Letter pubs.acs.org/Langmuir
Virtual Walls Based on Oil-Repellent Surfaces for Low-SurfaceTension Liquids Riberet Almeida and Jae Wan Kwon* Department of Electrical Engineering, University of MissouriColumbia, Columbia, Missouri 65211, United States S Supporting Information *
ABSTRACT: Manipulating and controlling water-based aqueous solutions with the use of virtual walls is relatively simple compared to that of nonaqueous low-surface-tension liquids, which pose greater challenges to microfluidic devices. This letter reports a novel technique to form a virtual wall for various low-surface-tension liquids. A microfluidic channel with virtual walls has been made to guide low-surface-tension liquids by using a specially designed oil-repellent surface. Unlike generic superoleophobic surfaces, our oil-repellent surface exhibited strong repellency to the lateral flow of low-surface-tension liquids such as hexadecane and dodecane. A plasma-assisted surface micromachining process has been utilized to form the oil-repellent surface. The use of combined features of re-entrant geometries on the surface played an important role in promoting its repellence to the lateral flow of low-surface-tension liquids. We have successfully demonstrated how low-surfacetension liquids can be well confined by the virtual walls.
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INTRODUCTION Microfluidic devices and systems, in general, encompass the flow of small amounts of liquids within three-dimensionally constructed tiny channels bounded by physical walls on all sides. Not so long ago, several interesting microfluidic channels with fewer bounding surfaces were introduced.1,2 Liquid streams were bounded by only two flat solid surfaces on opposite sides (i.e., top and bottom substrates) and exhibited large liquid−air interfaces elsewhere (i.e., left and right sides). Patterned hydrophobic coatings were used to create relatively well controlled boundaries with clearly standing liquid−air interfaces, which have been often termed as virtual walls1 or surface-tension-confined (STC) interfaces.3,4 Superhydrophobic surfaces5−7 also have high repellence to the lateral spreading of a water droplet because of the inherently hydrophobic surface chemistry8 and easily form virtual walls to limit the spreading boundaries of high-surface-tension liquids (aqueousbased solutions). Another commonly used method for the formation of virtual walls is to employ patterned self-assembled hydrophobic monolayers2,9,10 on various substrates such as silicon, glass, and plastic. In addition, several other less common methods such as microplasma jet writing11 and sol− gel inkjet printing3 have also been reported. All of these methods can precisely define the spatial locations of the virtual walls between two flat substrates but are very limited for only water-based aqueous solutions. However, to the best of our knowledge, there have been no reports of such virtual walls used for nonaqueous low-surface-tension liquids such as oils and other organic liquids. In this letter, we report how low-surface-tension liquids can be effectively bounded, confined, and transported along only © 2013 American Chemical Society
paths predefined by newly designed oil-repellent surfaces. Our unique oil-repellent surface, distinguished from generic superoleophobic surfaces, has shown outstanding repellence for laterally spreading low-surface-tension liquids. Conventional superoleophobic surfaces generally exhibit excellent repellence to vertically approaching liquid droplets as seen from sessile droplet contact angle measurements.12−17 It has been well characterized that the re-entrant and overhanging mushroom-like structures in superoleophobic surfaces as shown in Figure 1a−c are able to keep the vertically approaching low-surface-tension liquids from penetrating the patterned cavities on the surfaces.12,15−18 An oil droplet contacts and gets pinned on the surface of individually isolated and elevated tiny islands having re-entrant structures underneath (Figure 1a). The presence of the air pockets under the oil droplet and between the re-entrant structures helps the surface become superoleophobic. Interestingly, the surface chemistry of the superoleophobic surfaces is actually slightly oleophilic.8 The oil droplet can easily wet a fraction of the surface area of the reentrant structures but does not penetrate the air pockets. However, the stability of the liquid−air boundary is not very strong, and the oil can easily spread laterally when forced by external pressure18 or be perturbed by electric fields18,19 and can readily make a transition to the Wenzel state. Micro/ nanostructured superhydrophobic surfaces prevent the lateral spreading of aqueous solutions because of the inherently hydrophobic surface chemistry.8 However, conventional superReceived: October 9, 2012 Revised: January 10, 2013 Published: January 14, 2013 994
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wafer was isotropically etched in HNA for 2 to 3 min. The patterned disc-shaped SixNy features formed by a large undercut resulted in mushroom-like microstructures (Figure S1f). After being carefully rinsed with deionized (DI) water, the sample was gently dried with N2 and dehydrated at 150 °C for a few minutes. Finally, the surface was treated with Cytop (Asahi, Japan) by spin-coating at 3000 rpm and then curing at 175 °C for 1 h. In addition, we have also fabricated our new oil-repellent surface. A glass substrate was first cleaned using a piranha solution (1:1 v/v H2O2/sulfuric acid) and then thoroughly rinsed using DI water. The substrate was then dried and dehydrated on a hot plate at 150 °C for 5−10 min. After a 10-μm-thick layer of SU-8 3010 was spin-coated and cured, a 0.3-μm-thick aluminum layer was deposited and patterned. This patterned Al layer was used as an etch-mask for the subsequent O2 plasma etching of the SU-8 layer. The RIE etching conditions and time were adjusted to obtain an isotropic etch profile for the re-entrant curvature. After the selective RIE etching of SU8, the Al etch-mask layer was stripped away. The bottom substrate was finally spin-coated with Cytop. Cytop is a fluoropolymer that is very similar to Teflon. Its chemical structure has been highlighted in Figure S4. Cytop has a contact angle of 53° with hexadecane and 112° with water according to its data sheet.22 The top coverslip, which was also rendered hydrophobic with a Cytop layer, was placed over the bottom substrate with a 25-μm-thick spacer layer. The fabrication details have been highlighted (Figure S2 in Supporting Information). Various test liquids that were used for the surface characterization include n-nonane (surface tension of 22.37 mJ/cm2), dodecane (25.35 mJ/cm2), benzene (28.88 mJ/cm2), o-xylene (30.1 mJ/cm2), chlorobenzene (33.28 mJ/cm2), N,N-dimethylformamide (37.1 mJ/ cm2), furfural (41.9 mJ/cm2), nitrobenzene (43.9 mJ/cm2), 1,1,2,2tetrabromoethane (47.1 mJ/cm2), formamide (58.2 mJ/cm2), glycerol (64 mJ/cm2), and DI water (72 mJ/cm2) with surface tensions in the range of 22 to 72 mJ/cm2. Four contact angle measurements were taken and averaged for each test liquid. For the microscopic view by SEM for the triple-phase contact line (TCL) interaction of an oil droplet with the boundary between the micromachined area and the flat oleophilic micropath, a PDMS droplet was placed at the boundary and then completely cured. A very thin Pt layer was later sputterdeposited on the cured PDMS droplet on the surface for SEM imaging.
Figure 1. Illustration of the conventional superoleophobic surfaces (a−c). Oil can be pinned on (a) mushroom-like structures, (b) sharp edges, and (c) bumplike structures without wetting the entire surface. The newly developed oil-repellent surface has (d) re-entrant curvatures oriented in the vertical direction and (e) re-entrant circular geometries oriented in the lateral direction.
oleophobic surfaces cannot effectively contain the lateral spreading of a liquid droplet in the Wenzel state by pinning the contact line. Patterned micro/nanostructured superhydrophobic surfaces can be used to confine and guide liquid streams along hydrophilic paths,4,5 but conventional superoleophobic surfaces such as mushroom-structured surfaces do not exhibit similar confinement to low-surface-tension liquids. In particular, we have found from our experiment that when an oil droplet laterally transits from an oleophilic area onto a superoleophobic area it can easily wet the superoleophobic area because the contact line cannot be pinned (illustrated in Figure S1 in Supporting Information). This clearly shows the difficulty in confining low-surface-tension liquids within a predefined boundary by conventional superoleophobic surfaces. In previous work, however, another method for creating superoleophobic surfaces was introduced by electrospinning blends of fluorodecyl polyhedral oligomeric silsesquioxane (POSS) and poly(methyl methacrylate) (PMMA) polymers.14 Although this technique exhibits excellent superoleophobicity, patterning these surfaces to produce well-defined microfluidic paths using conventional photolithography seems challenging. The surface that we have developed formed a clear boundary and showed an excellent capability for confining various lowsurface-tension liquids. These surfaces are not superoleophobic but were characterized as being oleophobic through contact angle measurements. As shown in Figure 1d,e, our new surface comprises a flat, solid surface with a periodic array of micromachined holes. The array of micromachined holes can be considered to be a periodic array of defects. 20,21 Furthermore, the periodically arranged holes provide 2D reentrant features especially in the lateral direction. Surface micromachining of the holes forms vertically oriented reentrant curvatures under the edges of the holes, which can be effectively used to prevent the triple-phase contact line (TCL) of a liquid droplet from advancing across the surface. This plays an important role in achieving repellence to laterally spreading low-surface-tension liquids.
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RESULTS AND DISCUSSION Creating virtual walls for supporting the liquid−air interface of low-surface-tension liquids begins with designing surfaces that can prevent the lateral spreading of oils. Superoleophobic surfaces with re-entrant structures (Figure 1a−c) were initially fabricated and evaluated for this purpose. The ineffectiveness of the conventional superoleophobic surfaces in preventing lateral flows of low-surface-tension liquids has been confirmed and illustrated (Figure S1 in Supporting Information). The oil droplet very easily wets the mushroom-like structures and quickly travels over the superoleophobic surface area. The oil-repellent behavior of our newly designed surfaces arises primarily from the interaction of the liquid droplet’s triple-phase contact line (TCL) with the circular rim boundaries of holes and the re-entrant curvatures of the micromachined features below the surface. The array of micromachined holes can be considered to be a periodic array of defects.20,21 These defects can be seen as energy barriers that pin the contact line and prevent its further advancement.21 Because surface tension acts in a way to minimize the free surface, the wetted surface of the drop becomes as compact as possible. Both the periodic distance between holes and the hole diameters determine the pinning force. Furthermore, the circular rim boundaries along with the vertically oriented re-entrant curvatures underneath act as reentrant features on the 2D plane simultaneously with the
EXPERIMENTAL SECTION
To begin, we have built up the structures that are essential to the conventional superoleophobic surfaces as demonstrated in previous work.15,16 For the fabrication of the superoleophobic surface, 0.7-μmthick silicon nitride on a silicon wafer was first patterned to leave circular features with a diameter of 20 μm and a spacing of 15 μm in between. CF4 plasma was used to etch the nitride away, and then the 995
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at the boundary, it was completely cured. Clear SEM images of the PDMS droplet (representing the oil droplet’s behavior) at the boundary were then obtained (Figure 3). It can apparently be seen in Figure 3c that the TCL cannot advance further beyond the first row of etched holes. We have tested two different oils with different viscosities and surface tensions to look into the oil-repellent phenomenon. The robustness of the oil-repellent nature of the surfaces was evidenced by this experiment. Even when pushed by a pneumatic force (155°). This can be explained by treating the holes as defects (patches of air where the liquid has a contact angle of 180°). The corresponding contact angle on the oil-repellent surface can be predicted using the Cassie−Baxter relationship.23 The predicted apparent contact angle of 103.4° matches the measured value of 106.6° very closely. A further study of the TCL’s interaction with the boundary between a flat oleophilic region and a region comprising circular etched features using scanning electron microscopy (SEM) was carried out by using poly(dimethylsiloxane) (PDMS), which has a similar surface tension (19.8 mN/m) to that of silicone oil (21.5 mN/m). A droplet of PDMS was placed on a flat, oleophilic region of the surface and pneumatically pushed toward the micromachined (oil-repellent) region. Once it stopped proceeding any further
between the oil-repellent regions and remain confined. Even in a top-covered device, the oil can remain confined along the oleophilic micropath between the bottom and top surfaces (Figure 4e). Because there are no physical walls on the sides of the micropath, the liquid−air interface is pinned precisely at the boundary between the oleophilic and oil-repellent regions and can be looked upon as being confined by the presence of virtual walls. The low-surface-tension liquid moves along the virtual microfludic channel by capillary forces and remains within the oleophilic areas by surface tension. When the confined liquid’s Young−Laplace internal pressure exceeds a particular threshold, the TCL can advance beyond the first row of micromachined holes and will stop at the next row. If the internal pressure is still beyond the threshold pressure, then further advancement will occur until the internal pressure falls below
Figure 3. (a) SEM image of a poly(dimethylsiloxane) (PDMS) droplet placed at the boundary of an oil-repellent surface to mimic the behavior of an oil droplet. (b) Side-view image of a hexadecane droplet on this surface for contact angle measurements. (c) Magnified image of TCL depicting how such re-entrant features can act as barriers to prevent the lateral spreading of low-surface-tension liquids. 996
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(Figure 5b,c depicting liquid on the convex side of the liquid− air interface). For the case of an elongated droplet (such as the hexadecane droplet shown in Figure 4e), R2 for the sides where virtual walls are present is infinite and the Laplace equation is simplified to ΔP = γ/R1. This is under the assumption that the TCL at the oleophilic/oil-repellent boundary is a straight line that in reality is actually curved as seen in the SEM image in Figure 3a,c. The many small localized curves of the TCL do not appreciably alter the overall curvature of R1 at the boundary. The value R1 can be expressed by the equation R1 = h/[cos θint + cos θn] where h is the channel depth (Supporting Information). The maximum pressure that the virtual walls can sustain is Pmax = ΔP = γ/R1 = γ[cos θint + cos θn]/h. Theoretical calculations predict the corresponding maximum Pmax to be −348 N/m2. This value can increase up to 628 N/m2 when both top and bottom surfaces are patterned and aligned. The capillary force generated from the oleophilic surfaces drives liquid forward and produces a concave side-view profile as shown in Figure 5c with a negative curvature (R1), whereas the retraction force generated from the oil-repellent virtual walls restrains the liquid from spreading further beyond the boundary and produces a much less concave side-view profile as shown in Figure 5b. We believe that this is the underlying mechanism that drives low-surface-tension liquids along the oleophilic paths in top-covered devices.
the threshold breakthrough pressure. Even after a period of a few weeks, no further advancement of the oil beyond the first row of micromachined holes was observed. Because the pinning force at the circular rims that the TCL contacts is related to the circular rim diameters and the distance between the centers of the circular rims,20 tuning the threshold/breakthrough conditions for certain microfluidic applications is possible. Moreover, it was observed that less viscous fluids such as hexadecane (3 mPa·s @ 298 K) flow easily along the virtual microfluidic channel as compared to more viscous fluids such as silicone oil (50 mPa.s) and castor oil (650 mPa·s). Thus, by utilizing oil-repellent surfaces to form virtual walls, selective wetting by nonaqueous, low-surface-tension liquids can be precisely controlled. Figure 5 describes the top view (a) and cross-sectional views (b, c) of a liquid stream flowing along the micropath. As soon
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CONCLUSIONS In this study, we have introduced our newly designed oilrepellent surface. Virtual walls for supporting the liquid−air interface of low-surface-tension liquids along predefined paths in microfluidic devices have been successfully demonstrated by using these oil-repellent surfaces. Conventional superoleophobic surfaces were fabricated and compared to our oil-repellent surfaces with respect to the lateral flow of low-surface-tension liquids. Outstanding performance in containing the spread of liquids when introduced from the lateral direction exhibited by the newly designed surfaces has been highlighted in this study. These oil-repellent surfaces can easily and effectively halt the lateral flow of various low-surface-tension liquids. The oilrepellent surfaces were exploited to form virtual walls for confining and guiding low-surface-tension liquids along predefined microfluidic paths. The oil-repellent surfaces can be widely utilized for many industrial applications, and the virtual walls for the low-surface-tension liquids also have immense potential for many lab-on-a-chip (LOC) devices. One potential application is a microfluidic aqueous solution/oil separator. Mixtures of an aqueous solution and oil can be guided along microfluidic paths defined by the oil-repellent surfaces. In the middle of the path, an oil-only penetrable mesh can be placed for oil separation. Oils can fall though the mesh and be separated while the aqueous flow can just pass over the mesh. Another application is for guiding low-surface-tension liquids for organic synthesis in microreactors.
Figure 5. Illustration of important parameters for the virtual walls formed by oil-repellent surfaces in a top-covered microfluidic device such as Young−Laplace radii of curvature and contact angles. (a) Top view of a liquid stream confined by virtual walls. (b) Cross-sectional view of the liquid stream at A−A′. (c) Cross-sectional view of the liquid stream at B−B′.
as a low-surface-tension liquid is introduced along the micropath in a top-covered device, two different contact angles can be seen at the oil-repellent/oleophilic boundary and at the advancing front of the liquid. The liquid’s internal pressure is released at the advancing front by allowing the liquid to propagate along the oleophilic channel whereas it increases the contact angle at the oil-repellent/oleophilic boundary and results in a larger radius of curvature as compared to that of the advancing front. When the contact angle of the free surface of the liquid θint at the boundary between the oleophilic and oilrepellent areas exceeds the contact angle on the oil-repellent surface (which is 106.6° for the case of hexadecane), the virtual walls cannot be sustained along the boundary and the TCL will move beyond the next row of micromachined holes. A pressure drop is always present when the liquid−air interface is curved because of surface tension. According to the Young−Laplace equation that is given by ΔP = γ(1/R1 + 1/R2), where ΔP, γ, R2, and R1 are the pressure difference, liquid surface tension, and radii of curvature of the liquid droplet from the top and side views, respectively, the pressure on the concave side (Figure 5b,c depicting air on the concave side of the liquid−air interface) is always greater than the pressure on the convex side
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ASSOCIATED CONTENT
S Supporting Information *
Fabricated superoleophobic surface, schematic of fabrication steps of the oil-repellent surface and virtual walls in a topcovered device, characterization of the surface using different test liquids, and theoretical estimation of breakthrough pressures. This material is available free of charge via the Internet at http://pubs.acs.org. 997
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(21) Cubaud, T.; Fermigier, M. Faceted drops on heterogeneous surfaces. Europhys. Lett. 2007, 55, 239. (22) Asahi Glass Company. Cytop amorphous fluoropolymer technical data sheet. (23) Cassie, A. B. D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−551.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +1 573-882-0762. Notes
The authors declare no competing financial interest.
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