(677i) Thermodynamics of Cassie-Wenzel Transitions on Nanotextured Surfaces

In hydrophobic environments, water can become metastable with respect to its vapor; the resulting dewetting transitions influence numerous phenomena ranging from bubble coalescence and colloidal assembly to surface nanobubbles and superhydrophobicity. On superhydrophobic surfaces, the surface texture provides such an environment, and water can either wet the texture in the Wenzel state or sit atop an air cushion in the Cassie state. It is only in the Cassie state that the surface is superhydrophobic, with properties like water-repellency, self-cleaning, interfacial slip, and fouling resistance. However, the Cassie state is often a kinetically trapped metastable state and at elevated pressures, water can descend into the surface texture yielding the non-functional Wenzel state; superhydrophobicity is thus fragile. A promising strategy for improving the robustness of the superhydrophobic Cassie state is to reduce the texture dimensions. As texture size shrinks to the nanoscale, water density fluctuations can become increasingly important. Using molecular simulations in conjunction with enhanced sampling methods, here we investigate how fluctuations affect the free energetics of Cassie-Wenzel transitions on nanotextured surfaces. Our results suggest that highly co-operative water density fluctuations play an important role in determining the thermodynamics, kinetics, and thereby the phase behavior of Cassie-Wenzel transitions on nanotextured surfaces. Our results also allow us to propose improved surface morphologies, which not only destabilize the Wenzel state, but also render it unstable under ambient conditions, thereby facilitating the reversible recovery of the superhydrophobic Cassie state.