Corrosion influence on the evaporation of sessile droplet

Corrosion-driven droplet wetting on iron nanolayers

The classical Evans' drop describes a drop of aqueous salt solution, placed on a bulk metal surface where it displays a corrosion pit that grows over time producing further oxide deposits from the metal dissolution. We focus here on the corrosion-induced droplet spreading using iron nanolayers whose semi-transparency allowed us to monitor both iron corrosion propagation and electrolyte droplet behavior by simple optical means. We thus observed that pits grow under the droplet and merge into a corrosion front. This front reached the triple contact line and drove a non radial spreading, until it propagated outside the immobile droplet. Such chemically-active wetting is only observed in the presence of a conductive substrate that provides strong adhesion of the iron nanofilm to the substrate. By revisiting the classic Evan's drop experiment on thick iron film, a weaker corrosion-driven droplet spreading is also identified. These results require further investigations, but they clearly open up new perspectives on substrate wetting by corrosion-like electrochemical reactions at the nanometer scale.

Rupture of ultrathin solution films on planar solid substrates induced by solute crystallization

On-line optical imaging of continuously thinning planar films in a spin cast configuration reveals the rupture behavior of ultra-thin films of binary mixtures of a volatile solvent and a nonvolatile solute. The pure solvents completely wet the silica substrates whereas the solution films rupture at certain film thicknesses, h_rupture, which depend on, c₀, the initial weighing in solute concentrations. With small c₀, h_rupture increases proportional to c₀. With high c₀, all films rupture at h_rupture≈50nm, independent of c₀. The findings can be explained by the solute enrichment during the evaporative thinning. Solute crystallization at the liquid/substrate interface upon reaching solute supersaturation leads to locally different wetting properties. This induces locally the rupture of the film as soon as it is sufficiently thin. A proper data rescaling based on this scenario yields a universal rupture behavior of various different solvent/solute mixtures.