Droplet Impact on Surfaces with Asymmetric Microscopic Features.

Susumu Yada,Gustav Amberg,Blandine Allais,Shervin Bagheri,Fredrik Lundell,Wouter Van Der Wijngaart

doi:10.1021/acs.langmuir.1c01813

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 drops impacting at low velocities onto surfaces textured with asymmetric (tilted) ridges. We found that the difference between impact velocity and the capillary speed on a solid surface is a key factor of spreading asymmetry, where the capillary speed is determined by the friction at a moving three-phase contact line. The line-friction capillary number Caf = μfV0/σ (where μf,V0, and σ are the line friction, impact velocity, and surface tension, respectively) is defined as a measure of the importance of the topology of surface textures for the dynamics of droplet impact. We show that when Caf ≪ 1, the droplet impact is asymmetric; 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 ridges. When Caf ≫ 1, the geometric details of nonsmooth surfaces play little role.

Highlights

The complex fluid−surface interaction during the impact which includes splashing[7−11] and trapping of a thin gas film underneath the droplet[12−15] has been studied theoretically,[16−19] numerically,[18−21] and experimentally.[22−28] These studies have established useful scaling laws of maximal deformation, which among other things are reviewed in ref 2, 29
These studies have reported that surface topology influences the spreading and even small roughness delays spreading at a low impact velocity.[25]
It is not completely understood which microscopic features of a complex surface texture have the largest influence on droplet impact

Summary

Introduction

The impact of droplets on a solid surface is essential in technological applications such as spray coating and cooling,[1,2] pesticide deposition,[3,4] and inkjet printing.[5,6] The complex fluid−surface interaction during the impact which includes splashing[7−11] and trapping of a thin gas film underneath the droplet[12−15] has been studied theoretically,[16−19] numerically,[18−21] and experimentally.[22−28] These studies have established useful scaling laws of maximal deformation, which among other things are reviewed in ref 2, 29. The influence of surface roughness and microstructures on drop impact has been studied extensively focusing on different aspects, such as splashing,[23,29−31] bouncing,[32−35] trapped gas film under the droplet,[14] rolling speed after the impact on inclined substrates,[36] and maximum spreading radius.[25,26,37,38] These studies have reported that surface topology influences the spreading and even small roughness delays spreading at a low impact velocity.[25] it is not completely understood which microscopic features of a complex surface texture have the largest influence on droplet impact. One example of a complex surface is an asymmetric textured surface, i.e., where the unit structure (post, ridge, rising, etc.) is not symmetric to the vertical line passing through the center of the structure. These asymmetric surface structures have been mimicked for technical applications such as oil−water separation[43] and raindrop shielding,[42] their influence on droplet impact is not fully understood

Methods

Results

Conclusion