Highlights

Correlation between Hydrophobicity of a Surface and its Roughness

Scientific Achievement

Controlling hydrophobicity of solid surfaces is technically important due to a wide range of applications, including anti-sticking, anti-contamination, self-cleaning, and oil/water separation.  However, understanding of the correlation between hydrophobicity of a surface and its surface roughness is still rudimentary.  This is particularly true when considering roughness over a wide range of length scales.  We have tackled this problem by creating a new class of composite surfaces with different levels of roughness.  The composite surfaces consist of gallium arsenide substrates coated with silver nanoplates, which have controlled size and thickness, as well as varying degrees of nanoscale roughness.  The resulting composite surfaces exhibit various hydrophobicities after the Ag nanoplates are coated with self-assembled monolayers of alkyl thiol molecules.  For example, water droplets on composite surfaces with flat Ag nanoplates exhibit Cassie impregnating wetting state, i.e., the contact angles between water droplets and composite surfaces are very high and the droplets strongly stick to the surfaces.  On the other hand, water droplets on composite surfaces with rough Ag nanoplates also exhibit high contact angles, but the adhesion between water and the composite surfaces is very weak, resulting in a Cassie non-wetting state.

Significance

The results, for the first time, clearly indicate that the second level roughness of surfaces with hierarchical roughness determines the adhesion between water and the surfaces and that the first level roughness mainly determines the contact angles between the water droplets and the surfaces.  These findings provide a guideline to engineer surfaces with precise wetabilities.  This work has been published in Nano Research 1 (4), 292-302 (2008).

Performers

Y. Sun (Argonne-CNM)