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Using the Fact that Wetting Is Contact Line Dependent Dalton F. Cheng and Thomas J. McCarthy* Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003, United States ABSTRACT: Two series of experiments, both involving contact line pinning, are reported that were designed using the contact line perspective of wetting and require this perspective to explain the observed results. Perspectives based on contact areas, for example, Wenzel’s and Cassie’s, are not useful in either of these experimental situations. In the first type of experiment described, sessile water drops were pinned on low contact angle hysteresis surfaces using 40 different shape/size lithographed hydrophilic features. Hydrophilic arcs (sections of circles), short wedges (pointed to the center of the circle), long wedges (pointed to the opposite side of the circle), and the upper outlines of the short and long wedges were prepared and studied. These features were based on circles with diameters of 4 and 6 mm and arcs of 30°, 60°, 90°, and 120°. The volume of water that could be pinned depends on the linear shape of the portion of the feature that interacts with the receding contact line and not on the feature area. In the second type of experiment, thin hydrophilic contact lines were used to support films of water (puddles and kinetically trapped thin films) on water-repellent surfaces and used to control the shape (both 2D and 3D) of these thin films and puddles. Elongated water puddles, 60 mm long and 4 mm wide, were prepared using contact line patterns with line widths of 500, 250, and 100 μm. Curved puddles, geometric shapes, letters of the English alphabet, and puddles with variable liquid thicknesses (heights) were also prepared.
’ INTRODUCTION We recently reviewed1 our papers that were published over the past decade that emphasize the fact that liquid drop motion on solid surfaces involves events (advancing and receding) that occur at the three-phase solid/liquid/vapor contact line. In particular, we reviewed experiments2,3 that disproved the Wenzel and Cassie theories (that involve contact areas and not contact lines) and noted other work4-6 that had questioned (or had disproved) these theories. Here we describe experiments that were carried out with the objective of demonstrating that the contact line perspective is useful. These experiments further demonstrate situations where the Wenzel and Cassie perspectives are not relevant, but our intent is not to further prove points that we have already made. Instead we emphasize that the contact line perspective is required to address real problems in wetting and adhesion. Two types of experiments were performed: (1) water droplets were pinned on low contact angle hysteresis surfaces using different shape/size lithographed features and (2) different and unusually shaped puddles of water were prepared on water repellent surfaces using only thin hydrophilic contact lines of the desired shape. Figure 1 describes the simple contact line perspective, why it is useful and why contact areas are not important. Figure 1a depicts a 6 mm diameter hydrophilic (θA/θR = 0°/0°) spot and a 6 mm diameter (O.D.) hydrophilic ring (θA/θR = 0°/0°, line width =0.5 mm, I.D. = 5.0 mm) that are patterned on a hydrophobic, water repellent surface (θA/θR = 106°/102°). The colors red and green are used to denote wetting and nonwetting surfaces, respectively. When sufficient water is applied to both of these features to form sessile drops with the same contact angle (many angles are possible), the two drops are indistinguishable. The same force r 2011 American Chemical Society
is required to move either drop from their circular footprints. When the surface is tilted, even to 90°, both drops remain pinned. The contact line structure determines the wetting behavior, and the structure and contact angle of the surface under the drops are irrelevant. Figure 1b shows one way in which this perspective may be useful. The letter “Y” could be printed using ink and a printing plate with either a “solid Y” or the outline of a “Y”. Again, red and green are used to denote wetting and nonwetting surfaces, respectively. Experiments described in this paper show that in fact only a portion of the ring indicated in Figure 1a is required to pin a drop, that very low contact angle sessile drops can be stable over very hydrophobic surfaces, and that the “Y” could support ink over a very hydrophobic interior with only a thin perimeter outline that need not be continuous.
’ EXPERIMENTAL SECTION General Information. Silicon wafers (100 mm diameter, ∼500 μm thickness) were obtained from International Wafer Service (100 orientation, P/B doped, resistivity from 20 to 40 Ω cm). Wafers were cleaned using a Harrick plasma cleaner operated at 30 W for 15 min. The chamber was evacuated to less than 50 mTorr and bled with oxygen to maintain a pressure of 100 mTorr during the plasma cleaning. Shipley S1813 photoresist and Microposit MF-321 developer were obtained from Microchem Corp. UV exposure was carried out using a 500 W (365 nm) OAI UV lamp. Photolithography masks were printed on polyester film by CAD/Art Services, Inc. 1,3,5,7-Tetramethylcyclotetrasiloxane Received: January 15, 2011 Revised: February 14, 2011 Published: March 07, 2011 3693
dx.doi.org/10.1021/la2001893 | Langmuir 2011, 27, 3693–3697
Langmuir
ARTICLE
Figure 1. (a) A hydrophilic (red) circular spot and a hydrophilic ring on a hydrophobic (green) background. Water drops on these features are indistinguishable and require the same force to be displaced. (b) A solid hydrophilic “Y” and the hydrophilic outline of a “Y.” Both could be used to print ink. (D4H) and tridecafluoro-1,1,2,2-tetrahydrooctyldimethylchlorosilane (RFSiMe2Cl) were obtained from Gelest and used as received. Water was purified by reverse osmosis and filtering steps (18 MΩ). Ellipsometric thicknesses were determined using a Rudolph SL-II automatic ellipsometer and parameters described in a previous publication.7 Patterned Surfaces. An oxygen plasma cleaned silicon wafer was spin-coated with photoresist for 60 s at 4000 rpm and then heated at 115 °C for 60 s to remove solvents. This yields a uniform ∼1.3 μm thick coating. After exposure to UV for 15 s through a particular mask, the wafer was submerged in developer for 50 s to remove resist from the exposed regions, rinsed with deionized water for 1 min, and then further cleaned with oxygen plasma for 5 min. The wafer was placed in a 5 in. diameter pyrex reactor (fabricated from O-ring joints) that contained (at the bottom) a small amount (∼0.5 mL) of either D4H or RFSiMe2Cl (liquids). The reactor was sealed and heated (oil bath) at 80 °C for 72 h. The liquid reagents did not contact the silicon wafers and chemical reaction occurred at the solid-vapor interface. After removing the wafer from the reactor, the photoresist was removed by rinsing with acetone, exposing the hydrophilic silica patterns. Drop and Puddle Application. Water drops on pinning features (D4H-modified wafers) were applied on horizontal wafers using a 10 μL syringe. The wafer was carefully rotated to 90° from the horizontal (vertical). The contact line suspended sessile drops were increased in volume in 1 μL increments by careful addition of water using a 10 μL syringe. We report the maximum volume that was pinned by each feature; an additional microliter caused the drop to depin. Data are exclusively from either the first or second drop tested on all features. Generally, the maximum volume of pinned drops on a given feature decreased with multiple experiments, likely due to contamination that occurs from impurities in the water or the air. Puddles were applied to hydrophilic line enclosed hydrophobic (RFSiMe2Cl) areas by depositing water at one end of a pattern while elongating/pulling the drop/puddle to the other end of the pattern using a hydrophilic microscope slide. With the water puddle pinned at one end of a pattern, pulling the puddle with the microscope slide causes the puddle to narrow and confine itself within the hydrophilic lines.
’ RESULTS AND DISCUSSION Two general types of experiment were performed and are described and discussed here. In the first type, hydrophilic lines and spots (features), with specific size and shape on low hysteresis background surfaces, were designed and prepared to quantitatively correlate water drop pinning with hydrophilic
Figure 2. (a) Patterns of the hydrophilic features used for pinning water drops: arcs (sections of circles, 1), short wedges (pointed to the center of the circle, 2), long wedges (pointed to the opposite side of the circle, 3), upper outline of short wedge (4), upper outline of long wedge (5). (b) The nine drops of water in the photograph on the right are hanging from the features shown at the left.
contact line length and hydrophilic area. The second type involved the use of hydrophilic line patterns to control the shape of the three-phase contact line of water puddles, thus the overall shape of the water puddles. These experiments demonstrate that puddles with a range of controllable shapes can be prepared on nonwetting (high receding contact angle) surfaces by controlling only the contact line. Several of the preparative aspects of this research warrant comment. (1) The photolithography is standard and can be carried out in a reasonably clean preparative lab with a spin coater, UV lamp, and commercially available photoresist and developer solutions. A clean room is not required. The low resolution (the smallest features we studied were 100 μm wide lines) masks that we used are laser-printed (carbon black based toner) polyester film sections that are essentially the same as “overhead transparencies”. We generally use masks prepared on our office printer for this type of lithography, but a commercial mask printer was used to prepare masks for the features shown here because their printer has higher resolution, their ink has higher contrast than office printers, and these masks are relatively inexpensive (60 cm2) can be formed on a surface that exhibits water contact angles of θA/θR = 114°/106° using only a 0.5 mm width circular line demonstrates and emphasizes the importance and utility of contact lines. We prepared a variety of unusually shaped puddles on this perfluoralkyl surface using thin hydrophilic contact lines. Figure 7a shows contact line-stabilized elongated water puddles that are 60 mm long and 4 mm wide. Regardless of the line thickness ranging from 500 to 100 μm, the elongation of water
’ SUMMARY We demonstrate, using two types of experiments, situations in which the contact line perspective of wetting is required to either predict or explain the observed behavior. Contact area perspectives are not relevant or useful in these situations. The pinning of sessile water drops by hydrophilic features depends on the linear shape of the portion of the feature that interacts with the receding contact line. The pinning of water puddles by hydrophilic contact lines can be used to control the 2D and 3D shape of the puddles. Unusual shapes for liquids to take can be designed and prepared. Controlling the 3D shape of puddles using 2D patterns has greater implications than for water puddles. Ferrofluids could be used to create complex magnetic fields; polymerizable fluids could form solids with controlled topography; high surface tension solutions could be spread as kinetically stabilized thin films on low surface tension solids. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the group of Prof. Ken Carter and Dr. Isaac Moran for help with photolithography. We thank the Materials Research Science and Engineering Center (DMR-0213695) and the Center for Hierarchical Manufacturing (CMMI-0531171) at the University of Massachusetts for support and 3M, Henkel and Shocking Technologies for unrestricted funding. ’ REFERENCES (1) Gao, L.; McCarthy, T. J. Langmuir 2009, 25, 14105. (2) Gao, L.; McCarthy, T. J. Langmuir 2007, 23, 3762. (3) Gao, L.; McCarthy, T. J. Langmuir 2009, 25, 7249. (4) Pease, D. C. J. Phys. Chem. 1945, 49, 107. (5) Bartell, F. E.; Shepard, J. W. J. Phys. Chem. 1953, 57, 455. (6) Extrand, C. W. Langmuir 2003, 19, 3793. (7) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268. (8) Hozumi, A.; Cheng, D. F.; Yagihashi, M. J. Colloid Interface Sci. 2011, 353, 582. (9) deGennes, P.-G.; Brochard-Wyart, F.; Quere, D. Capillarity and Wetting Phenomena; Springer: New York, 2004; p 37. 3697
dx.doi.org/10.1021/la2001893 |Langmuir 2011, 27, 3693–3697
