The role of viscosity ratio in Janus drop impact on macro-ridge structure

Abstract

An interaction of liquid and solid surfaces upon impact has made great progress in understanding the principle behind impinging compound drops, such as single-interface Janus and core–shell configurations, for controlling drop mobility on the surfaces. Despite advancement of recent technologies, fundamentals of how viscosity ratios of Janus drops affect post-impact dynamics on anisotropic surfaces are still unknown. Here, we numerically investigate the asymmetric impact dynamics of Janus drops on a non-wettable ridged surface to demonstrate the feasibility of the separation of the low-viscosity part from the high-viscosity part by reducing the residence time. The separation is investigated for various viscosity ratios, Weber numbers (We), and initial angle, which are discussed in terms of the temporal evolution of the mass and momentum distributions. A regime map for the separation reveals that the low-viscosity parts are more likely to be separated from high-viscosity parts as the viscosity ratio increases. The phenomenon can be related to a retraction time, which is explained by a hydrodynamic model for the low-viscosity part. This study suggests that We thresholds for the separation can be significantly reduced with the help of center-assisted retraction along the ridge. The asymmetric bouncing of Janus drops on a ridged surface can open up possibilities for the efficient control of liquid separation.