Yield stress and elasticity of aqueous foams from protein and surfactant solutions – The role of continuous phase viscosity and interfacial properties

We discuss the effect of solvent viscosity ηL and interfacial elasticity (E′, G′) on apparent yield stress τy and storage modulus G0 of protein and surfactant foams made from solutions of these amphiphiles in various water/glycerol mixtures. The critical volume fraction at which τy and G0 occur is calculated from the bubble size distribution and is related to the adsorption kinetics of the corresponding amphiphile. Dependence of τy on ηL is weak (τy ∼ ηL0.3). Generally, higher interfacial moduli correspond to higher τy and G0, but the relationship is non-trivial when protein interaction and structure formation get relevant. Increasing glycerol fraction reduces electrostatic interaction range and solvent quality for the proteins. This leads to an increase in E′ as well as G0 for casein. For whey protein isolate (WPI) at 1% concentration, this results in an increase in G0, a decrease in E′, and a broad transition region between linear and non-linear stress response. These findings are consistently attributed to protein aggregation finally resulting in network formation across lamellae. This network does not form at 0.1% WPI concentration and accordingly G0 increases with WPI concentration. In contrast, τy remains constant suggesting that this network is destroyed at this stress level.