METHOD FOR RAPID AND PRECISE MANIPULATION OF A TINY VOLUME OF LIQUID DROPLETS

Abstract

An apparatus and method are provided for rapid and precise manipulation and transfer of tiny liquid droplets. by dynamically introducing microstructures with relatively high surface energy to a non-wettable surface, which surface has in-situ switchable adhesion to liquid droplets. By penetrating microstructures on the background surface, the chemical property of the surface is locally modified. Capillary bridges will form between microstructures and liquid droplets which lead to high adhesive forces. When the microstructures are retracted, the capillary bridges either pinch-off or recede, which drastically reduces the adhesion. With proper chemical modification, the surface can either manipulate a liquid droplet in air or in an immiscible carrier liquid. Tiny droplets with volumes down to nanoliter scale can be prepared and dispensed by using the surface.

Claims

1. Apparatus for the manipulation of micro/nanoliter water droplets, comprising: a syringe; a polyester mesh forming a superhydrophobic surface located at a distal end of the syringe; a plunger slidably located within the syringe behind or proximally of the mesh; at least one fiber located on the distal side of the plunger and being movable with the plunger, said at least one fiber being positioned so that it aligns with an open space or pore in the mesh, and whereby in a first position of the plunger the end of the at least one fibers is behind the mesh and in a second position of the plunger the end of the at least one fiber penetrates through the mesh and beyond its superhydrophobic surface.

2. The apparatus of claim 1 wherein the mesh comprises: a weave of between about 40 and 80 μm thick threads having an inter-thread distance of between 150 and 200 μm; a coating on the weave of a solution containing a mixture of polydimethylsiloxane (with crosslinker), graphene nanoplatelets, diethyl ether, and ethanol that has been cured at 60 to 100° C. for 1½ to 2½ hours; and a cladding layer of graphene nanoplatelets and PDMS composites with micro- and nanoscale surface roughness.

3. The apparatus of claim 1 wherein the mesh is fixed to the syringe by adhesives.

4. The apparatus of claim 1 wherein there are a plurality of fibers forming a fiber bundle, which fibers are optical fibers with peeled tips that expose an inner quartz fiber.

5. The apparatus of claim 4 wherein there are six 125 μm fibers forming a fiber bundle.

6. The apparatus of claim 4 wherein the fibers are one of intrinsic hydrophilic and octadecyltrichlorosilane (ODS) modified hydrophobic ones.

7. The apparatus of claim 1 wherein there are a plurality of fibers forming a fiber bundle, each fiber is arranged on the plunger to be aligned with openings or pores in the mesh so that the fiber-to-fiber distance is about the same as the mesh pore-to-pore distance, which facilitates alignment between the fiber tips and mesh pores; and wherein the distal ends of each of the fibers is bound by a metal tube where they contact the plunger.

8. The apparatus of claim 7 wherein there are four fibers.

9. The apparatus of claim 1 wherein there are a plurality of fibers forming a fiber bundle, each fiber is arranged on the plunger to be aligned with openings or pores in the mesh so that the fiber-to-fiber distance is about the same as the mesh pore-to-pore distance, which facilitates alignment between the fiber tips and mesh pores; and wherein the fibers are made of Dupont Tynex bristles that form a protruded fiber array when the plunger is in the second position so that the bristles penetrate through the mesh and beyond its superhydrophobic surface.

10. The apparatus of claim 1 wherein there are a plurality of fibers in the form of hydrophilic micro-fibers; wherein the micro-fibers form a fiber bundle and a liquid column is formed by being trapped among the micro-fibers; and wherein by withdrawing the micro-fibers, the liquid column is repelled out and a tiny droplet is formed.

11. A method for rapidly and precisely manipulating tiny volumes of liquid droplets, comprising the steps of: providing a movable microstructure with a background surface having a switchable wettability to non-wettability contrast; moving the microstructure to the location of a droplet; switching on the wettability to attract the droplet to the microstructure; moving the microstructure with the attached droplet to a different location; and switching off the wettability in order to deposit the droplet at the different location.

12. The method of claim 11 wherein the strength of attraction of the droplet to the microstructure is tuned by controlling number and surface chemistry of microstructures.

13. The method of claim 11 wherein the microstructure is a superhydrophobic mesh with pores and quartz fibers that can be moved behind the mesh to make the microstructure non-wettable and through and in front of the mesh to make the microstructure wettable.

14. The method of claim 11 wherein the manipulation is in air or an immiscible carrier liquid.

15. The method of claim 14, wherein the manipulation is under water and the mesh is modified to be superoleophobic under water, wherein the mesh is prepared by the following steps: dip coating the mesh in poly(ethyleneimine) (PEI) (0.1 g/mL) solution; allowing evaporation of the PEI solution; dip coating the mesh in sodium alginate solution (0.02 g/mL) and then immersing it in CaCl.sub.2 solution (1 M) for 10 min; and after treatment, covering the mesh with a layer of calcium alginate, which leads to underwater superoleophobicity.

16. A method of making a mesh for the apparatus of claim 1, comprising the steps of: weaving a polyester mesh with 40-80 μm thick threads having an inter-thread distance of 150-250 μm to form a background surface; dipping the background surface in a solution containing a mixture of 0.5 g polydimethylsiloxane (with 10% crosslinker), 0.5 g graphene nanoplatelets, 8 mL diethyl ether, and 7 mL ethanol to make the mesh superhydrophobic; and curing the mesh at 80° C. for 2 hours so as to form a cladding layer of graphene nanoplatelets and PDMS composites with micro- and nanoscale surface roughness on the cured mesh.

17. A method of making the hydrophobic fiber bundles for the apparatus of claim 1, comprising the steps of: immersing quartz fiber tips in 10 mM ODS toluene solution for 15 min; and curing the fibers in a furnace at 100° C. for 30 min.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The foregoing and other objects and advantages of the present invention will become more apparent when considered in connection with the following detailed description and appended drawings in which like designations denote like elements in the various views, and wherein:

[0018] FIG. 1A is an illustration of a device for practicing the method of the present invention, FIG. 1B is a microscope image of an assembly of micro-fibers with background mesh according to the present invention, FIGS. 1C and 1D are scanning electron microscope images of treated superhydrophobic mesh surface at a 200 micron level and a 700 nanometer level, respectively, FIG. 1E is a microscope image of a single micro-fiber according to the invention, and FIG. 1F is a microscope image of a bundle of micro-fibers according to the invention.

[0019] FIG. 2A is a plot of adhesive force against the number of micro-fibers with different surface chemistry, FIG. 2B is a microscope image of capillary bridges formed between micro-fibers and water droplets, FIG. 2C is a plot of liquid capacity against the number of micro-fibers with different surface chemistry, and FIG. 2D is a plot of the volume of the droplets transferred by the surface against the volume of the droplets deposited directly from a micropipette, all with respect to the present invention.

[0020] FIG. 3A is a series of photographs showing the capturing and releasing of a 5 μL water droplet, FIG. 3B is a series of photographs showing the preparation of a 0.5 μL water droplet from a commercial quantitative micropipette, and FIG. 3C is a series of photographs showing the capturing and releasing of a 5 μL oil droplet under water from a surface according to the present invention.

[0021] FIG. 4A is a plot of adhesive force and sliding angle against cycles of extending and withdrawing, and FIG. 4B is a plot of contact angles against pH values of aqueous droplets.

[0022] FIGS. 5A-5C are a series of photographs showing manipulation processes (capture, merging and release) utilizing the surface for micro-reactor of the present invention.

[0023] FIG. 6A is an image of a liquid column trapped between hydrophilic fibers, FIG. 6B is a plot of the volume of the nanoliter droplets against the protruding length for surfaces with varied numbers of hydrophilic fibers, and FIG. 6C is a series of photographs showing the preparation and deposition of nanoliter droplets by using the present invention.

[0024] FIGS. 7A-7E are a series of photographs showing the manipulation of multiple droplets using the expanded surface.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

[0025] The assembly of the surface is illustrated in FIGS. 1A, 1B and 1E. A piece of polyester mesh 10, which has been weaved with 60 μm thick threads having an inter-thread distance of 200 μm serves as the background surface. The mesh is modified to be superhydrophobic by dip coating in a solution containing a mixture of 0.5 g polydimethylsiloxane (with 10% crosslinker), 0.5 g graphene nanoplatelets, 8 mL diethyl ether, and 7 mL ethanol. After curing at 80° C. for 2 hr, the mesh is cladded by a layer of graphene nanoplatelets and PDMS composites with micro- and nanoscale surface roughness. High surface asperities and low surface energy of the composite modify the mesh to be superhydrophobic with a water contact angle of about 151°. Then, the superhydrophobic mesh is fixed on top of a syringe 12 by commercial adhesives.

[0026] Small bundles 14 of 9/125 μm optical fibers (e.g., six fibers per bundle) with peeled tips are utilized as movable microstructures with contrast wettability. Inner quartz fibers 11 are exposed on fiber tips by peeling the protective jacket 17 with a fiber stripper. Then a specific number of optical fibers are tightly bound by a metal tube 15. The quartz fiber tips are spaced by 60 μm thick protective jacket 17 on the lower part of the optical fibers. Therefore, the fiber to fiber distance is about the same as the mesh pore to pore distance, which facilitates the alignment between fiber tips and mesh pores. The optical fiber bundle is then fixed on top of a plunger 16 of the syringe by using commercial adhesives. By extending or withdrawing the plunger, quartz fibers can either penetrate or retract from the superhydrophobic mesh 10. While 9/125 μm optical fibers are preferred, 50/125 and 62.6/125 and others may also be used.

[0027] An optical micrograph showing the structure of the mesh surface with 4 penetrating fibers is shown in FIG. 1B. FIG. 1C shows a scanning electron micrograph of a typical coated polyester mesh where the spacing is about 200 μm. FIG. 1D is a magnified scanning electron micrograph at a 700 nm level showing nanoscale roughness of the coating.

[0028] FIG. 1A exhibits two contrastive states of water droplet on such surface. With the quartz fiber array penetrating the mesh as in the right side view, the mesh surface has local high adhesion; water droplets will be firmly pinned by quartz fibers and cannot slide even when the surface is inverted as shown at the bottom of the right view. By withdrawing the quartz fiber bundles 14, droplets are only in contact with the superhydrophobic mesh 10 as shown in the left view of FIG. 1A. In this position an air cushion below the Cassie-state droplet leads to high mobility with a sliding angle as low as 4.9° as illustrated at the bottom of the left view in FIG. 1A. The switching of states of the water droplet on the surface is in-situ and rapid, and is controlled by the movement of the fiber bundles 14 due to movement of the plunger 16.

[0029] The adhesion of the adhesive state of the mesh surface has been measured. Two kinds of quartz fibers, intrinsic hydrophilic ones and octadecyltrichlorosilane (ODS) modified hydrophobic ones, were used in the surface measurement, respectively. The ODS treatment process is conducted by immersing quartz fiber tips in 10 mM ODS toluene solution for 15 min and followed by curing in a furnace at 100° C. for 30 min. By referring to FIG. 2A it can be seen that for both kinds of fibers, adhesive forces grow with an increasing number of fibers. Without exposed fibers, the superhydrophobic mesh exhibits adhesion of about 8.9 μN. Such a low adhesion allows easy detachment of droplets from the surface. With 6 fibers penetrating the mesh, adhesive forces of the surface for intrinsic and ODS modified fibers rise drastically to 100.2 μN and 72.3 μN, respectively. The high adhesion of the surface can firmly hold a tiny water droplet even when the surface is inverted.

[0030] When the water droplet contacts the exposed fiber a capillary bridge is formed. The high adhesion is the resultant force of interaction between surface tension around the base of the capillary bridge and the force generated by Laplace pressure, i.e., the pressure difference between the inside and the outside of the curved surface that forms the boundary between air and the liquid. When hydrophilic fibers are used, water wets and wraps the fiber array, which forms a liquid column. The base periphery of the capillary bridge is defined by the liquid column. By considering the interaction between surface tension and the force of Laplace pressure, the maximum adhesion is as following:

F.sub.hydrophilic=γds−ΔP.Math.A

where s is circumference of the liquid column at base of the capillary bridge. γ is surface tension of water. ΔP is the Laplace pressure, which is roughly calculated by the contour of the stretched droplet. A is the cross-section area of base of the capillary bridge. The measured values are in good agreement with the proposed model.

[0031] By using hydrophobic ODS treated fibers, only a small capillary bridge forms at the top facet of each fiber. During stretching of the droplet, according to Gibbs criterion, the contact line of the capillary will recede on fiber facet only when the receding contact angle θ.sub.r is reached. The receding contact angle θ.sub.r for ODS modified fibers is measured to be about 96.4°. Thus, the adhesion for ODS modified fibers is as following:

[00001]$F_{\mathrm{hydrophobic}}=\underset{i=1}{\overset{n}{.Math.}}.Math..Math.{\oint }_{S_i}.Math.\gamma .Math..Math.\sin .Math..Math.{\theta }_i^d.Math.S_i-\Delta .Math..Math.P.Math.A.Math.n$

where s is the circumference of each quartz fiber, n is number of quartz fibers, and θ is the localized contact angle. Because of the interaction between neighboring capillary bridges, contact angles along the periphery of the quartz fiber will vary in the range of 96.4°≦θ<180°. FIG. 2B shows optical micrographs of the capillary bridges formed with 3 intrinsic or ODS treated quartz fibers.

[0032] As illustrated in FIG. 2C, the maximum liquid capacities of the surface have also been measured. The maximum droplet volume that the surface can hold when being inverted also grows with an increasing number of fibers. For 6 penetrating fibers, the liquid capacities are 11.8 μL and 7.3 μL for hydrophilic and hydrophobic quartz, respectively. The gravitational forces of the maximum droplet capacity correspond to adhesive forces of the surface. When the liquid volume exceeds the maximum value, the capillary bridge of the droplet will pinch-off due to instability.

[0033] To evaluate the droplet loss after the transfer process using the surface of the present invention, a comparison is made of the volume of droplets transferred by using the hydrophilic-fiber surface with that of the volume of droplets deposited directly from a micropipette. Droplets with a volume varying from 0.5-6 μL have been compared. The tests show that the volume of water droplets transferred by the surface is almost the same as that directly deposited. Thus, no obvious loss can be observed.

[0034] The insets in FIG. 2D show under a microscope the water residue after the withdrawal. For hydrophilic fibers, similar to some superhydrophobic surfaces, there is a minute amount of water residue on top of each fiber, which is caused by rupture of the capillary bridge. During the withdraw process, the superhydrophobic mesh repels water out of the spaces between the hydrophilic fibers, then the capillary bridge finally pinches off at the fiber facets. For a surface with hydrophobic fibers, the water contact line recedes on fibers once the receding contact angle has been reached. Therefore, as a result, the remaining amount of water on top of hydrophobic fibers is much less than that of hydrophilic ones. Compared with the volume of droplets, the volume loss during the transfer process by the surface of the present invention is negligible.

[0035] The surface is used for manipulation and preparation of micro/nano-liter water droplets. The capture and release process of a microliter water droplet is illustrated in FIG. 3A. First, a 5 μL water droplet is placed on a superhydrophobic surface. Then the surface with 4 hydrophilic fibers is used to pick up the water droplet in air. The adhesion of the droplet to the fibers balances the gravitational force on the droplet, which guarantees reliable manipulation. To disengage the droplet, the fibers are withdrawn from the superhydrophobic mesh and the droplet is released back onto the superhydrophobic surface.

[0036] FIG. 3B is a series of photographs showing the preparation of 0.5 μL water droplet from a micropipette. For preparation of a nanoliter droplet, a 0.5 μL water droplet is first measured by a commercial quantitative pipette. Then the surface with 4 hydrophilic fibers is applied to capture the tiny water droplet directly from the tip of pipette. To transfer the 0.5 μL water droplet onto a superhydrophobic surface, the hydrophilic fibers are withdrawn. The droplet is then prepared on the target surface.

[0037] To manipulate oil droplets under water, the mesh is modified to be superoleophobic under water. The mesh is first dip coated in poly(ethyleneimine) (PEI) (0.1 g/mL) solution. After evaporation of PEI solution, the mesh is dip coated in sodium alginate solution (0.02 g/mL) and then immersed in CaCl.sub.2 solution (1 M) for 10 min. After treatment, the mesh is covered by a layer of calcium alginate which leads to underwater superoleophobicity. Four ODS treated hydrophobic quartz fibers are used in the surface. Referring to FIG. 3C, a 5 μL 1,2-dichloroethane droplet is first placed on a piece of calcium alginate coated silicon wafer under water. Then, the surface with penetrating ODS treated fibers is utilized to stably pick up the oil droplet under water. The secure holding allows arbitrary manipulation and transportation of the oil droplet under water. To disperse it, the ODS treated fibers are withdrawn and the oil droplet is released back on the calcium alginate coated silicon wafer.

[0038] The application of the surface for droplet transfer is complemented by its good cycling durability and chemical resistance. In-situ rapid switching adhesion of the surface facilitates dexterous and precise liquid droplet handling, which is a critical step for droplet-based micro-reactors. Dynamic cyclic switching of the surface is illustrated in FIG. 4A. In that figure nine (9) cycles have been repeated. For each cycle, the sliding angles for 5 μL water droplets are 180° (pinned) and less than 10° for the adhesive and slippery states, respectively. Good cyclic switching ability can enable repeat and multistep operation of the surface. The slippery state of the surface is also proven to be resistant to droplets having pH values varying from 1 to 14. By referring to FIG. 4B, contact angles of droplets with pH values within such range are larger than 150°. The chemical resistance of the surface is empowered by good chemical stability of the superhydrophobic coating mixture.

[0039] The surface used for a droplet-based micro-reactor according to the present invention has proven effective for droplet-based micro-reaction as illustrated in FIG. 5. In FIG. 5A the release of a 2 μL FeCl.sub.3 (0.75 M) solution droplet on a superhydrophobic surface is shown. The inset image shows the capture of the 2 μL FeCl.sub.3 solution droplet from a pipette. FIG. 5B illustrates the merging (capture and mixing) of a 4 μL NaOH (1.5 M) solution droplet with the FeCl.sub.3 solution droplet. The inset image in FIG. 5B shows the capture of the 4 μL NaOH solution droplet from a pipette. FIG. 5C illustrates the merged droplet containing generated red Fe(OH).sub.3 precipitates, released back on to the superhydrophobic surface.

[0040] By using hydrophilic micro-fibers, a liquid column can be trapped among fibers as illustrated in FIG. 6A. Then, by withdrawing fibers, the liquid column is repelled out and a tiny droplet is formed as illustrated in FIG. 6C. Such a process can be utilized to prepare liquid droplets with volumes down to the nano-liter scale, which is difficult for a commercial micro-pipette to achieve. The volume of a prepared liquid droplet is determined by the space among micro-fibers. Such space increases with increasing numbers and protruding lengths of micro-fibers. As illustrated in FIG. 6B, liquid droplets with volumes of 20-200 nL can be prepared by this method.

[0041] The surface can be expanded through utilization of a large fiber array. As shown in FIG. 7A, a brush composed of Dupont Tynex bristles can be used instead of a quartz fiber array. With this brush there is a protruded fiber array. Such an expanded surface can be used to manipulate multiple droplets in parallel. FIGS. 7B-7D shows three droplets of different volume being successively captured by using the expanded surface with the protruding fiber array. By retracting the fiber array, the three droplets are simultaneously released to the substrate below as shown in FIG. 7E.

[0042] While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.