Foam coarsening in a yield stress fluid

Abstract

Foams coarsen because of pressure differences between bubbles of different sizes. We study the coarsening of quasi-2D foams made from model yield stress fluids: concentrated oil-in-water emulsions. We show that increasing the yield stress of the foamed emulsion continuous phase leads to both slower coarsening and irreversible structural change. The impact of the continuous phase rheology is stronger when the foamed emulsion is wetter or more confined. The bubble growth and organisation both become highly heterogeneous with an excess of small bubbles. We present a model that rationalises the impact of these three parameters by taking into account a resisting pressure required to displace the yield stress fluid around the bubbles.