How does water wet a hydrosoluble substrate?

Abstract

Thin layers made of water soluble polymers are good candidates to improve the wettability of surfaces as they are theoretically expected to be totally wettable. Yet, in this work we show that the wetting of a droplet of a volatile solvent, water, advancing on such films strongly depends on the degree of coating hydration beyond the contact line. The phenomenon originates not from the diffusion of water molecules through the coating, but from the evaporation from the advancing drop itself and re-condensation beyond the contact line. The faster a droplet is travelling on a coating, the less extensive the phenomenon of evaporation/recondensation beyond the contact line: macroscopically, the more hydrophobic the coating will appear. Expectedly, the contact angle θ strongly depends on the coating thickness e and the droplets advancing velocity U, two parameters that regulate the solvent uptake. More precisely, θ depends on product eU, which we predicted theoretically in a previous paper and verify here experimentally. The practical outcome of these findings is that hydrophilizing a substrate with a hydrophilic, fully water-soluble polymer will be more or less successful depending both on its thickness and on whether the substrate is static or moving during its use.