# Geometry-induced interface pinning at completely wet walls

## TL;DR

This study reveals how nanoscale patterning of solid walls influences wetting behavior, showing that surface geometry can induce pinning or depinning of the liquid interface, thus altering wetting properties in ways not predicted by classical laws.

## Contribution

The paper introduces a non-local density functional theory analysis demonstrating geometry-induced interface pinning and depinning transitions at nanostructured walls, contrasting with macroscopic wetting laws.

## Key findings

- Complete wetting depends on microscopic length-scales and surface patterning.
- Nano-modification can switch surface from hydrophilic to hydrophobic.
- Identifies a surface phase diagram with depinning and prewetting transitions.

## Abstract

We study complete wetting of solid walls that are patterned by parallel nanogrooves of depth $D$ and width $L$ with a periodicity of $2L$. The wall is formed of a material which interacts with the fluid via a long-range potential and exhibits first-order wetting transition at temperature $T_w$, should the wall is planar. Using a non-local density functional theory we show that at a fixed temperature $T>T_w$ the process of complete wetting depends sensitively on two microscopic length-scales $L_c^+$ and $L_c^-$. If the corrugation parameter $L$ is greater than $L_c^+$, the process is continuous similar to complete wetting on a planar wall. For $L_c^-<L<L_c^+$, the complete wetting exhibits first-order \emph{depinning transition} corresponding to an abrupt unbinding of the liquid-gas interface from the wall. Finally, for $L<L_c^-$ the interface remains pinned at the wall even at bulk liquid-gas coexistence. This implies that nano-modification of substrate surfaces can always change their wetting character from hydrophilic into hydrophobic, in direct contrast to the macroscopic Wenzel law. The resulting surface phase diagram reveals close analogy between the depinning and prewetting transitions including the nature of their critical points.