DIRECT EVIDENCE FOR VISCOSITY-INDEPENDENT SPREADING ON A SOFT SOLID

The kinetics of a spreading drop is usually controlled by conversion o f capillary potential energy into viscous dissipation within the liqui d when the solid is rigid. However, if the solid is sufficiently soft, a ''wetting ridge'' near the solid/liquid/vapor triple line can also be a dissipative sink as the wetting front moves. In this paper we pro vide evidence to show that when the substrate is soft, this local defo rmation near the triple line may lead to an energy dissipation that ma y outweigh effects due to viscosity over a large range of speeds and t herefore spreading kinetics becomes viscosity independent.