Abstract:
A method and apparatus for controlling multi-fluid flow in a micro channel is disclosed. The apparatus has a first inlet for a first fluid; a second inlet for a second fluid; a first outlet; and a second outlet. The micro channel is operatively and fluidically connected to the first inlet, the second inlet, the first outlet and the second outlet. The micro channel is for receiving the first fluid and the second fluid under pressure-driven flow; there being an interface between the first fluid and the second fluid when in the micro channel. The apparatus also includes a pair of electrodes for having a first electric field applied thereto for a controlling the fluid flow velocity of the first fluid along the micro channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from U.S. Provisional Patent Application No. 60/618,603 filed Oct. 15, 2004, which is hereby incorporated herein in its entirety for all purposes. 

   FIELD OF THE INVENTION 
   This invention relates to a method and apparatus for controlling multi-fluid flow in a micro channel and refers particularly, though not exclusively, to such a method and apparatus that operates on electrokinetic and hydrodynamic principles. In a preferred aspect the present invention relates to a method and apparatus for controlling a position of an interface of fluids in the micro channel for switching, mixing and/or cytometering. In a more preferred aspect the present invention is also for controlling the form and position of the interface. 
   BACKGROUND OF THE INVENTION 
   Most solid surfaces acquire an electrostatic charge when in contact with polar liquids. As a result, a difference in potential is developed across the interface between the negative and positive phases. The charged interface attracts ions of opposition charge (counter-ions) and repels ions of like charge (co-ions) in the liquid. The arrangement of charges that occurs near the interface leads to the development of an electric double layer. When a tangential electric field is applied along the capillary along which the liquid flows, liquids are pumped due to electroosmostic flow. The two widely used methods for the transportation of a single fluid in microfluidics are electroosmostic flow, and pressurized flow. 
   In microfluidics, the Reynolds number is small and fluid flow is laminar. Laminar fluid diffusion interfaces are created when two or more streams flow in parallel within a single micro-structure. Since the flows are laminar, there is no mixing between them. No mixing may be very useful because only diffusion occurs between the different streams of flow. Therefore, it is able to be used for extraction or separation in biological analysis. Diffusion-based microfluidic devices, such as the T-sensor® and the H-filter® have been developed for commercial use by Micronics, Inc. of Redmond, Wash., USA. 
   The variable viscosity of biological fluids can be problematic when the two streams of flows have different viscosities. The fluid with higher viscosity will occupy a wider portion of the channel while having a smaller velocity; whereas the fluid with lower viscosity flows at a larger velocity within a narrow portion of the channel. The two fluids will still have the same volumetric flow rate. The unmatched viscosity affects diffusion due to differences in residence time. The average residence time of the more viscous fluid will increase, while that of the less viscous fluid will decrease. To overcome this problem, it has been proposed to measure the viscosity of the fluid, and to add a viscosity-enhancing solute to the less viscous fluid. Another proposal is to control the ratio of the volumetric flow rate of the two fluids. By increasing the flow rate of the less viscous fluid, it is possible to maintain the interface of the two streams at the center of the channel. However, the unmatched average residence time remains unsolved because the less viscous fluid flows even faster, and has even shorter average residence time within the channel. 
   SUMMARY OF THE INVENTION 
   In accordance with a first preferred aspect there is provided an apparatus for controlling fluid flow in a micro channel, the apparatus comprising: 
   a first inlet for a first fluid; 
   a second inlet for a second fluid; 
   a first outlet; 
   a second outlet; 
   a pair of electrodes; and 
   the micro channel, wherein the micro channel is operatively and fluidically connected to the first inlet, the second inlet, the first outlet and the second outlet, the micro channel receives the first fluid and the second fluid under pressurized flow, a first interface is between the first fluid and the second fluid in the micro channel, and the pair of electrodes apply a first electric field that controls a flow velocity of the first fluid along the micro channel. 
   According to a second aspect there is provided a method for controlling fluid flow in a micro channel, wherein the micro channel is operatively and fluidically connected to a first inlet, a second inlet, a first outlet and a second outlet, the method comprising: 
   supplying a first fluid through the first inlet under pressurized flow to the micro channel; 
   supplying a second fluid through the second inlet under pressurized flow to the micro channel; 
   flowing the first fluid along the micro channel to the first outlet; 
   flowing the second fluid along the micro channel to the second outlet; 
   providing a first interface between the first fluid and the second fluid in the micro channel; and 
   applying a first electric field using a pair of electrodes to control a flow velocity of the first fluid along the micro channel. 
   The first electric field may control a location of the first interface across a width of the micro channel, and a residence time of the first and second fluids in the micro channel. 
   The first pair of electrodes may comprise a first electrode and a second electrode, wherein the first electrode is in the first inlet and the second electrode is in the first outlet. 
   The apparatus may comprise a third inlet for a third fluid, wherein the second inlet is between and spaced from the first inlet and the third inlet, a second interface is between the second fluid and the third fluid, and the third inlet and the third outlet are operatively and fluidically connected to the micro channel. 
   The apparatus may comprise a second pair of electrodes that apply a second electric field that controls a velocity of the third fluid along the micro channel. The second electric field may also control a location of the second interface across the width of the micro channel, and a residence time of the first, second and third fluids in the micro channel. The second pair of electrodes may comprise a first electrode and a second electrode, wherein the first electrode is in the third inlet and the second electrode is in the third outlet. 
   The apparatus may comprise a fourth outlet operatively and fluidically connected to the micro channel. The second electrode of the second pair of electrodes may be in the fourth outlet. 
   The apparatus may comprise a fifth outlet operatively and fluidically connected to the micro channel. The second electrode of the second pair of electrodes may be in the fifth outlet. 
   The first electric field and the second electric field may direct the second fluid to at least one of the first outlet, the second outlet, the third outlet, the fourth outlet and the fifth outlet. 
   The apparatus may be and the method may be used for at least one of an electrokinetic flow switch, a micromixer, a micro-flow cytometer, an interface position controller, and an in-channel fluidic lens. 
   The apparatus may include a fourth inlet for a fourth fluid that is operatively and fluidically connected to the micro channel. The fourth inlet may be between and spaced from the second and third inlets. Alternatively, the apparatus may include top and bottom fourth inlets, wherein the top fourth inlet is between and spaced from the first inlet and the second inlet, and the bottom fourth inlet is between and spaced from the second inlet and the third inlet. The fourth fluid may be a protection fluid that separates the first fluid from the second and third fluids. Alternatively, the fourth fluid may be two sample fluids and the second fluid may be a protection fluid that separates the two sample fluids. 
   The first and second electric fields may narrow a width of the second fluid in the micro channel, thereby focusing a flow of the second fluid in the micro channel. The apparatus may be a micro-mixer and the method may be used for mixing at the micro scale, and the first and second electric fields may narrow a width of the second and fourth fluids in the micro channel, thereby controlling a diffusion path and a diffusion time in the micro channel. 
   The apparatus may include a controller for controlling the first electric field and the second electric field, and the first electric field and the second electric field may control the locations of the first interface and the second interface. 
   The apparatus may include a pair of additional electrodes located at opposite axial ends of the micro channel, and a pair of further electrodes located at a top and a bottom of the micro channel, wherein the further electrodes control a curved shape of the first interface, and the additional electrodes control a focal length and a position of the curved shape. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be fully 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 with reference to the accompanying illustrative drawings. 
     In the drawings: 
       FIG. 1  is a schematic top view of a preferred micro channel arrangement; 
       FIG. 2  is an enlarged vertical cross-sectional view along the lines and in the direction of arrows  2 - 2  in  FIG. 1 ; 
       FIG. 3  is two graphs of the relationship between NaCl holdup and (a) different applied voltage for the same volumetric flow rates, and (b) volumetric flow rate under the same applied voltage; 
       FIG. 4  is a schematic illustration of a second preferred form of flow switch at a first operational state; 
       FIG. 5  is a schematic illustration of the second preferred form of flow switch at a second operational state; 
       FIG. 6  is a schematic illustration of the second preferred form of flow switch at a third operational state; 
       FIG. 7  is a schematic illustration of the second preferred form of flow switch at a fourth operational state; 
       FIG. 8  is a schematic illustration of the second preferred form of flow switch at a fifth operational state; 
       FIG. 9  is a schematic illustration of a third preferred form of flow switch at a first operational state; 
       FIG. 10  is a schematic illustration of the third preferred form of flow switch at a second operational state; 
       FIG. 11  is a schematic illustration of a fourth preferred form of flow switch at a first operational state; 
       FIG. 12  is a schematic illustration of the fourth preferred form of flow switch at a second operational state; 
       FIG. 13  is a schematic illustration of the fourth preferred form of flow switch at a third operational state; 
       FIG. 14  is a schematic illustration of a micro mixer, 
       FIG. 15  is a schematic illustration of a microflow cytometer; 
       FIG. 16  is a schematic illustration of an interface position controller; and 
       FIG. 17  is a schematic illustration of an in-channel fluidic lens. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The first embodiment is shown in  FIGS. 1 and 2  and includes H-shaped microfluidics structure  10 , syringes  31 ,  32  driven by pumps (not shown), and electrodes  14  for the application of an electric field. Preferably electrodes  14  are a metal such as platinum. Syringes  31 ,  32  supply fluids  16 ,  17  respectively to straight micro channel  20 , and fluids  16 ,  17  flow side-by-side in micro channel  20  from left to right. Fluids  16 ,  17  flow under the action of pressure from syringes  31 ,  32  respectively. Inlets A and C are for fluids  16 ,  17  respectively, and outlets B and D are for the collection of products or wastes from micro channel  20 . Between inlet A and outlet B, electrodes  14  are inserted for the application of the electric field and electrodes  14  are supplied by power supply  22 . The electric field from inlet A to outlet B is positive, and from outlet B to inlet A is negative. Micro channel  20  may have any suitable size and configuration such as a cross sectional area of 1000 μm×100 μm and a length of 5 mm. This gives micro channel  20  a width to depth ratio of 10:1. 
   Fluids  16 ,  17  are introduced through inlets A and C respectively into micro channel  20 . The schematic cross-sectional view of fluids  16 ,  17  flow inside micro channel  20  is shown in  FIG. 2 . Interface  24  is between fluids  16 ,  17 . Fluid  16  may be an aqueous NaCl solution (concentration 0.7×10 −3 M) and the fluid  17  may be an aqueous glycerol (volume concentration 14%). The widths occupied by NaCl solution  16  and aqueous glycerol  17  are denoted as w 2  and w 1  respectively. The holdup of NaCl solution  16 , e 2 , is the ratio of the area occupied by NaCl solution  16  to the whole area of the cross-section of micro channel  20 . As the height is common, this becomes: 
   
     
       
         
           
             e 
             2 
           
           = 
           
             
               
                 w 
                 2 
               
               
                 
                   w 
                   1 
                 
                 + 
                 
                   w 
                   2 
                 
               
             
             . 
           
         
       
     
   
   Similarly, the holdup of aqueous glycerol  17  is e 1 =1−e 2 . 
   When fluids  16 ,  17  contact the channel wall of micro channel  20 , the negatively charged channel wall influences the distribution of free ions in NaCl solution  16  to form an electrical double layer near the channel wall. But aqueous glycerol  17  only minimally forms an electrical double layer as there are few free ions. Thus the electroosmotic flow will only affect NaCl solution  16 . When a positive voltage is applied between inlet A and outlet B (inlet A at the positive electrode  14 , outlet B at the negative electrode  14 ), the electroosmotic force forces NaCl solution  16  to flow in the same direction as the pressurized flow. If a negative electric field is applied (inlet A is at the negative electrode  14 , outlet B is at the positive electrode  14 ), an opposite electroosmotic flow results which is against the pressurized flow. 
   A fluorescent dye such as fluorescein disodium salt C 20 H 10 Na 2 O 5 , (also called Acid Yellow 73) may be added to NaCl solution  16  for image collection. When the fluorescein is illuminated by a mercury lamp, a coupled charge device (CCD) camera or other similar device may be used for image capturing to enable measurements to be taken. The same volumetric flow rates at inlets A and C may be ensured through the use of identical syringes  31 ,  32  driven by a single syringe pump. 
   The parameters considered in the graphs of  FIG. 3  are inlet volumetric flow rates, and electric voltage applied between inlet A and outlet B. The holdup of NaCl solution  16  was obtained by normalizing its width w 2  to the whole channel width (w 2 +w 1 ) of micro channel  20 . As shown in  FIG. 3(   a ), when the electric field changes in magnitude and direction, the holdup of NaCl solution  16  changes accordingly. When no voltage is applied across inlet A and outlet B, the flow is simply a pressurized two-phase flow. As aqueous glycerol  17  is about 1.5 times more viscous than NaCl solution  16 , the less viscous NaCl solution  16  occupies a smaller portion of the channel width of micro channel  20 . NaCl solution  16  has a holdup of 0.35 without an externally applied voltage, as shown in  FIG. 3(   a ). When a negative electric field is applied across inlet A and outlet B, the holdup of NaCl solution  16  increases as the electroosmotic flow is against the pressurized flow by the use of the negative electric field. One explanation is that NaCl solution  16  becomes more “viscous” due to the electroosmotic effect. As such it occupies a larger proportion of the channel width of micro channel  20 —w 2  increases and w 1  decreases. The holdup of NaCl solution  16  increases with an increase in the negative electric field. 
   Due to the same pressure drop across sections E and F of micro channel  20  (see  FIG. 1 ), in order to achieve the same volumetric flow rates, the more viscous fluid has to spread to a larger width, i.e. a higher liquid holdup. When a positive electric field is applied, NaCl solution  16  has a lower “viscosity” since the electroosmotic flow is the same direction as the pressurized flow so that the electroosmotic effect aids the flow of NaCl solution  16 . 
     FIG. 3(   a ) also shows that as the inlet volumetric flow rates of fluids  16 ,  17  increase, the electroosmotic flow effect on the pressurized flow weakens. At the flow rate of 1.2 ml/h, the holdup of NaCl solution  16 , e 2 , remains constant even though the voltage varies from −0.8 kV to 0.6 kV. For typical electroosmotic flows, in which hundreds of volts per centimeter of electric field are applied, the resultant flow rate is of the order 0.1 to a few mm/s. But for pressurized flow in micro channels, the flow rate can be controlled over a wider range. When the pressurized flow rate is set at 0.4 ml/h, the average velocity for NaCl solution  16  through micro channel  20  is 3.17 mm/s with no external applied electric field. This is comparable to that of electroosmotic flow.  FIG. 3(   a ) shows that by adjusting the electric field, the position of interface  24  between fluids  16 ,  17  can be controlled. As such, variation of NaCl solution  16  holdup e 2  from 0.25 to 0.50 is controlled. 
   The relationship between NaCl solution  16  holdup e 2  at different flow rates under a fixed electric field is shown in  FIG. 3(   b ). Holdup e 2  remains the same (0.35) for different volumetric flow rates in the absence of an externally applied electric field. This is because the volumetric flow rates ratio between fluids  16 ,  17  is kept unchanged at 1:1. As the flow rate increases, holdup e 2  converges to a constant value, 0.35. This is the value without an externally applied electric field. The reason is that the larger, pressurized flow speed makes the electroosmotic effect virtually negligible. 
   Therefore, by adjusting the magnitude and the direction of the applied electric field, the position of interface  24  between fluids  16 ,  17  can be controlled, as can be the average residence time for fluids  16 ,  17 . The H-shaped microfluidics structure  10  can therefore be used as a diffusion-based analysis device as it provides the same average residence time for fluids  16 ,  17 . 
   A second preferred from of microfluidic flow switch is shown in  FIG. 4 . Microfluidics device  400  has three inlets  401 ,  402  and  403  with respective syringes  431 ,  432  and  433 , and five outlets  411  to  415 . Inlets  401  and  403  are spaced apart and introduce control fluids  416  and  418  such as aqueous NaCl. Sample fluid  411 , which can be a biological fluid of interest, is introduced from inlet  402  between and spaced from inlets  401 ,  403 . A first pair of electrodes  421  is located between inlet  401  and outlet  411 , and a second pair of electrodes  422  is located between inlet  403  and outlet  415  for the application of electric fields. Electrodes  421  are supplied by first power supply  423 , and electrodes  422  are supplied by second power supply  424 . 
   Without changing the flow rate, the spread widths of the three laminar streams of fluids  416 ,  417  and  418  can be adjusted by adjusting the direction and strength of the electric field, based on the working principle described above. Sample fluid  417  can therefore be guided into different outlets by controlling the direction and strength of the voltage applied to electrodes  421  and  422 . 
   In  FIG. 4 , electrodes  421  and  422  apply equal electric fields so that fluids  416  and  418  occupy an equal portion of the channel width of micro channel  420 . In that way, sample fluid  417  is guided down the centre of micro channel  420  and thus exits through the centrally-aligned outlet  413 . 
   In  FIG. 5 , if electrodes  421  apply a positive electric field and electrodes  422  apply a negative electric field, control fluid  416  occupies a reduced portion of the channel width of micro channel  420 , and control fluid  418  occupies an increased portion of the channel width of micro channel  420 , thereby guiding sample fluid  417  to outlet  412 . A similar effect may be achieved by having electrodes  421  apply a strong, positive electric field and electrodes  422  apply no electric field. The effect is created by having electrodes  421  apply an electric field that is more positive than that applied by electrodes  422 . 
     FIG. 6  is the reverse of  FIG. 5 , so that sample fluid  417  flows to outlet  414 , and  FIG. 7  is the same as  FIG. 5  except that the difference in the applied electric fields is greater so that sample fluid  417  flows to outlet  411 . 
   To get sample fluid  417  of high purity, the electric fields can be adjusted so that sample fluid  417  has a width that is slightly larger than the outlet width. 
   Besides flow switching, microfluidics device  400  can be used for flow focusing. Sample fluid  417  can be squeezed into a very thin flow to allow only a single cell or several cells to pass as in  FIG. 8 . This is useful for cell detection. If the electric field is remotely controlled such as by using a computer, a programmable sample injection device or programmable dispensing device can be provided. Microfluidics device  400  can also be used as a valve since the desired outlet  411  to  415  can be selected by controlling the electric field. 
   To reduce diffusion or reaction between control fluids  416 ,  418  and sample fluid  417 , another protection fluid  419  is introduced to separate the two in  FIG. 9 . Preferably, protection fluid  419  is relatively inert with both control fluids  416 ,  418  and sample fluid  417 . Protection fluid  419  is introduced by syringes  434 ,  435  and respective inlets  404 ,  405 . 
   Multiple sample fluids  417  are switched in  FIG. 10 . Between sample fluids  417 ( a ) and  417 ( b ), protection fluid  419  or a buffer fluid is introduced for separation of sample fluids  417 ( a ) and  417 ( b ). 
   Other designs based on the working principle of the present invention can be employed.  FIGS. 11 to 13  show a Y-shaped flow switch under different work modes, e.g. switching sample fluid  417  to one or more outlets. In  FIG. 11 , the Y-shaped microfluidic flow switch has two inlets  401 ,  402  and four outlets  411  to  414 . Control fluid  416  and sample fluid  417  are introduced from inlets  401  and  402 . The electric field is applied through two electrodes  421  inserted between inlet  401  and outlet  411 . Sample fluid  417  can be directed to outlets  412 ,  413  and  414 . For example, the flow switch directs sample fluid  417  to outlet  412  as shown. In  FIG. 12 , sample fluid  417  is passed to outlets  412 ,  413 , and in  FIG. 13 , sample fluid  417  is passed to outlets  411  to  414 . This may be simultaneously, or sequentially. 
     FIG. 14  shows microfluidics device  400  as a micro mixer. The diffusion distance, according to the square dependency, affects the diffusion time between the laminar flows of sample fluids  417 ( a ) and  417 ( b ). As diffusion is the main mechanism through which mixing occurs between the two laminar streams, by adjusting the electric field across the control fluids  416  and  418 , sample fluids  417 ( a ) and  417 ( b ) are squeezed into a narrow stream to thus reduce the diffusion path and diffusion time and increase the mixing efficiency. 
     FIG. 15  shows microfluidics device  400  as a microflow cytometer. A conventional microflow cytometer uses hydrodynamic focusing. Instead of focusing the sample flow hydrodynamically through the sheath flow rate, by combining the pressure driven and the electrokinetic effects, microfluidics device  400  provides a microflow cytometer that focuses the cells in sample fluid  417 . The fluid flow along micro channel  420  is smaller in width than inlet  403 , and is preferably the same as, or only slightly greater than outlet  413 . In this way the focusing takes place along micro channel  420 . 
   Although the electrodes  14 ,  421  and  422  are described and illustrated as being in the inlets and outlets, they may be located in micro channel  20 ,  420  adjacent the inlets and outlets, or at the junction of the inlets and the micro channel, and/or at the junction of the outlets and the micro channel. 
     FIG. 16  illustrates an interface position controller for determining and controlling the positions of interfaces between fluids. When fluids in micro channel  1620  are excited with laser  1640 , fluorescent light signals are emitted. Band-gap filter  1642  is placed on the other side of micro channel  1620  so that only light of the emitted wavelength is passed to CCD array  1644 , or other photosensor. The fluorescent signal detects the presence of the fluid interfaces and thus enables the position of the fluid interfaces to be determined as the output signal  1646  is proportional to the bright area of micro channel  1620 . The interface position is compared to the desired position  1648  in controller  1650 , and if they are different, controller  1650  outputs control signal  1652  to amplifier  1622 . The power supply to terminals  1614  is adjusted to adjust the applied electric field to micro channel  1620  thereby controlling the interface position. 
     FIG. 17  illustrates an in-channel fluidic lens. Additional electrodes  1760  and  1762  are located at opposite axial ends of micro channel  1720  and provide axial control, and two further electrodes  1764  and  1766  are placed at the top and bottom of micro channel  1720  at the detection area of micro channel  1720 . The electrodes may be transparent material such as indium tin oxide. Further electrodes  1764  and  1766  apply a potential that, in turn, controls contact angle  1768 . Therefore, interface  1770  becomes curved as shown. Interface  1770  acts as a cylindrical lens and focuses the incoming excitation laser  1772  to a sheet with high intensity. This allows for a large fluorescence detection area within micro channel  1720 , and for emitted signal  1776  to have higher intensity. Focal length and position  1774  is controlled by the potential applied by additional electrodes  1760 ,  1762 . Therefore, by selective excitation of electrodes  1760 ,  1762 ,  1764  and  1766 , improved performance may result. 
   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 in details of design or construction may be made without departing from the present invention.