  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 the substrate elasticity, impact velocity and the 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 then
gradually decreases for softer substrates. We experimentally determine the
dependence of the maximum Weber number allowing non-wetting impacts on the
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.
