The role of surfactant type and bubble surface mobility in foam rheology

Abstract

This paper is an overview of our recent understanding of the effects of surfactant type and bubble surface mobility on foam rheological properties. The focus is on the viscous friction between bubbles in steadily sheared foams, as well as between bubbles and confining solid wall. Large set of experimental results is reviewed to demonstrate that two qualitatively different classes of surfactants can be clearly distinguished. The first class is represented by the typical synthetic surfactants (such as sodium dodecylsulfate) which are characterised with low surface modulus and fast relaxation of the surface tension after a rapid change of surface area. In contrast, the second class of surfactants exhibits high surface modulus and relatively slow relaxation of the surface tension. Typical examples for this class are the sodium and potassium salts of fatty acids (alkylcarboxylic acids), such as lauric and myristic acids. With respect to foam rheology, the second class of surfactants leads to significantly higher viscous stress and to different scaling laws of the shear stress vs. shear rate in flowing foams. The reasons for these differences are discussed from the viewpoint of the mechanisms of viscous dissipation of energy in sheared foams and the respective theoretical models. The process of bubble breakup in sheared foams (determining the final bubble-size distribution after foam shearing) is also discussed, because the experimental results and their analysis show that this phenomenon is controlled by foam rheological properties.

Damping of liquid sloshing by foams

“…43 Thus, the friction force of the Plateau borders on two solid walls can be estimated as f b = K γ f Ca 2/3 , where Ca = µ f v b /γ f is the Capillary number and K is a constant for given geometrical parameters describing the Plateau borders. 35,37 The typical Capillary number considered in our experiments is of order Ca ∼ 3 × 10 −3 , where µ f and γ f are the viscosity and surface tension of the foaming solution, respectively.…”

“…The surfactant molecules lead to a stable foam with low surface modulus. 43 Glycerol is used to slow down the aging of the foam associated with the drainage and the coarsening processes. 30 In addition, the experiments are performed on short time scales, typically less than few minutes, to avoid such aging.…”

Response of a two-dimensional liquid foam to air injection: Influence of surfactants, critical velocities and branched fracture

“…We have measured the viscosity of the solutions η, the surface tension σ, and the surface modulus E D quantifying the interfacial dilatational viscoelasticity [3]. For the SDS solution, η = 1.2 mPa.s, σ = 36.…”

“…First, we will study the influence of surface rheology, yet unexplored in the configuration of air injection in foam. Two limiting regimes have been identified [3]: the "mobile" limit, where surfactants confer to the gas/liquid interfaces a negligible viscoelasticity, and the incompressible (or "immobile") limit, where surfactants at interfaces display a high viscoelasticity, hence a strong resistance to interfacial compression. In terms of hydrodynamic boundary conditions, the "mobile" limit gives free shear, and the incompressible one no slip.…”

“…Understanding this intricate macroscopic behaviour in relation with considerations at the scale of single bubbles and films motivates active research [3,4,5], with open questions on e.g. shear localisation [6] or nonlocal effects [7].…”