Viscous droplet impingement on soft substrates.

Viscous droplets impinging on soft substrates may exhibit several distinct behaviours including repeated bouncing, wetting, and hovering, i.e. , spreading and retracting after impact without bouncing back or wetting. We experimentally study the conditions enabling these characteristic behaviours by systematically varying substrate elasticity, impact velocity and liquid viscosity. For each substrate elasticity, the transition to wetting is determined as the dependence of the Weber number We, which measures the droplet's kinetic energy against its surface energy, on the Ohnesorge number Oh, which compares viscosity to inertia and capillarity. We find that while We at the wetting transition monotonically decreases with Oh for relatively rigid substrates, it exhibits a counter-intuitive behaviour in which it first increases and then gradually decreases for softer substrates. We experimentally determine the dependence of the maximum Weber number allowing non-wetting impacts on substrate elasticity and show that it provides an excellent quantitative measure of liquid repellency for a wide range of surfaces, from liquid to soft surfaces and non-deformable surfaces.