Shear modulus and yield stress of foams: contribution of interfacial elasticity

Abstract

The link between interfacial elasticity of foaming solutions and the elasticity and yield stress of their aqueous foams is probed for a variety of surfactant, block-copolymer, protein, food, and particle-stabilized (Pickering) foams. We measured interfacial tension σ and interfacial elastic moduli of foaming solutions in dilation E_∞ as well as in shear at concentrations suitable for foaming and compared them to the shear modulus and yield stress of corresponding foams normalized by bubbles’ Sauter radius R₃₂ and foams’ gas volume fraction. The interfacial shear modulus was only measurable for the foaming solutions including proteins or nanoparticles. For these systems the foam shear modulus scaled reasonably well with . The interfacial dilational modulus was accessible for all investigated systems and the foam shear modulus as well as yield stress scaled with a generalized Laplace pressure (σ + 2E_∞)/R₃₂. But foams stabilized by nanoparticles or aggregated proteins exhibited even higher shear modulus and yield stress values not captured by the proposed scaling with the generalized Laplace pressure and also show an unexpectedly high dependence of these characteristics on the gas volume fraction. We attribute this to attractive forces between particles and/or structure formation across the lamellae that become increasingly dominant as the lamellae narrow down during foam drainage.