Differences between protein and surfactant foams: Microscopic properties, stability and coarsening

We report results on foamability, stability and coarsening of foams made either of surfactant (SDS) or of milk protein (casein) solutions. Studies have been performed at the scales of the gas-liquid interface, thin liquid film and bubble size, in order to find the correlations between these different scales, and to elucidate the microscopic origins of the macroscopic features. For both systems, foamability concentration thresholds have been measured, and a bubble size dependence has been found. A clear correlation between the stability of an isolated thin film and the foam stability is always evidenced. However, the mechanism of stability of the casein thin films is different from the surfactant one, and related to the confinement and percolation of casein aggregates. We also report results on coarsening at constant liquid fraction, showing that the protein foams coarsen more slowly than the surfactant ones, and that it is due to differences in thin film thickness.