Droplet dynamics on viscoelastic soft substrate: Toward coalescence control

Abstract

We study the dynamical behavior of droplets on a viscoelastic soft substrate. Using thin film approximation for the hydrodynamics and time-dependent Winkler’s substrate model, we show numerically how droplet growth depends strongly on the viscous damping characteristic of the substrate, leading to asymmetric stick-slip dynamics corroborated by experimental observations. Scaling arguments are presented to rationalize radial growth and the underlying substrate response to viscoelastic limits. Using an adjacent pair of inflating droplets, we report strongly diverse coalescence outcomes with non-linear coalescence times, including attraction, repulsion, and remarkably, a separation regime, within which the two droplets grow away from each other and remain separated due to intervening wetting ridges. Together, our results indicate strong interactions between the substrate and the droplet across viscoelastic and capillary timescales, with practical implications for smart surface engineering, condensation, and coalescence control.