Understanding superhydrophobic behaviors on hydrophilic materials: a thermodynamic approach

Abstract

Some experiments have proved that superhydrophobic behaviors can be achieved on inherently hydrophilic substrates without low surface energy modification at micro-scale. However, the thermodynamic mechanisms about these results have not been well-understood. In this work, a 2D analytical model was reported to analyze this unexpected experimental observations and wetting behaviors on trapezoidal, vertical and inverse-trapezoidal microstructure surfaces. Theoretical results showed that intrinsic contact angle, which was restricted by sidewall angle of micropillars, was not an independent parameter to affect superhydrophobicity. And re-entrant structures were critical in the realization of microstructures alone inducing transition from hydrophilicity to superhydrophobicity. The wetting transition criterion was that sidewall angle should be less than intrinsic contact angle. On this occasion, a positive energy barrier could support liquid/vapor interfaces and separate Wenzel and Cassie state on hydrophilic substrates. And the physical explanations can be found that the positive energy barrier mainly came from the growth of the high-energetic solid/vapor interfaces to be wetted by the drop with liquid/vapor interfaces moving down inverse-trapezoidal pillars. As for the optimal design of microstructures, considering the limitation of pillar width and the ‘sag’ transition caused by pillar height, T-shape microstructures could be a good choice.

Keywords

• re-entrant structures
• superhydrophobic surfaces
• sidewall angle
• inherently hydrophilic substrates
• wetting transition criterion