STABILITY OF EVAPORATING 2-LAYERED LIQUID-FILM IN THE PRESENCE OF SURFACTANT - II - LINEAR-ANALYSIS

In this study we perform a linear analysis of the stability of a two-l ayered liquid him which is evaporating upon a horizontal solid substra te. The film contains surfactant that is soluble in both liquid phases . The problem was solved in general form, in our previous work [cf. Da nov et al. (1997b) Chem. Engng Sci., submitted] by using lubrication a pproximation. The governing equations for the film thickness and the s urfactant concentration derived there are treated using linear analysi s in the present study. The resulting system of ordinary differential equations describes the evolution of small long-wave disturbances of t he film shape and the surfactant distribution during the film evaporat ion. The model allows one to investigate the role of different factors on the film stability. To illustrate the effects we consider a partic ular system: water-hexane him evaporating on a PVC plate. The limiting cases of pure liquids and tangentially immobile surfaces, as well as the general case are studied numerically. Both cases when a low- and a high-molecular weight surfactants present into the film are investiga ted. The numerical results in the case of non-linear stability analysi s of a PVC/tetrachlorethane/water/vapour evaporating film are subject of part III [cf. Paunov et al. (1997) Chem. Engng Sci., submitted]. (C ) 1998 Elsevier Science Ltd. All rights reserved.