Surface structure determines dynamic wetting

TL;DR

This study investigates how micro-scale surface roughness affects the initial dynamic spreading of droplets, revealing a quantifiable relationship through line friction and providing a criterion to predict dominant spreading mechanisms.

Contribution

It introduces a simple formula linking surface roughness geometry to line friction, advancing understanding of how surface structure controls wetting dynamics.

Findings

Roughness influence can be quantified by a line friction coefficient.

A simple formula relates roughness geometry to energy dissipation.

A criterion predicts whether spreading is dominated by roughness or inertia.

Abstract

Liquid wetting of a surface is omnipresent in nature and the advance of micro-fabrication and assembly techniques in recent years offers increasing ability to control this phenomenon. Here, we identify how surface roughness influences the initial dynamic spreading of a partially wetting droplet by studying the spreading on a solid substrate patterned with microstructures just a few micrometers in size. We reveal that the roughness influence can be quantified in terms of a line friction coefficient for the energy dissipation rate at the contact line, and that this can be described in a simple formula in terms of the geometrical parameters of the roughness and the line-friction coefficient of the planar surface. We further identify a criterion to predict if the spreading will be controlled by this surface roughness or by liquid inertia. Our results point to the possibility of selectively controlling the wetting behavior by engineering the surface structure.