Water Wetting Transition Parameters of Perfluorinated Substrates with Periodically Distributed Flat-Top Microscale Obstacles

Abstract

Superhydrophobicity is obtained on photolithographically structured silicon surfaces consisting of flat-top pillars after a perfluorosilanization treatment. Systematic static contact angle measurements were carried out on these surfaces as a function of pillar parameters that geometrically determine the surface roughness, including pillar height, diameter, top perimeter, overall filling factor, and disposition. In line with thermodynamics models, two regimes of static contact angles are observed varying each parameter independently: the "Cassie" regime, in which the water drop sits suspended on top of the pillars (referred to as composite), corresponding to experimental contact angles greater than 140-150 degrees, and the "Wenzel" regime, in which water completely wets the asperities (referred to as wetted), corresponding to lower experimental contact angles. A transition between the Cassie and Wenzel regimes corresponds to a set of well-defined parameters. By smoothly depositing water drops on the surfaces, this transition is observed for surface parameter values far from the calculated ones for the thermodynamic transition, therefore offering evidence for the existence of metastable composite states. For all studied parameters, the position of the experimental transition correlates well with a rough estimation of the energy barrier to be overcome from a composite metastable state in order to reach the thermodynamically favored Wenzel state. This energy barrier is estimated as the surface energy variation between the Cassie state and the hypothetical composite state with complete filling of the surface asperities by water, keeping the contact angle constant.

Physics and applications of superhydrophobic and superhydrophilic surfaces and coatings

“…64,[66][67][68] An energy barrier unfavorable to liquid wicking may be maximized in this manner. 59,[69][70][71][72][73] This energetic barrier, if it is larger than the inherent thermal energy 7 needs to be overcome by mechanical means such as vibrations, 74,75 impact 76,77 or load imposed on the drop.…”

Wetting properties of cosmetic polymeric solutions on hair tresses

“…A hair tress demonstrates hydrophobic properties not only because of the hydrophobic nature of the hair surface, but also because of air pockets in between the fibres. That is, there are three possible wetting regimes: (a) Cassie wetting, where liquid is sitting on the hair tress, without penetration inside the tress, (b) Wenzel wetting, characterized by penetration of liquid into the hair tress, (c) transition from Cassie wetting to Wenzel wetting, after some critical contact angle is reached [15][16][17][18]. All three mentioned regimes were observed.…”

Partial Wetting of Aqueous Solutions on High Aspect Ratio Nanopillars with Hydrophilic Surface Finish

“…11 Obviously poor wetting would affect the ef- * Electrochemical Society Active Member. * * Electrochemical Society Fellow.…”