Self-Movement of Liquid Drops on Swollen Polydimethylsiloxane Films of Varying Thickness

Self-movement of a liquid drop on a surface is controlled by surface tension forces, which can be quantified by a difference in contact angles between two "sides" of a drop. Differences in contact angles are typically induced by chemical, electrical, or topographical gradients. However, swelling and thickness gradients on a soft surface may also induce drop motion. When a drop is placed on soft and swollen elastomers, network deformation and potential fluid separation occur. This means that there exist both pull-up of the polymer network (a solid wetting ridge) and pull-up of the swelling solvent (an oil ridge) at the periphery of the drop. The heights of these ridges are determined by a balance of surface tension of the drop and restoring forces of the polymer network, which thus impact contact angles. Here we study the dependence of contact angles on the elastomer film thicknesses, the degree of network crosslinking, and the degree of network swelling using polydimethylsiloxane (PDMS) swollen with silicone oil. Above a certain swelling ratio, we find that the contact angles for 4 µL water drops noticeably decrease as film thickness increases within a range of 1-250 microns. Our results suggest that a thickness gradient imparted to highly swollen elastomer films can promote spontaneous movement of water drops without the need to apply external energy.