Designing Omniphobic Surfaces for Super Liquid Repellency and Friction Reduction
Lecture by Gareth H. McKinley
Department of Mechanical Engineering
Massachusetts Institute of Technology
Tuesday, May 4, 2010
Room 1610 Engineering Hall
Refreshments at 9:15 a.m.
Lecture at 9:30 a.m.
Many different structured surfaces with a wide range of surface chemistries and topographies have been investigated for controlling the wetting (or non-wetting) properties of a fluid/solid interface. Experimental advances in nanofabrication have led to the ability to achieve unprecedented control over the microtexture of a substrate, and this can result in almost perfect ultrahydrophobicity. No structured surfaces to date, however, have been able to achieve super-oleophobicity (oil-repellency, or resistance to wetting by low interfacial tension liquids such as hydrocarbons). Silsequioxanes are nanometer-scale caged molecules that can be heavily fluorinated and molecularly dispersed in a range of polymers to systematically control both the hydrophobicity and oleophobicity of polymer substrates. Electrospinning polymer fiber mats from these dispersions enables tight control over local surface topography (in conjunction with surface chemistry and overall roughness) to significantly enhance repellency to a wide range of liquids. The chemical and topographic mechanisms behind this repellency are elucidated by lithographically producing model surfaces in silicon, which feature strongly re-entrant structures (referred to as ‘micro-hoodoos’ because of their similarity to geomorphological features). Microtextured re-entrant structures coated with FluoroPOSS are the most oleophobic surfaces produced to date, with alkane contact angles greater than 160° and low wetting hysteresis. Applications of these coatings include fabrics with enhanced solvent/oil resistance, surfaces for reducing biofouling or friction and separation of oil/water dispersions. Non-wettable surfaces also offer the possibility of ‘giant liquid slip’ over the microscopic air pockets or ‘plastron film’ trapped in the re-entrant textured surface. Using parallel-plate rheometry, we explore the degree of friction reduction that can be achieved as the geometric details of the re-entrant surface structures are varied, and compare the experimental results with recent scaling theories. Slip lengths of greater than 500 μm can be observed for optimal textures and coatings.
Gareth H. McKinley
Gareth H. McKinley is the School of Engineering Professor of Teaching Innovation and Associate Head for Research with the Department of Mechanical Engineering at MIT, as well as a cofounder of Cambridge Polymer Group, a contract research laboratory specializing in polymers. He received his B.Sc. in Natural Sciences/Chemical Engineering from Cambridge University and his Ph.D. in Chemical Engineering from MIT. Professor McKinley’s research activities focus on rheology, fluid dynamics, mathematical modeling and experimental design for rheological characterization.
He is a Fellow of the American Physical Society. His awards include the 2006 Society of Rheology Publication Award and the 2002 Frenkiel Award of the APS Division of Fluid Dynamics, and he was the MIT Class of 1960 Fellow for Teaching Excellence from 2005 to 2007. He edited the Journal of Non-Newtonian Fluid Mechanics from 2000 to 2009.