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, which depend on, c, the initial weighing in solute concentrations. With small c, h increases proportional to c. With high c, all films rupture at h≈50nm, independent of c. 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.