ROLE OF SURFACE FORCES IN THE STABILITY OF EVAPORATING THIN LIQUID-FILMS THAT CONTAIN SURFACTANT MICELLES

We consider an evaporating liquid him which lies on a planar heated so lid substrate. The film contains a dissolved surfactant at a high conc entration, so that micellar aggregates exist in the bulk. Linear stabi lity analysis of this system is performed by investigating the time ev olution of the amplitude of fluctuation waves. The liquid-vapor interf ace is regarded as a two-dimensional continuum characterized by intrin sic viscosity, specific adsorption, and surface tension. The latter qu antities depend on the instantaneous subsurface concentration of surfa ctant monomers (subject to fluctuation) and upon the temperature. At s mall Reynolds and large Peclet numbers, and for thin films, the lubric ation approximation model can be applied to solve the hydrodynamic pro blem. In the balance of normal stress at the fluid interface, we accou nt for the contribution of intermolecular forces. There are van der Wa als, electrostatic, steric, and oscillatory structural interactions, w hich are described in terms of separate components of disjoining press ure. The oscillatory structural forces are due to the presence of surf actant micelles or other colloidal particles in the film. These forces turn out to have the highest magnitude, and are of great importance f or the stability. We solve numerically the evolution equation for the fluctuation, thus finding the critical thickness of film rupture and t he critical lateral wave number. The influence of the surfactant type and concentration and the relative significance of the particular inte ractions under different conditions are discussed in detail. (C) 1998 Academic Press.