The shape and dynamics of deformations of viscoelastic fluids by water droplets

Many studies on the deformation of soft films by liquids confirmed the increase in the radius of the deformation and the decrease in the apparent contact angle. However, due to the thinness, the dynamics of the deformation could not be observed until the thermodynamic equilibrium. Thus, the dynamics on thick soft materials was studied until equilibrium to contrast the effect of different interfacial energy between different soft materials and water. Therefore, we prepared two different polymeric fluids with similar rheology by cross-linking monomers, yet with different contact angles with water. Sometime after water droplets were placed on these thick polymers, 3D profiles of the deformation were recorded. Though the effect of the surface tension was not verified, the same trend in the dynamics was observed as with thin films, except for the decrease in the radius after the initial increase. The three-phase boundaries (TPBs) were found not at the apex of the ridges formed during the transition to equilibrium. By calculating the surface tensions and angles of each interface at the equilibrium, we found that the temporary imbalance among surface tensions induced the slip of the TPBs toward the center of water droplets, thus dislocating the TPBs and decreasing the radius.