Droplet impact on surfaces with asymmetric microscopic features

TL;DR

This study experimentally investigates how asymmetric microscopic surface features influence droplet impact dynamics, revealing the role of line friction and surface topology at different impact regimes.

Contribution

It introduces the line-friction capillary number to quantify the impact of surface texture on droplet dynamics and elucidates the effects of asymmetric ridges at various impact velocities.

Findings

Contact line speed against ridge inclination is governed by line friction at low Ca_f.

Pinning occurs at ridge corners when Ca_f is small, affecting impact behavior.

Surface geometry becomes negligible at high Ca_f, reducing the influence of microscopic features.

Abstract

The impact of liquid drops on a rigid surface is central in cleaning, cooling and coating processes in both nature and industrial applications. However, it is not clear how details of pores, roughness and texture on the solid surface influence the initial stages of the impact dynamics. Here, we experimentally study drop impacting at low velocities onto surfaces textured with asymmetric (tilted) ridges. We define the line-friction capillary number $Ca_f={\mu_f V_0}/{\sigma}$ (where $\mu_f$, $V_0$ and $\sigma$ are the line friction, impact velocity and surface tension, respectively) as a measure of the importance of the topology of surface textures for the dynamics of droplet impact. We show that when $Ca_f \ll 1$, the contact line speed in the direction against the inclination of the ridges is set by line-friction, whereas in the direction with inclination the contact line is pinned at acute corners of the ridge. When $Ca_f \sim 1$, the pinning is only temporary until the liquid-vapor interface reaches to the next ridge where a new contact line is formed. Finally, when $Ca_f\gg 1$, the geometric details of non-smooth surfaces play little role.