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Non-wetting surfaces and estimation of slip lengths
Monday, Jun 11, 2012, 9:50 AM -10:10 AM
, R. E. Cohen, G. H. McKinley;
Massachusetts Institute of Technology, Cambridge, MA.
We summarize a technique to fabricate large-area superhydrophobic and oleophobic surfaces by spray coating a polymer blend of poly(methyl methacrylate) (PMMA) and the low surface energy molecule 1H,1H,2H,2H-heptadecafluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) at various concentrations using an air brush with a pressurized nitrogen stream. The surface morphology can be systematically tuned from a spherical or corpuscular microstructure to beads-on-string and a fiborous non-woven mesh. Scanning electron micrograph show the formation of micro-textured surfaces possessing re-entrant curvature, which is critical in obtaining liquid repellency with low surface tension liquids. An operating diagram is developed to predict the surface morphology based on the polymer solution concentration and molecular weight, and the non-wetting behavior is established with contact angle measurments with liquids over a range of surface tensions.
Using these surfaces, we then demonstrate a systematic approach to determining the effective contact angle of drops on non-wetting surfaces. We use a perturbation solution of the Bashforth-Adams equation to investigate the effect of gravity-induced sagging in amplifying variations in goniometric measurements of the contact angles of sessile drops. We illustrate the inherent sensitivity of goniometric contact angle measurement techniques to drop dimensions as the apparent contact angle approaches 180 degrees.
Finally, the robustly liquid-repellent spray-coated surfaces lead to very high apparent contact angles, low contact angle hysteresis and the possibility of ‘liquid slip’ over the microscopic air pockets or ‘plastron film’ trapped in the textured surface. The drag-reducing ability of the spray-coated superhydrophobic surfaces is quantified using torque measurements in a parallel plate rheometer over a range of shear rates and a slip length of b
= 38 μm is obtained, and is related to an averaged microstructure length-scale using the framework provided by Ybert. Finally, the ability of dual-scale spray-coated mesh surfaces to enable both large slip-lengths (as high as b
=253 μm) and high robustness against wetting transition is demonstrated.
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