Drop bouncing by micro-grooves

Abstract

Micro-textures are a well-known measure to increase surface hydrophobicity. Here, we experimentally investigate the impact of falling water droplets (diameter 2.1 mm, impact speed 0.62 m/s) on flat and structured surfaces made of the same hydrophobic material. While on the flat surface the drop settles with deposition, it bounces from the micro-grooved surface. Numerical simulations with a phase-field method mimicking the experiments do reproduce the different impact outcomes (deposition vs. bouncing) observed on both substrates. The axisymmetric simulation for the flat surface and the three-dimensional simulation for the structured surface employ the same grid size. In addition, the values for capillary width (chosen to be about 1% of the drop diameter) and mobility are the same in both simulations, where in the wetting boundary condition the static contact angle on the flat surface (100.3°) is identically used. Recovering the distinct experimental impingement outcomes in the simulation, though limited to one specific combination of drop diameter and impact speed, highlights the potential of the phase-field method for correctly predicting drop impact phenomena on flat and micro-structured surfaces under adequate resolution. Concerning the instantaneous droplet shape, the agreement between computations and experiments on both substrates is, however, only good till the beginning of the receding phases, whereas thereafter, significant differences are obtained.