Dynamics of Trisiloxane Wetting: Effects of Diffusion and Surface Hydrophobicity

The excellent wetting and spreading behavior of superspreader solutions has been known and extensively studied over recent years. However, explanations for wetting dynamics and accompanying mathematical models have not yet proved completely successful. Many attempts have been made to quantify the time exponents, but none of the models so far has been able to describe successfully the whole wetting process of trisiloxane solutions, especially on hydrophobic surfaces. We have investigated partial wetting of Silwet L-77 superspreader solutions of high concentrations (well above cmc) on polymer-coated substrates of varying hydrophobicity. Results obtained can be explained in terms of diffusion at the “subsurface” as the key factor for trisiloxane enhanced wetting. A theory, involving concentration and surface tension gradients governing the dynamic wetting process, has been developed in order to explain the evolution of drop radius and consequent decrease in contact angle. The proposed model, which may explain exceptional wetting of trisiloxanes, was found to be in good agreement with experimental results. Coefficients obtained from application of the model to experimental results lend strong evidence supporting the importance of substrate hydrophobicity as one of the crucial aspects for wetting quality.