Water Droplet Equilibration on Silicone Elastomer Substrates Containing Mobile Silicones

Understanding interfacial behavior at the water/silicone elastomer interface is vital in many applications, including microfluidics, antibiofouling, and self-cleaning surfaces. Silicone elastomers are not always static systems, however. Unreacted silicone molecules within a substrate may change the water-wetting behavior compared to fully reacted substrates. To investigate the impact of free silicone species at the interface, we systematically studied water wettability as a function of contact time with various silicone elastomer substrates. One set of Sylgard 184 substrates was prepared from curing at optimal stoichiometry and included doses of silicone molecules of varying molecular weights. Another set of substrates was made by mixing Sylgard 184 in imbalanced ratios to provide incomplete cross-linking. In the absence of added silicone oils, we observe a linear decrease in contact angle with time due to the evaporation of water. However, within certain molecular weight and loading levels of additional inert oils, nonlinear wetting behavior occurs as oils migrate to the interface and the system stabilizes. Similar nonlinear wetting behavior occurs without including silicone oils by mixing pure Sylgard 184 in very imbalanced mixing ratios, resulting in incomplete cross-linking. Such substrates contain unbound mobile silicone species that diffuse to the water-substrate interface. We show how the water contact angle technique can elucidate the presence of unbound silicone oils in the substrate.