Rheology of colloidal gas aphrons (microfoams) made from different surfactants

This paper extends our previous study on microfoam rheology made from non-ionic (Tween 20) surfactants to ionic surfactants. Anionic (sodium dodecyl sulfate) and cationic (cetyl trimethylammonium bromide) surfactants were used to generate microfoams by stirring an aqueous surfactant solution at high speed in a baffled beaker. Pipe flow experiments were performed in cylindrical stainless steel pipe 1.5 mm in diameter under adiabatic and fully developed laminar flow conditions. The porosity ϕ, bubble size distribution, Sauter mean radius r32, surface tension σ, and pH was reported for each solution. The porosity varied between 0.54 and 0.72 while the Sauter mean radius ranged from 28 to 48 μm. Zero slip velocity was assumed to prevail at the foam–wall interface as previously observed and reported in the literature for stainless steel pipes. Volume equalized method was used to analyze the data obtained from pipe flow viscometer. In all cases, microfoams behave as a shear thinning fluid. The results suggest that the dimensionless wall shear stress τw*=τwr32/σε is proportional to (Ca*)m defined as Ca*=μℓr32γ˙a/σε where τw is the wall shear stress, γ˙a is the shear rate, σ is the surface tension, μℓ is the liquid velocity, and ɛ = 1/(1 − ϕ) is the specific expansion ratio. The average value of the power–law index m was found to be 0.64 ± 0.04 with 95% confidence interval.