Droplet Bouncing and Breakup during Impact on a Microgrooved Surface.

We experimentally investigate the 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 the transverse and longitudinal directions. The effect of the pitch of the grooved surface and Weber number on the 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 a function of the pitch and Weber number: droplet completely bounces off the surface (CB), bouncing occurs with droplet breakup (BDB), or no bouncing because of a Cassie to Wenzel wetting transition (NBW). In BDB and NBW, the liquid partially or completely penetrates the grooves beneath the droplet as a result of the wetting transition. The former results in droplet breakup alongside bouncing, while the latter suppresses the bouncing. These outcomes are demarcated on the 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 the coefficient of restitution of the bouncing droplet and the volume of the 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 into the mechanism of liquid penetration into the grooves. The present results on microgrooved surfaces are compared with published results on micropillared surfaces in order to assess the water-repelling properties of the two surfaces.