Activity: Talk or presentation › Oral presentation
We report a hitherto not observed wetting phenomenon, namely the retraction of a drop after deposition on a solid substrate, caused by an interfacial reaction between cations in the aqueous drop and fatty acids in the ambient oil. We investigate how this process depends on the concentrations of the reactants, the chain lengths of fatty acid and alkane solvent, and the solid substrate material. We demonstrate with contact angle and AFM measurements that deposition of amphiphilic molecules on the solid takes place only after assembly at the oil-water interface. Deposition of material by the moving contact line leads to an increasingly hydrophobic local substrate and a concomitant increase in contact angle, which we call autophobing. This phenomenon is observed both on mica and on silica substrates, for several fatty acids and alkane solvents. The time-dependent contact angle can be described with a theoretical model in which the adsorption reaction at the oil-water interface is rate-limiting, and transfer to the solid is determined by a mass flux balance (similar to a Langmuir Blodgett transfer). The ensuing time-dependent oil-water and solid-oil interfacial tensions then produce the evolution of the contact angle.