A Study on the Dynamics of Compound Materials on Superhydrophobic Surfaces: Effects of Droplet’s Size and Shape

Compound materials have two or more unmixable parts that retain a shared surface with one another for engineering purposes. Such compound materials, like the Janus or core–shell configurations, create opportunities for relevant applications because they offer diverse combinations of complex impinging materials and complex surfaces. However, previous studies have only assumed spherical configurations, or focused on the bouncing dynamics, without considering the effect of the material size. The current work numerically studies the dynamic characteristics of Janus materials with ellipsoidal shapes for various impact speeds and viscosity ratios, to analyze the effect of the size and shape of the material on bouncing and separation behavior. The threshold Weber numbers at which separation starts after the collision are investigated as a function of the droplet size, ellipticity, and viscosity ratio. In addition, a regime map of the separation efficiency of the Janus droplets is established under various viscosity ratios and Weber numbers to investigate the effects of droplet shape on the asymmetric bouncing and separation behavior. It is found that the separation efficiency and mechanism of two prolate spheroids are different from each other at the same ellipticity. This study will provide an efficient strategy to control the bouncing of compound materials in applications, such as drug delivery, liquid purification, and bio- or multi-material printing.