Pressure-sensitive conversions between Cassie and Wenzel wetting states on a nanocorrugated surface

D Vanzo and A Luzar and D Bratko, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 128, 323 (2022).

DOI: 10.1007/s00339-022-05458-4

Molecular dynamics simulations were used to study pressure-controlled wetting behavior of a nanostructured surface. Model graphene-like material functionalized by nanosized asperities was shown to support reversible dynamic transitions between superhydrophobic Cassie states that minimize contact with water and completely wetted Wenzel states. In practical applications, similar reversibility has been achieved by hierarchical corrugations, air trapping, or tailored geometry of surface posts. Nano corrugations alone are shown to secure a robust Cassie state at low pressures, support transition to Wenzel state at pressure of O(10(2)) atm and can recover the Cassie regime without prohibitive hysteresis upon decompression. For O(10) nm surface fragments, timescales of dynamic response to compression deduced from the relaxation rates of water uptake fluctuations were estimated in the range of 0.1-1 ns. On small surfaces, an essentially barrier-free transition to Wenzel state is typically initiated at surface edges, followed by cooperative spreading across the entire surface. Conversely, the recovery of the Cassie state, which involves mild hysteresis, relies on nucleation away from the edges and should therefore be essentially independent of surface size. In conventional picture, cycling rate is determined by the latter process. This suggests subnanosecond responses of surface wettability could also be realized on macroscopic samples, leaving pressure control as the practical rate determinant in eventual application.

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