Asymmetry of wetting and de-wetting on high-friction surfaces originates from the same molecular physics

TL;DR

This study uses molecular dynamics simulations to explore the molecular physics behind wetting and de-wetting asymmetry on high-friction surfaces, emphasizing the role of contact line friction and molecular transport.

Contribution

It introduces a contact line friction model that captures molecular transport effects and explains wetting asymmetry on high-friction hydrophilic surfaces.

Findings

Wetting and de-wetting dynamics are governed by a contact line friction relation.

Hydrophilic substrates are easier to wet than de-wet, showing asymmetry.

A new approach to distinguish contact line friction models beyond experimental limits.

Abstract

The motion of three-phase contact lines is one of the most relevant research topics of micro- and nano-fluidics. According to many hydrodynamic and molecular models, the dynamics of contact lines is assumed overdamped and dominated by localised liquid-solid friction, entailing the existence of a mobility relation between contact line speed and microscopic contact angle. We present and discuss a set of non-equilibrium atomistic Molecular Dynamics simulations of water nanodroplets spreading on or confined between silica-like walls, showing the existence of the aforementioned relation and its invariance under wetting modes (`spontaneous' or `forced'). Upon changing the wettability of the walls, it has been noticed that more hydrophilic substrates are easier to wet rather than de-wet; we show how this asymmetry can be automatically captured by a contact line friction model that accounts for the molecular transport between liquid layers. A simple examination of the order and orientation of near-contact-line water molecules corroborates the physical foundation of the model. Lastly, we propose an approach to discriminate between contact line friction models which overcomes the limitations of experimental resolution. This work constitutes a stepping stone towards demystifying wetting dynamics on high-friction hydrophilic substrates and underlines the relevance of contact line friction in modelling the motion of three-phase contact lines.