Spreading and dynamics of liquid drops involving nanometric deformations on soft substrates

When a liquid spreads spontaneously on a rigid solid, a dynamic equilibrium is set up in which the non-equilibrated Young force causes triple line motion. Most solids may be considered as rigid when submitted to capillary forces. With soft solids, however, a 'wetting ridge' is formed as the wetting front advances or recedes. This ridge accompanies the motion leading to energy dissipation due to the hysteretic nature of the soft substrate and the strain cycle undergone. It has been established that the wetting ridge (of the order of tens of nanometers in height on silicone rubbers) can be so important in its consequences that viscous dissipation within the liquid becomes negligible. We propose to review the impact of solid deformation in the kinetics of spreading of liquids on soft materials, in wetting and dewetting modes, and we will consider also liquid drop motion due to gravity. In this last case, liquid drops running down a rubber plane move at a speed largely determined by the shear modulus of the polymer. The possibility of tailoring wetting, dewetting or draining kinetics on soft materials to a specific need is clearly evidenced. (C) 2002 Elsevier Science B.V. All rights reserved.