Abstract:
A flow of liquids is carried out on a microscale utilizing surface effects to guide the liquid on flow paths to maintain laminar flow. No sidewall confining structure is required, minimizing resistance to flow and allowing laminar flow to be maintained at high flow rates. The guiding structure has flow guiding stripes formed on one or both of facing base and cover surfaces which are wettable by a selected liquid to direct the liquid from a source location to a destination location. The regions adjacent to the guiding stripes on the base and cover surfaces are non-wettable. The smooth interface between the gas and liquid along the flowing stream allows gas-liquid reactions to take place as a function of diffusion across the interface without mixing of the gas and liquid. Liquid-liquid flows may also be guided with such structures.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of provisional patent application No. 60/267,692, filed Feb. 9, 2001, the disclosure of which is incorporated by reference. 
    
    
     REFERENCE TO GOVERNMENT RIGHTS 
     This invention was made with United States government support awarded by the following agency: DOD ARPA F30602-00-2-0570. The United States government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains generally to the field of fluid control devices and particularly to the formation and use of microfluidic systems. 
     BACKGROUND OF THE INVENTION 
     The manipulation of fluids in small volumes is required or desirable in many applications of microfluidic devices, including rapid bioassays, microchemical reactions, and chemical and biological sensing. For a review of such applications, see M. Freemantle, “Downsizing Chemistry,” Chem. &amp; Eng. News, Vol. 77, No. 8, 1999, pp. 27-36. A microfluidic handling system utilizing microchannels is described in U.S. Pat. No. 6,193,647 to Beebe, et al. A variety of techniques have been used to pump, transport, position, and mix small liquid samples. Examples of such techniques include electro-osmotic flow, electrowetting, electrochemistry, and thermocapillary pumping. Surface properties, particularly surface wetting properties, have a significant effect on liquid behavior when very small volumes of liquid are manipulated. Such surface effect or capillary force is the basis of capillary pumping. Studies of structured surfaces consisting of either hydrophilic and hydrophobic stripes or patterned positive and negative surface charges show phenomena which can be exploited to control liquid motions in microfluidic devices. See, H. Gau, et al., “Liquid Morphologies on Structured Surfaces: From Microchannels to Microchips,” Science, Vol. 283, 1999, pp. 46-49; A. A. Darhuber, et al., “Morphology of Liquid Microstructures on Chemically Patterned Surfaces,” J. Appl. Phys., Vol. 87, 2000, pp. 7768-7775; A. D. Strook, et al., “Patterning Electro-Osmotic Flow with Patterned Surface Charge,” Phys. Rev. Lett., Vol. 84, 2000, pp. 3314-3317. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, flow of liquids is carried out on a microscale utilizing surface effects to control and direct the liquid. Liquid flow is guided by surface flow paths to maintain full laminar flow. The liquid stream can be supported and guided entirely by a flow path formed on facing surfaces such that the flowing stream is in contact only with the flow paths on the surfaces. Because the flowing stream is not in contact with other confining structure, such as sidewalls or the interior walls of channels or tubes, resistance to flow is minimized and laminar flow conditions may be maintained at higher flow rates. The surface tension of the flowing liquid provides vertical support for the stream of liquid, resulting in maximum exposure of the flowing stream to the atmosphere that surrounds it, thereby maximizing the surface area of gas-liquid interactions. Such microfluidically controlled streams may be utilized for applications such as chemical analysis, drug research, chromatography, cooling of electronic chips, flow sensors, air borne sample collection, and various medical applications including implantable drug dispensing systems and dialysis systems. 
     A microfluidic flow guiding structure for carrying out the invention includes a base having a surface and a cover with a surface facing the base surface. Adjacent facing regions on the base surface and cover surface define a flow path from a source position to a destination position on the base surface and cover surface, with at least a region on each of the base surface and cover surface being wettable by and having a wetting angle of less than 90° with respect to a selected liquid, the wettable region on at least one of the base surface and cover surface formed as a flow guiding strip and a region adjacent to the guiding stripe on the at least one of the base surface and cover surface being non-wettable by and having a wetting angle of greater than 90° with respect to the selected liquid. At such dimensions, a liquid injected onto the guiding stripe will be held by surface energy forces between the guiding stripe(s) on the base and cover surfaces and will flow along the stripe(s) from the source position to the destination position without flowing onto the non-wettable regions adjacent to the stripes. Further, the flow will remain laminar along the guiding stripes up to relatively high flow rates. The surface tension of the liquid itself creates a virtual wall that separates the liquid from the surrounding gas. Because of the smooth laminar flow of the liquid, there is no turbulence at the interface between the flowing liquid and the surrounding gas, and thus no intermixing of the liquid and gas occurs. The smooth interface between the gas and liquid phases along the flowing stream allows gas-liquid reactions to take place as a function of diffusion across the interface. Because the flowing stream is supported only at its bottom and top, the surface area of the flowing stream that is exposed to the ambient gas is maximized, and is much larger than the surface area per unit flow rate that can be obtained with liquid flowing through channels in contact with sidewalls or through channels formed in permeable membranes. Thus, chemical reactions between the gas and liquid can occur much more rapidly per unit volume by utilizing the present invention. 
     Two or more parallel guiding stripes that are separated by a non-wettable region may be formed on the base and cover surfaces to guide adjacent streams of liquid that flow together and that contact each other without mixing, but with diffusion allowed across the boundary between the two liquids. 
     The flow guiding stripes may be formed by various techniques, including lithography and the deposition of self-assembled monolayers by appropriately controlling flowing streams of liquid material, such as trichlorosilanes, which will form a non-wettable layer on the surface of the base. The flowing streams of material that deposit the monolayers may be guided by channels having a bottom wall and vertical walls that are formed in the base and closed by the cover surface, with the bottom wall of the channels defining the surface of the base on which the flow guiding stripes are formed. 
     Valves which control the flow of liquid on the flow guiding stripes may be formed in various manners, including as a barrier on the flow guiding stripe which blocks liquid flow below a selected pressure level; above that pressure level liquid on the guiding stripes will flow around the barrier. The valves may also be formed of materials that change dimension in response to characteristics of the liquid or that change over time from hydrophobic to hydrophilic or vice versa. Such materials include hydrogels that swell in response to certain conditions in the liquid to selectively block the flow on the guiding stripes. 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a simplified perspective view of a portion of a microfluidic flow guiding structure in accordance with the invention. 
     FIG. 2 is a top view of the base portion of the structure of FIG.  1 . 
     FIG. 3 is a cross-sectional view of the structure of FIG. 1 illustrating parallel flow streams on the structure. 
     FIG. 4 is a view as in FIG. 3 showing guided flow streams at a higher pressure. 
     FIG. 5 is a cross-sectional view similar to FIG. 3 showing flow streams guided adjacent to sidewalls of a channel in the structure. 
     FIG. 6 illustrates the formation of guiding stripes in a channel utilizing streams of a liquid material from which self-assembled monolayers are deposited, confined by a solvent for the material. 
     FIG. 7 is a view similar to FIG. 6 illustrating an alternative arrangement of supply of solvent and depositing liquid. 
     FIG. 8 is a view of the formation of multiple guiding paths utilizing adjacent flowing streams of solvent and liquid from which self-assembled monolayers are deposited. 
     FIG. 9 is a further view illustrating the deposit of multiple flow guiding paths with multiple streams of solvent and liquid material from which self-assembled monolayers are deposited. 
     FIG. 10 is a view showing the formation of flow guiding paths using parallel streams of solvent and liquid from which self-assembled monolayers are deposited with an arrangement of channels to provide a structure in which the directionality of the flow of liquid is controlled by the pressure of the liquid. 
     FIG. 11 illustrates flow of liquid on a flow path guided by guiding stripes on which is formed a barrier made of a hydrogel material that reacts with the flowing liquid. 
     FIG. 12 is a view as in FIG. 11 illustrating the swelling and expansion of the hydrogel barriers to a position at which the flow of liquid on the guiding stripes is blocked. 
     FIG. 13 is a view similar to FIG. 12 illustrating the flow of a liquid around the barrier after a selected pressure of liquid flow has been reached. 
     FIG. 14 illustrates the patterning of hydrophilic regions in microchannels utilizing photolithographic techniques. 
     FIG. 15 is a diagram illustrating the angle of curvature θ b  when a pressure is applied to an aqueous solution adjacent to an organic liquid. 
     FIG. 16 is a diagram illustrating the angle of curvature θ b  when a pressure is applied to an organic liquid adjacent to an aqueous liquid. 
     FIG. 17 is a schematic illustration of a surface patterned channel with contacting streams of aqueous and organic liquids. 
     FIG. 18 is a schematic illustration of a polymer membrane fabricated inside the channel of FIG. 17 by interfacial polymerization. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For purposes of illustrating the principles of the invention, a microfluidic flow guiding structure is shown generally at  20  in FIG.  1 . The flow guiding structure includes a base  21  having a surface  22  on which at least one flow path extends from a source position (generally referenced at  23  in FIG.  1 ), to a destination position (referenced for illustrative purposes at  24  in FIG.  1 ). A cover  25  extends over the base  21 . The base  21  and cover  25  can be made of a variety of solid materials, including plastics, glass, silicon or other semiconductors, photosensitive polymers, etc. Although a separate cover  25  is shown for illustration, the base and cover may be formed integrally, or of multiple parts, and of the same or different materials. A channel  26  is defined by the surface  22 , as the bottom wall of the channel, by two sidewalls  27  which extend upwardly from the bottom wall  22 , and by a cover surface  28  which is spaced from and faces the base bottom wall surface  27 . In accordance with the invention, the channel  26  may be formed by any suitable technique, including micromachining techniques used in the production of microelectromechanical systems (MEMS) and in conventional semiconductor processing. The microfluidic structures of the present invention are generally formed at dimensions at which conventional machining processes are either not feasible at all or do not provide adequately precisely defined structures. It is also understood that a straight channel  26  is shown in FIG. 1 for illustrative purposes only, and that the microfluidic structures may have multiple channels of a wide variety of path configurations. 
     For purposes of illustrating the invention, two flow guiding stripes  30  and  31  are formed on the base surface  22  extending from the source position  23  to the destination position  24 . The flow guiding stripes  30  and  31  are selected to be wettable by and have a wetting angle of less than 90° with respect to a selected liquid. The parallel flow guiding stripes  30  and  31  are separated by a region  32  which is non-wettable by and has a wetting angle of greater than 90° with respect to the selected liquid. For illustration, additional regions  35  and  36 , outwardly adjacent to the guiding stripe  30  and the guiding stripe  31 , respectively, are formed on the base surface  22  and also are non-wettable by the selected liquid. Similar wettable flow guiding stripes  37  and  38 , separated by a non-wettable region  39 , with the stripes  37  and  38  spaced from the walls  27  by non-wettable regions  40  and  41 , may be formed on the cover surface  28  adjacently spaced from and facing the stripes  30  and  31 , respectively. The stripes  37  and  38  may be narrower, wider, or of the same width as the stripes  30  and  31 . The present invention may be utilized with a flow guiding stripe or stripes formed on only one of the base or cover surfaces, with the other surface then being wettable with respect to the selected liquid. For illustration, tubes  41  and  42  are mounted to supply liquid to the space between the stripes  30  and  37  and the stripes  31  and  38 , respectively, at the source position  23 , and may comprise, for example, the syringes of a syringe pump of conventional construction. It is understood that this arrangement is for purposes of illustration only, and any supply technique or device may be used. For example only, the channel  26  may be closed at its ends, with liquid supplied to and removed from the channel through openings in the cover  25 , or in the base  21 , formed at the source position and destination position, respectively. FIG. 2 shows the layout of the regions  30 ,  31 ,  32 ,  35  and  36  on the base surface  28 . 
     FIG. 3 illustrates the flow of liquid streams  44  and  45  which are confined between the guiding stripes  30  and  37  and between the guiding stripes  31  and  38 , respectively. Where the materials of the base  21  and the cover  25  are wettable by the selected liquid which forms the streams  44  and  45 , the flow guiding stripes  30 ,  31 ,  37  and  38  may be formed as exposed regions of the material of the base and cover surfaces, whereas the adjacent regions  32 ,  35  and  36  on the base and the regions  39 ,  40  and  41  on the cover may be formed of a layer of material on the base and cover surfaces which is non-wettable by the selected liquid. As the pressure of the liquid in the flowing streams  44  and  45  is increased, the cross-section of the streams increases to the point where the streams expand and contact each other, as illustrated in FIG.  4 . Because the streams  44  and  45  are flowing in smooth laminar flow, without turbulence, the streams  44  and  45  can flow together without mixing with each other, but the contact between the streams will allow diffusion across the interface between the two streams. Such an effect may be exploited for various purposes. Further, as best illustrated in FIG. 3, because the streams  44  and  45  are in contact with a solid only at the guiding stripes  30 ,  31 ,  37  and  38 , the cross-sectional area to exposed surface area of the streams  44  and  45  is very high—much higher than can be obtained utilizing conventional gas-liquid interface techniques, such as liquid flowing in thin sheets on a wettable surface, liquid flowing confined within a channel, or liquid flowing within walls of a channel formed of a semi-permeable membrane. 
     FIG. 5 shows a cross-sectional view of a similar structure in which flow guiding stripes  48  and  49  are formed on the surface  22  of the base  21  (formed as a bottom wall of the channel  26 ) which are separated by a region  50  which is formed to be non-wettable by the selected liquid. The flow guiding stripes  48  and  49  and sidewalls  27  of the channel  26  are wettable by the selected liquid, so that the streams of flowing liquid  52  and  53  remain in contact with both the sidewalls  27  and the bottom wall  22  at the guiding stripes  48  and  49 . Wettable guiding stripes  54  and  55 , separated by a non-wettable region  56 , are formed on the facing surface  28  of the cover  25 , and guide the streams  52  and  53  at the top of the streams. The flow of the liquid streams  52  and  53  under sufficient pressure can expand to provide contact between the two flowing streams  52  and  53  at an interface as illustrated in FIG. 5, allowing diffusion across the interface between the two streams. 
     The selected liquid which is guided by adjacent, facing wettable stripes in accordance with the invention may be essentially any flowable liquid, including organic as well as inorganic liquids. For example only, the selected liquid may be water or water based liquid solutions. For purposes of simplified terminology, the formation of the guiding stripes and the microfluidic structures in accordance with the invention is described below referring to hydrophobic and hydrophilic surface regions, it being understood that the microfluidic structures of the invention may be formed with materials that have the appropriate surface wetting characteristics with respect to any selected liquid. 
     The maximum height of the channels under which self-supported liquid streams will be maintained will vary with the characteristics of the flowing liquid and the wettability of the liquid with respect to the flow guiding stripes. Generally, channel heights of 1000 μm or less are preferred. Exemplary guiding stripe widths may be in the range of 500 μm, but may be smaller or larger. 
     Patterning hydrophobic and hydrophilic regions inside microchannels usually requires modifying surface properties in selected areas of a substrate first, followed by aligning and bonding of substrates to form microchannel networks. Such processes, consisting of a series of steps, are complicated and time-consuming. In accordance with the invention, one preferred approach to patterning surface free energies inside channel networks is to combine multiphase liquid laminar flows and self-assembled monolayer (SAM) chemistry. The whole process can typically be carried out within several minutes. Microfluidic flow inside sufficiently small channels is laminar, that is, multiphase liquid streams flow side by side without turbulent mixing, with diffusion across the interface being the only means for mixing of components between two neighboring streams. Multiphase liquid laminar flows have previously been explored to design diffusion-based extractors to fabricate microstructures, and to pattern cells and their environments inside preformed capillaries. See, Kovacs, G. T. A.  Micromachined Transducer Sourcebook,  Boston: McGraw-Hill, 1998; Weigl, B. H., et al.,  Science  1999, 283, 346-347; Kenis, P. J. A., et al. “Microfabrication inside Capillaries using Multiphase Laminar Flow Patterning,”  Science  1999, 285, 83-85; Takayama, S., et al.,  Proc. Natl. Acad. Sci. USA  1999, 96, 5545-5548. SAM is a simple method to modify surface wetting properties of a variety of materials, including silicate substrates, metals and polymers, and it has been extensively studied in terms of mechanisms and applications in the past two decades. See Ulman, A.  An Introduction to Ultrathin Organic Films , Academic Press: Boston, 1991; Ulman, A., “Formation and Structure of Self-Assembled Monolayers”  Chem. Rev.  1996, 96, 1533-1554. By controlling the water content in the solvent, SAMs of trichlorosilanes can be formed in a short period of time, e.g., several minutes or less. In the following examples, hexadecane (Aldrich, 99%) was used as a solvent and trichlorosilanes were used to form SAMs on silicate substrates. It was found from contact angle measurement that SAMs with full coverages were formed on glass substrates in less than two minutes. 
     The channels used were made from glass substrates and glass cover slips as described in Beebe, D. J., et al.,  Nature  2000, 404, 588. When hexadecane and the solution of octadecyltrichlorosilane (OTS) in hexadecane (0.5 w/v %) were brought together in channels by syringe pumps and the laminar flow was maintained for two to three minutes, SAM was formed in the area that trichlorosilane solution flowed through, with the other areas remaining hydrophilic (water advancing contact angle θ a ≈0°). Hexadecane and the solution of trichlorosilane in hexadecane (0.5 w/v %) were pumped into channels by two syringe pumps (Harvard Apparatus PHD 2000 Programmable). Syringes were connected to pipet tips fixed to channels by a polydimethylsiloxane (PDMS) tubing. Hexadecane was always brought into channels first and stopped last. The flow rates of solvent and solution were usually the same, either 1 ml/min or 2 ml/min; the flow time was two to three minutes. The channels were then cleaned by sequentially flushing with 10 ml hexane and 10 ml methanol followed by drying with a stream of clean nitrogen. Since the reactivity of trichlorosilane is high, solvent was always introduced into the channel first and stopped last to eliminate the formation of SAM in unwanted areas. The channels were then cleaned by sequentially flushing 10 ml of hexane and 10 ml of methanol followed by drying with a stream of clean air or nitrogen. 
     A series of hydrophobic and hydrophilic patterns were designed and are shown in FIGS. 6-9. Guiding stripes  56  were formed in a structure as discussed above having a main channel  57  and one or more side channels  58 . As expected, aqueous solutions flowed only on designed hydrophilic pathways on the guiding stripes under a pressure below a critical value, and the flow patterns were exactly the same as the solvent in laminar flows. The liquid will gradually retreat back to hydrophilic regions if it accidentally moves into the hydrophobic regions. Using these designs, liquids can be transported from one reservoir (source) to a designated container (destination) without going to other areas, as shown in FIG. 7, and two streams can be brought from two inlets into the channel and transported to a designated reservoir for mixing or reaction as shown in FIGS. 6,  8  and  9 . There are no physical walls on the sides of the liquid stream; the walls of the streams themselves, which are maintained by surface tension, may be considered “virtual walls.” Laser confocal microscopy was used to examine the vertical shape of virtual walls of a Rodamine B dilute aqueous solution, and it was found that the walls were convex and that the curvature increased with increasing pressure, as illustrated in FIGS. 3 and 4. When the pressure was above a critical value, the solution was no longer confined on hydrophilic pathways  56  and crossed the boundary between hydrophobic and hydrophilic regions. The essential requirement for water crossing the boundary is that the water wetting angle θ a  of virtual walls is larger than that on the hydrophobic region. The θ a  of water on SAM of OTS is 112°. When the water surface is curved, the pressures across the surface are different due to surface free energy; the pressure is always greater on the concave side than the pressure on the convex side. This is described by the Young-Laplace equation, ΔP=γ(1/R 1 +1/R 2 ), where ΔP is the pressure difference, γ is the surface free energy of aqueous solution, and R 1  and R 2  are radii of curvature in vertical and parallel directions relative to channel planes. Since the liquid surface at a certain height in the parallel direction is flat, R 2  is indefinite and the equation is simplified as follows: ΔP=γ/R 1 . Based on the essential condition for water crossing the boundary, R 1  can be calculated and expressed by the equation, R 1  sin(θ a −90°)=h/2, where h is the channel depth. Therefore, the maximum pressure that virtual walls can stand is expressed by the following equation: P max =ΔP=2γ sin(θ a −90°)/h. From this analysis, the maximum pressure is determined by the surface free energy of the aqueous solution γ, the θ a  of the aqueous solution on the hydrophobic region, and the channel depth h. This analysis also indicates that virtual walls cannot stand any pressure if the θ a  of the liquid on the hydrophobic region is smaller than 90°. This prediction was confirmed by use of bromoundecyltrichlorosilane instead of OTS to modify surface wetting property; the water wetting angle θ a  on SAM of bromoundecyltrichlorosilane is 83°. Carefully adding deionized (DI) water into the channel initially resulted in water being confined on hydrophilic regions; however, a little increase in pressure pushed water to cross the boundary and destroy the virtual walls. For example, for a channel depth of approximately 180 μm, a surface free energy of DI water of 72.1 mN/m, and θ a  of DI water on the OTS monolayer of 112°, calculations show that the maximum pressure that virtual walls can stand is 300.09 N/m 2 , corresponding to a pressure of 30.6 mm water. For SAM of heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane (HFTS), the θ a  of DI water is 118°, and the maximum pressure is 376.09 N/m 2  (38.4 mm water). The flow pattern design in FIG. 7 was used to measure the maximum pressures that the virtual walls of a DI water stream can stand. Although the measurement was complicated by kinetic issues and high humidity inside the channels, it was observed that bulges developed at a pressure of 31 mm water for OTS patterned channels and 37 mm water for HFTS patterned channels, which were close to theoretical results. A pipet tip (United Laboratory Plastics, 201-1000 μl pipet tips) with inner diameters of 8 mm at bottom and 6 mm at top was fixed onto channels, and the maximum pressure that the virtual walls can stand was measured by gradually adding deionized water into the pipet and measuring the water height when a bulge developed. 
     Based on the maximum pressure differences for OTS monolayer and HFTS monolayer, simple pressure-sensitive virtual valves may be fabricated to manipulate liquid flow direction inside channel networks  57  and  58 , as shown in FIG.  10 . In this design, the central part  56  is hydrophilic and the other two parts are modified with SAM of OTS and SAM of HFTS. Therefore, the maximum pressures that the two virtual walls of the two liquid streams can stand are different. At a low pressure, for example, 10 mm water, the Rodamine B dilute solution flowed through the central hydrophilic region from A to B. At a medium pressure P OTS &lt;P&lt;P HFTS  (e.g., 26 mm water) the virtual wall between the hydrophilic region and the OTS monolayer region was destroyed, leading to water flowing from A to B, C, and D. The surface free energy of Rhodamine B dilute solution is lower than that of deionized water, resulting in a lower maximum pressure. At a higher pressure P&gt;P HFTS , aqueous solution flowed through all channels from A to B, C, D, E, and F. Since the wetting angle θ a  of liquids on hydrophobic regions can be systematically adjusted by use of a mix of SAMS of two or more different trichlorosilanes, and different liquids have different wetting angle θ a  on one SAM, the liquid flow direction can be changed at any desired pressure. Such virtual valves may thus be used in microfluidic systems and microreactors. 
     An advantage of the present invention is that the virtual walls provide a large gas-liquid interface area, which enables gas-liquid reactions in microchips. The following is an example of a simple gas-liquid reaction. The aqueous solution confined in hydrophilic pathways was a pH=6.44 phosphate buffer solution containing acid-base indicator methyl red sodium salt. When acetic acid gas was carried into the channels by nitrogen, acetic acid diffused through the interface and reacted with buffer, and the color of the solution changed from yellow to pink. Under the experimental conditions the diffusion was rapid, which was revealed by the moving velocity of the pink color frontline. Gas-liquid and gas-solid reactions are a key component in biological and chemical reagents detectors. The virtual walls of the flow streams controlled in accordance with the invention thus offer a high sensitivity to chemical reagents. 
     By taking advantage of the large surface area of the virtual walls, functions can be achieved on microchips which are difficult by other methods. For example, concentrating samples in microfluidic systems is nontrivial; several methods have been reported in the literature but are not straightforward. The virtual walls of flowing streams in accordance with the invention provide a method to concentrate liquids on microchips, bring dry air or nitrogen through two inlets in the channel design as shown in FIG. 7, and pump solution slowly through hydrophilic pathway to concentrate solutions. The function of lungs in microfluidic systems can be mimicked for exchanging components between a liquid phase and a gas phase. In combination with stimuli-responsive hydrogels, more complex functions may be realized in microfluidic system; for example, a valve as shown in FIGS. 11-13. Two hydrogels barriers  60  and  61  were fabricated in the boundaries of hydrophilic and hydrophobic regions  64  and  65 , respectively, defining facing guiding stripes in a channel  67  on a base and a cover. When the solution is acidic, the gels allowed the solution to flow, as shown in FIG.  11 . When the solution was basic, the part of the gels in the hydrophilic regions swelled and blocked flow, as shown in FIG.  12 . Once the gels were fully swollen and the gel surfaces in hydrophobic regions became hydrophilic, the solution flowed around the gels, as shown in FIG.  13 . The time that the gels blocked flow is dependent on the pH value of the solution, the gel compositions, and the ratio of gels in hydrophilic and hydrophobic regions. 
     Pattern formation by multiphase liquid laminar flow as discussed above requires preformed channels, limiting the applicability of this technique to certain potential device applications. An alternative technique is the use of photolithography to pattern surface free energies inside microchannels. A photopatternable material is applied to the bottom wall of a channel in a base and is then photolithographically patterned to form wettable and non-wettable regions to define a guiding stripe or stripes. Similar photopatterning can be carried out on the cover. As an example, the photochemistry of the 2-nitrobenzyl group (see, e.g., D. H. Rich, et al., J. Am. Chem. Soc., Vol. 97, 1975, pp.1575, et seq.; V. N. R. Pillai, Synthesis, 1980) can be used to synthesize photocleanable SAMs, as illustrated in FIG.  14 . Ultraviolet (or other wavelength) radiation passed through a masks  90  placed on top of a photopolymer, e.g., SAM-modified, channel bottom walls  91  results in the production of hydrophilic carboxyl in the irradiated regions  92 , with the non-irradiated regions  93  remaining hydrophobic. Aqueous solutions will tend to be confined to the irradiated regions  92 . Other photopatternable chemistry may also be utilized. Such photopatterning provides flexibility in the design and generation of complex flow patterns and facilitates mass manufacturing of surface directed flow devices. A matching pattern may be formed by the same photochemistry techniques on the facing surface of the cover, which is then mounted onto the base in proper position so that the flow guiding stripes on the cover and base are adjacent to and face each other. The microchannels were cleaned by sequentially flushing with 10 ml hexane and 10 ml methanol after monolayer deposition from a 0.5 w/w % solution of the corresponding trichorosilane in hexadecane and then dried with a stream of nitrogen. A photomask was placed on top of the SAM-coated channel filled with pH=11.77 NaOH solution. The UV light source was an Olympus Epi-Fluorescent Microscope (BX-60) passed through a near UV filter cube (U-MNUA, type BP 360-370 nm) with 360-370 nm band pass. A 2× magnification lens was used and the irradiation time was 90 minutes. After irradiation, the channel was rinsed with 10 ml of methanol and then dried with a stream of clean air. A dilute Rhodamine B aqueous solution was added into the channels by a syringe and a pressure was applied to push the solution slowly through the hydrophobic region. When the aqueous solution reached the edge of the irradiated area, it wetted the hydrophilic region spontaneously and formed patterns identical to the photomask. 
     Liquids of the same or similar nature such as aqueous-aqueous solutions or hexadecane-hexadecane (HD) solutions exhibit stable laminar flow inside microchannels. This liquid behavior has been successfully used in microfluidic diffusion-based separation and detection, fabrication of various microstructures, and patterning surface free energies inside microchannels. Few studies have been reported on the flow behavior of immiscible liquids at the microscale. Owing to the different viscosities, densities, and interfacial free energies between such liquids, the flow behavior of immiscible liquids were observed to be different from liquids of the same or similar natures. Control of the flow is nontrivial, and separation of immiscible liquids is difficult once they are in contact. The present invention may be utilized to control the flow of immiscible liquids in microchannels by patterning surface free energies, and can be applied to the fabrication of a semipermeable membrane. An example of the invention carried out in this manner is given below. 
     Channels were made from “Piranha”-treated glass slides and cover slips, and were coated with a photocleavable SAM of 2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluoro-1-octyl 4-(11-trichlorosilyl-1-oxoundecyloxymethyl)-3-nitrobenzoate (F-SAM). A photomask and a UV light were employed to pattern the surface free energies. Upon exposure to UV irradiation, the o-nitrobenzyl-oxygen bond in F-SAM is cleaved, and the surface becomes hydrophilic. Aqueous solutions introduced to the surface-patterned channel flow only along the hydrophilic pathway when the pressure is maintained below a critical value. However, organic liquids such as HD are not confined to either the hydrophilic or hydrophobic regions. This is consistent with the fact that the advancing contact angles of these organic liquids in air (θ org/air ) on both hydrophobic and hydrophilic regions are smaller than 90°. By first introducing an aqueous solution in the hydrophilic region, HD and some other organic liquids subsequently introduced are confined to the hydrophobic region while aqueous solutions are confined to the hydrophilic region, provided the pressures are maintained below critical values. Since organic liquids are not confined to the hydrophobic region without an aqueous solution first filling the hydrophilic region, the organic liquids can be considered as being confined by liquid walls. 
     The interface of H 2 O (or an aqueous solution) and organic liquid is pinned precisely at the boundary between the hydrophilic and hydrophobic surface patterns. If a pressure is applied to H 2 O, the interface will curve toward the organic phase as shown in FIG. 15, and vice versa if a pressure is applied to the organic liquid as shown in FIG.  16 . Critical conditions for liquid wall rupture will occur when the angle of curvature, θ b , equals the advancing contact angle of H 2 O on the hydrophobic surface covered in the organic liquid (θ water/org ) (if the pressure is applied to H 2 O) or the advancing contact angle of the organic liquid on the hydrophilic surface covered in water (θ org/water ) (if the pressure is applied to the organic phase). When a liquid-liquid interface is curved, there is a pressure drop across the interface. This is described by the Young-Laplace equation, ΔP=γ(1/R 1 +1/R 2 ), where ΔP is the pressure difference, γ is the liquid-liquid interfacial tension, R 1  and R 2  are the radii of curvature in directions vertical and parallel to the liquid stream. For a straight stream, the equation is simplified to ΔP=γ/R 1  and since R 1  can be expressed by the equation, R 1 =h/[2 sin(θ b −90°)], where h is the channel depth (˜180 μm), the maximum pressures that liquid walls can sustain in a straight stream are P water/org =(2γ/h)sin(θ water/org −90°) above which H 2 O flows into the hydrophobic region and P org/water =(2γ/h)sin(θ org/water −90°) above which the organic liquid flows into the hydrophilic region. Obviously, θ water/org  and θ org/water  must be greater than 90° to confine H 2 O in the hydrophilic region and organic liquids in the hydrophobic region. We have measured θ water/org  on a F-SAM and θ org/water  on a UV-irradiated F-SAM. The experimentally determined maximum pressures that liquid walls can withstand are in good agreement with the calculated values by use of the independently measured values of θ water/org  and θ org/water  and the liquid-liquid interfacial tensions. 
     In comparison to multistream laminar flow, the ability to confine organic liquids to hydrophobic regions and aqueous solutions to hydrophilic regions makes it practical in accordance with the invention to manipulate immiscible liquids inside microchannels. For example, one liquid can remain static while the other liquid is flowing and the boundary remains constant. Moreover, two immiscible liquids can flow in the same direction (concurrent flow) or in opposite directions (countercurrent flow) while maintaining a stationary boundary. In contrast to laminar flow, the width ratio of the two streams is not determined by the relative flow velocities but rather by the width ratio of hydrophobic and hydrophilic pathways provided the pressure is subcritical. Countercurrent flow is widely adopted in nature for efficient mass transfer such as gas exchange between air and blood in bird lungs and between water and blood in fish gills. 
     In the present invention, liquid walls may be utilized to conduct an interfacial polymerization in a surface-patterned channel to fabricate a semipermeable membrane, as illustrated by the following example. An F-SAM coated channel was photopatterned as illustrated in FIG. 17 to provide a hydrophillic surface region  100  and a hydrophobic surface region  102 , with a boundary  103  at which the two regions meet. An aqueous solution containing hexamethylenediamine (62.5 mM) was brought into the hydrophilic region  100  followed by introduction of a solution of adipoyl chloride in xylenes (46.9 mM) into the hydrophobic region  102 . Interfacial polymerization occurred immediately when the two phases made contact at the boundary  103 , producing a polymer film at the hydrophilic-hydrophobic boundary as shown at  104  in FIG.  18 . The polymerization proceeded at room temperature for 8 min at which point the organic solution was flushed out of the channel with xylenes and the aqueous solution was flushed out of the channel with methanol. Both sides of the membrane were then rinsed with 10 mL methanol, and dried with nitrogen. Membrane permeability was studied on an Olympus fluorescent microscope BX 60 using an aqueous suspension of 0.2 μm fluorescent microspheres. The suspension was injected into the hydrophilic region and was retained by the membrane under ambient conditions. When a pressure was applied to the suspension, water gradually passed through the membrane while microspheres remained behind and became concentrated in the vicinity of the membrane. This indicated that the membrane&#39;s pore size is below 200 nm. 
     The following are the procedures utilized in the example given above. 
     1. A photomask was placed on top of an F-SAM coated channel filled with 0.1 M HCl, and a cover slip was placed on top of photomask to ensure it flat. The UV intensity of Novacure systems (EFOS, Model N2001-A1) was preset at 5600 mW/cm 2 , the distance between the lens and the channel was 40 mm, and the irradiation area was 20 mm in diameter. The irradiation time was 6 min. The introductory pathways on the two ends were also irradiated such that aqueous solutions can flow in spontaneously. The channel was sequentially flushed with 10 mL deionized water and 10 mL methanol, and then dried with a stream of clean air. In a watch glass was placed a piece of F-SAM-coated cover slip. 0.1 M HCl solution was filled into the space between the cover glass and the watch glass. After irradiation under the same conditions as for channels, the cover glass was rinsed with methanol, acetone, and methanol, and then dried with a stream of air. Contact angle measurement showed that the advancing contact angle of H 2 O in air (θ water/air ) on the UV-irradiated F-SAM decreased from 118° to 67°. The advancing contact angles of hexadecane on F-SAM and the UV-irradiated F-SAM are 74° and 18°, respectively. 
     2. Pipette tips with inner diameters of 8 mm at the bottom and 6 mm at the top were fixed onto a hydrophilic outlet and a hydrophobic outlet. The maximum pressure P water/org  was determined by gradually adding deionized water into the tip fixed to the hydrophilic region and measuring water height when the interface started moving. P org/water  was determined by gradually adding organic liquid into the tip fixed to hydrophobic region and measuring organic liquid height when the interface started moving. 
     3. Advancing contact angles were measured by use of a Rame Hart NRL contact angle goniometer (Model 100-00) with a microsyringe attachment. For measurement of θ water/org , A F-SAM coated cover glass was immersed in a 5 mm deep organic liquid, a water drop was brought into contact with F-SAM, and the advancing value was determined by adding water into the drop. For θ org/water , a UV-irradiated F-SAM coated cover glass was immersed in water, an organic liquid drop was brought into contact with the surface by microsyringe, and advancing value was determined by adding liquid into the drop. 
     4. Table 1 provides a comparison of the calculated maximum pressures and experimental results. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Comparison of the calculated maximum pressures and experimental results* 
               
             
          
           
               
                   
                   
                   
                 γ 
                 P water/org   C   
                 P org/water   C   
                 P water/org   E   
                 P org/water   E   
               
               
                   
                 θ water/org   
                 θ org/water   
                 (mN/m) 
                 (N/m 2 ) 
                 (N/m 2 ) 
                 (N/m 2 ) 
                 (N/m 2 ) 
               
               
                   
                   
               
             
          
           
               
                 Hexadecane 
                 143 ± 3 0   
                 123 ± 2 0   
                 52.5 1   
                 466 
                 318 
                 507 
                 326 
               
               
                 Cyclohexane 
                 149 ± 3 0   
                 123 ± 2 0   
                 50.2 2   
                 478 
                 304 
                 461 
                 267 
               
               
                 Toluene 
                 146 ± 3 0   
                 104 ± 2 0   
                 37.5 2   
                 345 
                 101 
                 294 
                 127 
               
               
                 Xylenes 
                 150 ± 4 0   
                 110 ± 1 0   
                 37.5 3   
                 361 
                 143 
                 304 
                 177 
               
               
                 Chloroform 
                 149 ± 2 0   
                  86 ± 2 0   
                 32.0 1   
                 305 
                 0 
                   
                 0 †   
               
               
                   
               
               
                 *P water/org   C , P org/water   C : the calculated maximum pressures by use of the independently determined values of θ water/org  and θ org/water  and the liquid-liquid interfacial tensions. P water/org   E , P org/water   E : the experimental results.  
               
               
                   † Chloroform was found not to be confined to the hydrophobic region  
               
             
          
         
       
     
     5. The aqueous suspension of 0.2 μm carboxylate-modified microspheres (yellow-green fluorescent, 505/515) was purchased from Molecular Probes, and was diluted with 2×volume of deionized water before used for permeability study. The fluorescent image was taken by use of WIBA cube and a 365 nm UV light. 
     It is understood that the invention is not confined to the particular embodiments disclosed herein as illustrative, but embraces all such forms thereof as come within the scope of the following claims.