the liquid column; a chamber containing a volume of gas therein and connected to one end of the capillary tube, to provide a restoring force due to an interfacial tension between the gas and the liquid column; a membrane confining the electrolyte solution and separating the electrolyte solution from the fluid; and a fluid transport tube, connected perpendicular to another end of the capillary tube, through which the fluid is pumped by periodically changing potentials due to the bidirectional movement of the electrolyte solution. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The above and other objectives and features of the present invention will become apparent from the following description of preferred embodiments, given in conjunction with the accompanying drawings, in which: 
           [0008]      FIG. 1  is a schematic of a micropump utilizing electrocapillary effects and the gas pressures as a restoring force in accordance with a preferred embodiment of the present invention; 
           [0009]      FIG. 2A  shows a micropump engraved channel and space necessary for a gas chamber, an electrolyte solution and a fluid transport tube on a polymer plate in accordance with another embodiment of the present invention; 
           [0010]      FIG. 2B  is a perspective view with a plunger exploded from the micropump of  FIG. 2A ; 
           [0011]      FIG. 3  shows a micropump including neck portions and a liquid layer membrane that is immiscible with an electrolyte solution as well as with the pumped fluid in accordance with still another embodiment of the present invention; 
           [0012]      FIG. 4  represents a micropump array including multiple capillary tubes in accordance with still another embodiment of the present invention; and 
           [0013]      FIG. 5  shows a central part of a U-shaped micropump based on gravitational restoring force in accordance with still another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
         [0015]    As shown in  FIG. 1 , a micropump  100  includes a gas chamber  112  of a given volume; a capillary tube  111  connected to the gas chamber  112 ; a fluid transport tube  131 , connected perpendicular to the capillary tube  111 , through which a fluid such as a liquid or a gas to be pumped moves; two check valves  132 ,  133  that control the flow of the fluid within the fluid transport tube  131 ; a metal pin  123  connected to a liquid column (e.g., mercury column)  121 ; an electrolyte solution  122  forming an interfacial boundary with the mercury column  121 ; an electrode  124  immersed in the electrolyte solution  122 ; a membrane  126  that confines the electrolyte solution  122  within the capillary tube  111 , and separates the electrolyte solution  122  from the fluid; and a voltage source  125  connected to the metal pin  123  and the electrode  124 . 
         [0016]    The gas chamber  112 , which is filled with dry air, nitrogen, inert gas, or the like, is connected to one side of the capillary tube  111 . The capillary tube  111  may be constructed with a glass or engraved into a solid polymeric material. The capillary tube  111  is filled with an appropriate amount of mercury column  121  and is provided with the metal pin  123 . 
         [0017]    Since mercury is hydrophobic and has a large surface tension, the mercury column  121  forms convex meniscuses on its both sides. It is desirable to use platinum or any other metal for the metal pin  123 , which does not dissolve in mercury to form an amalgam. The gas chamber  112  is isolated from the ambient air because of the mercury column  121  filled in the center of the capillary tube  111 . Instead of mercury, any liquid that does not mix or react with the electrolyte solution  122  may be used. However, an appropriate amount of salt need be added to make it electrically conductive in case that the liquid itself is non-conductive. 
         [0018]    The other side of the capillary tube  111  is filled with an electrolyte solution  122 , which does not react or mix with mercury column  121 . To separate the electrolyte solution  122  from the fluids being pumped, a membrane  126  is used. For the electrolyte solution  122 , an aqueous solution containing a salt, an acid, or a base can be used. The aqueous solution is inert to the electrochemical reaction within the employed potential range. The electrolyte solution  122  is required to be electrically conductive, and an electric double layer is formed at the interface between the mercury column  121  and the electrolyte solution  122 . The electrode  124  is installed around the middle of the electrolyte solution  122 , and may be formed of a bare silver wire or preferably a silver wire coated with silver chloride formed by chlorination of the silver wire surface. In order to maintain a constant potential at the electrode  124 , an appropriate amount of chloride may be added to the electrolyte solution  122 . Alternatively, another acid or base solution may be added for maintaining a constant potential at the electrode  124 . 
         [0019]    The fluid transport tube  131  transporting the fluid to be pumped is connected perpendicular to the capillary tube  111  on the opposite side of the gas chamber  112 . Within this fluid transport tube  131 , two check valves  132  and  133  are provided so that the fluid would flow in a designated direction only. In this connection, the capillary tube  111  is connected to the fluid transport tube  131  at a location between two check valves  132  and  133 . 
         [0020]    The metal pin  123  disposed within the mercury column  121  and the electrode  124  immersed in the electrolyte solution  122  are connected to the voltage source  125 . The voltage source  125  provides square or sine wave voltages to the system through the metal pin  123  and the electrode  124 . 
         [0021]    The operation of micropump  100  will now be explained. 
         [0022]    The surface tension (or interfacial tension) between a mercury and an electrolyte solution becomes maximum at a certain potential (potential of zero charge (PZC)) of the mercury relative to the electrolyte solution, and then, diminishes sharply as the potential is made higher or lower than PZC (electrocapillary phenomenon). The surface tension of the mercury column exerts a pressure toward the inside thereof. Referring to Eq. 1, this surface pressure P is proportional to the surface tension γ and is inversely proportional to the radius r of the capillary tube containing the mercury column: 
         [0000]        P= 2 γ/r Eq.  1. 
         [0023]    The surface pressure can, therefore, be changed by the applied potential, and hence the mercury column can be easily pushed or pulled by manipulating the applied potential. The reciprocal movement thus generated is the core mechanism of the micropump in the present invention as the piston in a cylinder. 
         [0024]    The fluid to be pumped stays still before a square or sine wave is applied, as both the check valves  132 ,  133  are closed. In order to operate the micropump  100 , a square or sine wave voltage is applied to the metal pin  123  and the electrode  124  by turning on the voltage source  125 , resulting in a periodic change in surface tension of the mercury column  121  in the capillary tube  111 . Due to the periodic change, the mercury column  121  moves back and forth along the axis of the capillary tube  111  at the frequency of the square or sine wave voltage. 
         [0025]    When the mercury column  121  moves towards the gas chamber  112 , the electrolyte solution  122  also moves towards the gas chamber  112 , thereby pulling the fluid and thus opening the check valve  132  while closing the check valve  133 . When the mercury column  121  moves in the opposite direction, the check valve  132  closes and the check valve  133  opens up due to the pressure built up in the fluid transport tube  131 . Repeated operations using the square or sine wave allow the fluid to be pumped effectively between the two check valves  132  and  133  of the fluid transport tube  131 . 
         [0026]    The bidirectional movement of the mercury column  121  causes the fluid transport tube  131  to be sucked in or out depending on the position of the flexible membrane  126  confining the electrolyte solution  122 . The position of the membrane  126  changes in unison with that of the interface between the mercury column  121  and the electrolyte solution  122 . As a result, the fluid in the fluid transport tube  131  is controlled to be moved towards one direction by the two check valves  132  and  133 . 
         [0027]    Meanwhile, the operating of the voltage source  125  can be a square or sinusoidal wave of a relatively low frequency and a small magnitude, typically about 0.5 V peak-to-peak, which may be directly applied to the metal pin  123  and the electrode  124 , overlaid on the open circuit voltage or a given DC (direct current) voltage. An appropriate range of voltage levels and the frequency can be determined depending on the desired rate and the amount of the fluid to be pumped, the types of the solvent and salt used in the electrolyte solution  122 , etc. The optimum bias DC voltage can be determined such that the potential of zero charge (PZC) is located one side of the potential range of the square or sinusoidal wave. If the signal has too high frequency, the rapid movement of the mercury column  121  may generate small mechanical waves on its surfaces, which may cause unwanted creeping of the electrolyte solution  122  between the mercury column  121  and the wall of the capillary tube  111 . 
         [0028]    When the gas in the gas chamber  112  is compressed due to the movement of the mercury column  121  towards the gas chamber  112 , the compressed gas pushes it back to release the pressure. This, along with the change in surface tension, leads to the periodic movement of the mercury column  121 , resulting in an effective operation of the micropump  100 . This feature, which is different from the other prior art pumps, allows the micropump  100  to be used in any situation regardless of the orientation. When the micropump  100  has to be spatially oriented such that the mercury column  121  would move along the axis of gravity, its pumping can be adjusted by controlling the volume of the gas chamber  112 . 
         [0029]    The micropump  100  thus formed operates without the effect of the gravitational force in all possible orientations independent of how the micropump  100  is situated in space. Further, the micropump  100  needs no electrical motor, consumes a very small amount of electrical energy, and is simple in its mechanical structure. 
         [0030]    Hereinafter, another preferred embodiment of the present invention will be explained. 
         [0031]    As shown in  FIGS. 2A and 2B , a micropump  200  includes a gas chamber  212  of a given volume; a capillary tube  211  connected to the gas chamber  212 ; a fluid transport tube  231  through which a pumped fluid is moved and connected perpendicular to the capillary tube  211 ; two check valves  232 ,  233  for controlling the flow of the fluid; a metal pin  223  disposed within the mercury column  221 ; an electrolyte solution  222  forming an interfacial boundary with the mercury column  221 ; an electrode  224  immersed in the electrolyte solution  222 ; a membrane  226  that confines the electrolyte solution  222  within the capillary tube  211 , and separates the electrolyte solution  222  from the fluid; and a voltage source  225  connected to the metal pin  223  and the electrode  224 . 
         [0032]    And, the micropump  200  may further include a pair of neck portions  214  provided on both sides of the capillary tube  211  to confine the mercury column  221  at a portion of the capillary tube  211  between the neck portions  214 ; and a plunger  213  fitted in the gas chamber  212  for adjusting the volume of the gas chamber  212 . 
         [0033]    Instead of the fixed volume gas chamber  112  ( FIG. 1 ), the plunger  213  is fitted into the gas chamber  212 , thereby enabling convenient adjustment of the gas volume. An elastic thimble may also be used instead of the plunger  213 . 
         [0034]    The membrane  226  may be formed of expandable/contractible solid material in any shape. 
         [0035]    Also, the neck portions  214 , of which diameters are slightly smaller than those of the capillary tube  211 , is provided on both sides of the mercury column  221  to prevent mercury column  221  from flowing into the gas chamber  212  and/or the fluid transport tube  231  in a case of an unexpected mechanical shock. 
         [0036]    The micropump  200  may be formed in small polymer blocks as shown in  FIGS. 2A and 2B . Upper and lower parts with engraved grooves correspond to the capillary tube  211 , the gas chamber  212  and the transport tube  231 . The parts can be fabricated on polymer blocks by means of a lithographic technique. The micropump  200  may be further formed by overlaying the top part over the bottom part. Variations in components are also possible. 
         [0037]    The operation of the micropump  200  is substantially identical to that of the micropump  100 , and therefore, will be omitted for the simplicity. 
         [0038]      FIG. 3  shows a micropump in accordance with still another embodiment of the present invention. 
         [0039]    A micropump  300  includes a gas chamber  312  of a given volume; a capillary tube  311  connected to the gas chamber  312 ; a fluid transport tube  331  through which a pumped fluid moves connected perpendicular to the capillary tube  311 ; two check valves  332 ,  333  for controlling the flow of the fluid disposed on both sides of the fluid transport tube  231 ; a metal pin  323  disposed within the mercury column  321 ; an electrolyte solution  322  forming an interfacial boundary with the mercury column  321 ; an electrode  324  immersed in the electrolyte solution  322 ; and a voltage source  325  connected to the metal pin  323  and the electrode  324 . 
         [0040]    Further, the micropump  300  includes neck portions  314  provided on both sides of the capillary tube  311  to confine the mercury column  321  at a portion of the capillary tube  311  between the neck portions  314 ; and a membrane  327  for confining the electrolyte solution  322  within the capillary tube  311 , and for separating the electrolyte solution  322  from the fluid. 
         [0041]    The membrane  327  is formed of, e.g., a liquid paraffin layer. The membrane  327  is immiscible with the fluid being pumped and the electrolyte solution  322 . The expandable/contractible membranes such as shown in  FIGS. 1 ,  2 A, and  2 B can also be used in this aspect instead. 
         [0042]      FIG. 4  shows a micropump array in accordance with still another embodiment of the present invention. 
         [0043]    A micropump arrays  400  includes a plurality of capiliary tubes  411 ; a centralized chamber  415  into which the capiliary tubes  411  are merged; a plurality of mercury columns  421  located within the respective capillary tubes  411 ; an electrolyte solution  422  in the capillary tubes  411  and the centralized chamber  415 ; a plurality of metal pins  423  disposed within the respective mercury columns  421 ; an electrode  424  disposed within the centralized chamber  415 ; a voltage source  425  connected to the metal pins  423  and the electrode  424 , for supplying square waves or alternating voltages; a plurality of gas chambers  412  connected to the respective capillary tubes  411 ; a fluid transport tube  431 , connected perpendicular to the chamber  415 , through which the fluid to be pumped; a pair of check valves  432 ,  433  provided inside the fluid transport tube  431 , for guiding the pumped fluid in a designated direction while preventing backflow of the pumped fluid; and a membrane  426  separating the electrolyte solution  422  from the fluid. 
         [0044]    The micropump  400  can also equipped a plurality of plungers (not shown) for adjusting the volumes of the corresponding gas chambers  412 ; and neck portions (not shown), whose diameters are smaller than those of the capillary tubes  411 , provided on both sides of the mercury columns  421 . 
         [0045]    By means of connecting the capillary tubes  411  in parallel and merging them into the chamber  415 , the pumping capacity per unit time increases. 
         [0046]    Meanwhile, based on gravitational restoring force against the electrocapillary tension as seen in  FIG. 5 , still another preferred embodiment of the present invention will be explained. 
         [0047]    A micropump  500  includes a U-shaped capillary tube  511  containing a mercury column  521  and an electrolyte solution  522 ; an electrode  524  disposed to contact the electrolyte solution  522 ; and a metal pin  523  disposed to contact the mercury column  521 . 
         [0048]    The micropump  500  further include a voltage source (not shown) connected to the electrode  524  and the metal pin  523 , for supplying square waves or alternating voltages; a fluid transport tube (not shown), connected perpendicular to the capillary tube  511 , through which the fluid to be pumped; and a pair of check valves (not shown) provided to the fluid transport tube, for guiding the pumped fluid in designated direction while preventing backflow of the pumped fluid. 
         [0049]    In this case, the mercury column  521  moves by the changes in surface tension, and the electrically initiated movement is restored due to the gravitational force. While this configuration allows only a given spatial orientation, more efficient design may be used. 
         [0050]    While two experiments for the construction and operation of the micropumps will be given below to demonstrate how effectively they work, their applications are not limited by what are shown by the two examples. 
       EXAMPLE 1 
       [0051]    The pumping rate (flow rate) of the micropump is determined by the moving rate of the mercury column and the cross sectional area of the capillary tube. The moving rate is determined by the distance of the mercury column movement multiplied by the frequency of the square or AC waves applied. The maximum pumping rate is then expressed by Eq. 2: 
         [0000]      Pumping rate= d·f·A   Eq. 2, 
         [0000]    where d is the distance of the mercury column movement, f is the frequency of the AC or square pulse wave, and A is the cross sectional area of the capillary tube. The pumping rate may be adjusted by controlling any of these parameters. 
         [0052]    When the change in surface tension of mercury was 5%, which was usually achievable with a half volt amplitude, the gas volume was 1.0 cm 3 , the radius of the capillary tube was 0.5 mm, and the frequency of the square or AC waves was 1 Hz, the pumped volume per one cycle was 0.79 μL/s at an atmospheric pressure of the gas and the distance of the mercury column movement of 1 mm, and the pumping rate was 47 μL/min. When the length of the mercury column is to be 2 mm, the amount of the mercury column to be used is 0.0016 cm 3 , or 21 mg. 
       EXAMPLE 2 
       [0053]    The same conditions were adapted as those in EXAMPLE 1 except that the gas volume was 0.1 cm 3  and the radius of the capillary tube was 0.1 mm, the pumped volume per one cycle was 0.4 μL/s, the distance of the mercury column movement was 13 mm, and the pumping rate was 24 μL/min. When the length of the mercury column is to be 2 mm, the amount of the mercury column to be used is 0.000063 cm 3 , or 0.85 mg. 
         [0054]    In summary, the micropump described in the present invention has the following characteristics: (1) a very small amount of liquids or gases can be pumped, (2) the size of the pump is small with its simple structure and the low construction cost, (3) the pump can be used to pump a wide variety of fluids including aqueous solution, nonaqueous solution, gases or the like, (4) no vibration and/or no noise is generated during its operation, (5) the flow and pumping rates can be easily controlled, (6) the pump can be arranged in any spatial orientation, (7) the pump may be applied to microanalysis, mixing/dividing fluids for chemical reactions, or any other purposes, (8) no significant consumption of energy due to the lack of frictional forces or other mechanical stresses, resulting in low consumption of the power and small operational variations due to the temperature changes, and (9) very low pollution or damages of the environment are expected due to mercury spills, if any, thanks to a very small amount of mercury used in a closed space. 
         [0055]    While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.