Simulation of surfactant transport during the rheological relaxation of two-dimensional dry foams

Micro-mechanical prediction of the effect of surfactant concentration and external friction on the visco-elasto-plastic response of an aqueous foam

We apply a combination of the Viscous Froth model and a surfactant transfer model [Zaccagnino et al., Phys. Rev. E, 2018, 98, 022801] to predict the rheological response of a two-dimensional dry aqueous foam. The model includes both the effect of friction between the foam and the boundaries of the container and also the dissipative effects on the film interfaces caused by surfactant motion. These dynamics are characterized by two free parameters: the Gibbs elasticity, relating surfactant concentration to interfacial tension, and the mobility of the surfactant molecules on the interfaces. We employ numerical simulations to evaluate the static shear modulus, yield stress and the storage and loss moduli of a foam and investigate the effect of our free parameters on these rheological properties.

Effect of surfactant redistribution on the flow and stability of foam films

The viscous froth model for two-dimensional (2D) dissipative foam rheology is combined with Marangoni-driven surfactant redistribution on a foam film. The model is used to study the flow of a 2D foam system consisting of one bubble partially filling a constricted channel and a single spanning film connecting it to the opposite channel wall. Gradients of surface tension arising from film deformation induce tangential flow that redistributes surfactant along the film. This redistribution, and the consequent changes in film tension, inhibit the structure from undergoing a foam-destroying topological change in which the spanning film leaves the bubble behind; foam stability is thereby increased. The system's behaviour is categorized by a Gibbs-Marangoni parameter, representing the ratio between the rate of motion in tangential and normal directions. Larger values of the Gibbs-Marangoni parameter induce greater variation in surface tension, increase the rate of surfactant redistribution and reduce the likelihood of topological changes. An intermediate regime is, however, identified in which the Gibbs-Marangoni parameter is large enough to create a significant gradient of surface tension but is not great enough to smooth out the flow-induced redistribution of surfactant entirely, resulting in non-monotonic variation in the bubble height, and hence in foam stability.

Dynamical Coupling between Connected Foam Films: Interface Transfer across the Menisci

The highly confined flow of the liquid phase, trapped between the gas bubbles, is at the origin of the large effective viscosity of the liquid foams. Despite the industrial relevance of this complex fluid, the foam viscosity remains difficult to predict because of the lack of flow characterization at the bubble scale. Using an original deformable frame, we provide the first experimental evidence of the interface transfer between a compressed film (or a stretched film) and its first neighbor, across their common meniscus. We measure this transfer velocity, which is a key boundary condition for local flows in foams. We also show the dramatic film thickness variation induced by this interface transfer, which may play an important role in the film thickness distribution of a 3D foam sample.