Rheological properties of wet foams generated from viscous pseudoplastic fluids

Foams generated from viscous non-Newtonian fluids have received insufficient attention in the literature despite their wide ranging industrial applications. We study the steady shear flow and viscoelastic properties of wet foams generated in a continuous multi rotor-stator pilot-scale device. The foams are generated from viscous pseudoplastic liquids consisting of aqueous solutions of Xanthan gum and caster sugar, and stabilised with a non-ionic food grade surfactant (PGE 55). Rheological measurements were conducted inside a parallel plate geometry on a controlled stress/strain rheometer. The effects of foam microstructural properties as defined by the process parameters (rotor speed, foam air fraction) and liquid composition (Xanthan gum concentration) are elucidated. Simultaneous in-situ visualisation of the foam during shearing revealed for the first time the existence of inward shear-induced radial migration of liquid at high shear rates which is responsible for the time-dependence behaviour of the foams. The foam flow curves were successfully fitted to a generalised dimensionless form of the Carreau-Yasuda model with data for all conditions investigated collapsing approximately on one single curve, which allows prediction of foam apparent viscosity as a function of air volume fraction.