Department of Chemical Engineering, MIT
The fabrication of non-wetting surfaces and coatings that repel low surface tension oils and organic liquids requires the presence of re-entrant topographies coupled with low surface energy coatings. Commercially available woven fabrics (e.g., nylon- or PET-based fabrics) possess inherently re-entrant textures in the form of cylindrical yarns and fibers. Various coating techniques including dip-coating, chemical vapor deposition and fluorosilane chemistry can been used to chemically modify the fabric surface energy and impart an oleophobic character. We present a nested model with n levels of hierarchy that is constructed from modular units of cylindrical and spherical building blocks to describe the repellency of low surface tension liquid drops on woven and nano-textured superoleophobic fabrics. At each level of hierarchy, the density of the topographical features is captured using a dimensionless textural parameter Dn*. For traditional plain-woven chemically treated fabrics (n = 2), the tight packing of individual fibers is shown to impose a geometric constraint on the maximum oleophobicity that can be achieved by modifying the surface energy of the coating. These fabrics are characterized by a lower bound on their equilibrium contact angle below which the Cassie-Baxter to Wenzel wetting transition spontaneously occurs. We demonstrate how the introduction of an additional micro/nano-textured length scale on the fibers (n = 3) is necessary to overcome this limit and create more robustly non-wetting fabrics. We extend previously developed ideas on predicting when a pressure driven Cassie-Baxter to Wenzel transition occurs, which yields a framework that guides the rational design of particle size, feature density and surface coatings. Finally, we show a simple experimental realization of the enhanced oleophobicity of fabrics by depositing spherical beads of poly(methyl methacrylate)/fluorodecyl polyhedral oligomeric silsesquioxane onto the fibers of a woven nylon fabric.
About the Speaker:
Justin is a Ph.D. candidate in Chemical Engineering working under the supervision of Professor Robert E. Cohen and Professor Gareth H. McKinley. His research focuses on investigating the influence of physical and chemical parameters on surface wettability and characterizing interfacial phenomena in a range of solid-liquid systems. This has led to the design and fabrication of functional coatings for a range of applications, such as drag reduction, fog harvesting, and oleophobic fabrics.
Date of Talk:
March 11, 2015