Droplet Bouncing and Breakup during Impact on Microgrooved Surface

TL;DR

This study experimentally investigates how water droplets impact microgrooved hydrophobic surfaces, revealing how surface pitch and Weber number influence bouncing, breakup, and wetting transitions, and proposing a regime map for these outcomes.

Contribution

The paper introduces a detailed regime map linking Weber number and pitch to droplet impact outcomes on microgrooved surfaces, supported by experimental data and a mathematical model.

Findings

Maximum droplet spreading is greater in the longitudinal direction at low pitch and Weber number.

Different impact outcomes (no bounce, bounce, breakup, wetting transition) depend on pitch and Weber number.

A regime map delineates transitions between bouncing, breakup, and wetting states.

Abstract

We experimentally investigate impact dynamics of a microliter water droplet on a hydrophobic microgrooved surface. The surface is fabricated using photolithography and high-speed visualization is employed to record the time-varying droplet shapes in transverse as well as longitudinal direction. The effect of the pitch of the grooved surface and Weber number on droplet dynamics and impact outcome are studied. At low pitch and Weber number, the maximum droplet spreading is found to be greater in the longitudinal direction than the transverse direction to the grooves. The preferential spreading inversely scales with the pitch at a given Weber number. In this case, the outcome is no bouncing (NB); however, this changes at larger pitch or Weber number. Under these conditions, the following outcomes are obtained as function of the pitch and Weber number - droplet completely bounces off the surface (CB), bouncing occurs with droplet breakup (BDB) or no bouncing due to Cassie to Wenzel wetting transition (NBW). In BDB and NBW, the liquid penetrates the grooves partially or completely beneath the droplet due to the wetting transition. The former results in droplet breakup alongside bouncing while the latter suppresses the bouncing. These outcomes are demarcated on Weber number - dimensionless pitch plane and the proposed regime map suggests the existence of a critical Weber number or pitch for the transition from one regime to the other. CB and BDB are quantified by plotting coefficient of restitution of the bouncing droplet and volume of daughter droplet left on the surface, respectively. The critical Weber number needed for the transition from CB to BDB is estimated using an existing mathematical model and is compared with the measurements. The comparison is good and provides insights in mechanism of the liquid penetration into the grooves.