Effects of Volume Conservation on Plateau Border Sag in Foams

In the sagging Plateau border model, borders which are loaded with the weight of the liquid that they contain are distorted normal to their length and in varying amounts along their length. Applied to a 3D foam structure, this would result in artificial variations in bubble size/volume, so a Surface Evolver model is developed for conserving the volume of bubbles adjacent to the sagging Plateau border. As a result of this modification to the sagging Plateau border model, the sag is reduced and the adjacent films bulge. An analytical solution for the case of conserved bubble volume shows good agreement with the Surface Evolver simulations. With increased loading the three-film structure collapses under its own weight–as is also the case for unconserved bubble volume. However, the loading that can be supported by the structure is greater with volume conservation than without. Finally, to extend these theoretical results and to obtain an outlook for the use of the sagging border model to simulate the limit of stability of foam structure, the onset of structural rearrangement or ‘convective (structural) instability’ is estimated from collapse in the Surface Evolver simulations.