Nonlinear behavior of an insoluble surfactant partially covering liquid during the transition to equilibrium

Abstract

The process of relaxation to the mechanical and thermal equilibrium state of an insoluble surfactant film partially covering liquid has been investigated theoretically by the method of direct numerical simulation. Thermometric experimental data, which describe the film dynamics along the upper free boundary of a shallow Hele – Shaw cell, are analyzed and explained. At the initial stage, the temperature along the surface is non-uniform. This distribution generates a large-scale convective motion throughout the volume. The volume of the carrying liquid is relatively small. Therefore, the mutual action of the surfactant film and moving liquid on the dynamics of each other is turned to be significant. Thermo- and concentration-capillary forces on the surface, jointly with the buoyancy one in the volume, initiate the convection but after the heating stops, the non-monotonous propagation of the thermal front can be observed on the surface for some values of governing parameters. The calculations show that the characteristics of the motion are essentially different for temperature and concentration fields. This effect can be explained by the serious distinction of the kinetic coefficients for surfactant transfer and thermal processes.