Condensed-Matter Physics & Materials Science Seminar

Ultrahydrophobicity and Wettability of Microtextured Surfaces

Gareth H. McKinley
Director, Hatsopoulos Microfluids Laboratory
Department of Mechanical Engineering, M.I.T., Cambridge, MA 02139, USA


It has long been known that the combination of surface chemistry and topology can strongly amplify the wetting (or non wetting) properties of a substrate. With recent developments in nanofabrication it has become possible to achieve unprecedented control over the microtexture of a surface and this can result in almost perfect ultrahydrophobicity. We discuss two recent developments in the area. First we show how fluorocarbon chemistry combined with surface roughness on vertically-aligned “nanotube forests” can result in robust ultrahydrophobicity even to condensing vapors. The high thermal conductivity of the nanotubes also suggests possible commercial applications (for example as ultra-conductive heat transfer surfaces or micromixers). We also briefly consider modifications to the impact dynamics at the air-water interface resulting from hydrophobic coatings. Secondly we demonstrate how molecular-level dispersions of perfluorinated nanoparticles can be used to systematically control both the hydrophobicity and oleophobicity (oil-repellency) of polymer substrates. The chemical and topographic mechanisms behind this repellency are elucidated by lithographically producing model microtextured surfaces in silicon that feature strongly re-entrant structures (referred to as 'micro-hoodoos' because of their similarity to geomorphological features). The resulting composite surfaces are the most oleophobic ever produced, with alkane contact angles greater than 160˚ and low hysteresis. These coatings are expected to have a wide range of commercial applications including the development of engineering surfaces with enhanced solvent resistance, reduction of biofouling and separation of oil/water dispersions.