Dynamics of free liquid films during formation of polymer foams

Determination of thinning rates of free liquid films is important for many applications involving formation and stability of foams. One of examples is self-foaming of polymer melts. The morphology of the resulting foam is governed by the thinning rates of interbubble films and by the thinning rates of free films separating the bubbles from the ambient gas. The film thinning dynamics depends on the rheological properties of the liquid. In this theoretical and numerical work a model describing the thinning of a free liquid film is developed for viscoelastic liquids. The upper-convected Maxwell model is used for the description of liquid rheology. We apply an asymptotic analysis to derive the equations governing thin film evolution. The evolution equation is solved for the lamellar region and for the region of transition between the thinning lamella and the Plateau border. The effect of the relaxation time on the film profile and its evolution is quantified. We find out that the film thinning rate increases with increasing relaxation time.