Effect of softness of polydimethylsiloxane on the hydrophobicity of pillar-like patterned surfaces

In this study, we have demonstrated that different wetting behavior of regular pillar-like patterned PDMS can be achieved by controlling the mixing ratio of polydimethylsiloxane's (PDMS's) prepolymer base and curing agent. The dynamic contact angles and sliding angles of water droplet on these patterned surfaces are carefully measured. It is found that the experimental results for PDMS substrates prepared at the mixing ratio of base to curing agent 10 : 1 are consistent with the theoretical predictions of the Wenzel model and Cassie model. However, for the PDMS substrates prepared at the mixing ratio of base to curing agent 20 : 1, the wetting behavior would change due to the deformation of the structure—collapse of pillars due to the softness. In addition, the dynamic phenomenon of the impregnating process of water penetrating into the pillars is observed through the optical microscope when a water droplet is placed on the regular pillar-like patterned PDMS substrates prepared at the mixing ratio of base to curing agent 20 : 1. The pillars are pushed down by water flow one after another like dominoes. The flow pattern of the impregnating process is discussed.