The Role of Diffusion in Superspreading

Rapid wetting kinetics and good surface coverage are two properties of so-called 'superspreader' solutions which have been recognised for some years. However, explanations for their efficiency have proved limited. Several attempts to quantify the behaviour have been made, but none of the models so far has been successfully able to describe the whole wetting process of trisiloxane solutions, particularly on hydrophobic surfaces. We consider here the partial wetting of Silwet L-77® superspreader solutions at relatively high concentrations (above the critical micelle concentration) on polymeric surfaces of varying hydrophobicity. Results obtained can be explained by diffusion to the 'subsurface', or zone in proximity to the surface or interface. A theory, involving concentration and surface tension gradients governing the dynamic wetting process, has been developed to explain the evolution with time of drop contact radius, and therefore decrease in contact angle.