Pressure effects on water slippage over silane-coated rough surfaces: pillars and holes

Nanoscale roughness in combination with surface chemistry modification can achieve large liquid slippage due to entrapment of air pockets in the surface asperities (superhydrophobic Cassie state). However, pressure increase may result in the collapse of the gaseous enclaves into the fully wet Wenzel state, and in the irreversible loss of the desirable superhydrophobic features. In order to explain the subtle effects of pressure, an extensive simulative campaign based on full-atoms molecular dynamics and aimed at reproducing a realistic superhydrophobic system comprising water, solid walls, and hydrophobic coating with octadecyltrichlorosilane is addressed in comparison with a continuum approach. The dramatic impact of pressure on slippage is twofold: on the one hand, it influences the shape of the liquid/vapor interface, thus indirectly affecting the flow. This is a purely geometrical effect well captured in principle by the continuum description. On the other hand, pressure controls the complex interplay between water and hydrophobic chains close to the triple line where it significantly alters the liquid structure, thus modifying apparent slippage. This is an intrinsically atomistic phenomenology that cannot be predicted at the continuum level. The combination of the two effects is found to explain the peculiar response of slippage to the applied pressure at the nanoscale, that may look at first sight in contradiction with most microscale literature results.