Abstract: Drainage of vertical foam film containing insoluble surfactant is investigated with consideration of combined effect of disjoining pressure and surface viscosity. Lubrication theory is applied to establish evolution equations of film thickness, surface concentration of insoluble surfactant and surface velocity. Film evolution under different initial surfactant concentration and gradient are simulated numerically. It shows that in the absence of initial surfactant concentration, film drainage process is very short and tends to break quickly, whereas lifetime of foam film is prolonged by adding surfactant to film surface. Under relatively low surfactant concentration, Marangoni effect is not enough to defeat gravity drainage, and "black film" cannot exist stably. Increasing concentration of initial surfactant reduces surface velocity of film and strengthens rigidity of film surface, and film stability is enhanced. Influence of concentration gradient of surfactant on drainage process is highlighted with evident reduction of surface velocity in initial stage of film drainage.

Key words: vertical film, drainage, disjoining pressure, surface viscosity, surfactant