The coupling between foam viscoelasticity and interfacial rheology

We study the impact of interfacial rheology on the linear viscoelastic relaxations of disordered 3D foams. Their complex shear modulus G* = G′ + iG′′ is measured as a function of frequency, bubble size, liquid viscosity and surface dilatational modulus. We show that for foams with very rigid interfaces the variations of G* with frequency f between 1 and 100 Hz deviate from the behavior previously predicted as a generic consequence of topological disorder and observed for foams with mobile or moderately rigid interfaces. Our experiments demonstrate that the relaxations slow down as the bubble size or the foaming liquid viscosity increases. We show under which conditions, depending on the interfacial rigidity, the loss factor G′′/G′ can be collapsed on master curves by applying a scale factor to the frequency. The dependence of this scale factor on the bubble size and viscosity constitutes a robust criterion helping to identify the dominant dissipation mechanism.