Dewetting of ultrathin surfactant-covered films

Abstract

Many industrially and technologically important situations involve thin films covered with either pre-existing or introduced surfactant, which can potentially affect dewetting and spreading processes. The two-dimensional dewetting dynamics of ultrathin liquid films are studied here in the presence of insoluble surfactant; surfactants can drive a flow due to surface tension gradients and additionally the coefficients of the intermolecular potential, which are usually assumed to be constant, can depend on the surfactant concentration. Coupled evolution equations for the film height and surfactant concentration are derived using lubrication theory. These equations are parameterized by a Marangoni parameter, ℳ, and the equilibrium film thickness, lc, obtained by setting the intermolecular potential to zero. A linear stability analysis of these equations shows that the presence of surfactant can widen the band of unstable wave numbers and that, for relatively large lc, the selected wave number is minimized for a certain value of ℳ. Numerical simulations of the evolution equations show that initially nonuniform distributions of surfactant can destabilize clean spinodally stable films. Our results also show that it is possible to destabilize these films using uniform initial surfactant distributions, although this is only possible when the intermolecular potential coefficients are concentration dependent.