Pressure-Oscillatory Defoaming for Shear-thinning Fluids

Abstract

Enhancement of the rising velocity of air bubbles in shear-thinning fluids under oscillatory pressure field was studied experimentally. Application of mechanical pressure oscillations to cause cyclical change of bubble diameters can give rise to strong local flows around the bubbles that are sufficient to decrease the apparent viscosity of shear-thinning fluids and hence bubble rising velocity. Enhancement of the natural air bubble rising velocity of over 400 times has been obtained with shear-thinning fluids, whereas almost no effect is observed with Newtonian fluids. Time-series data of cyclical bubble diameter were obtained experimentally using a strobe scope and a video system to estimate the local shear rate and the local shear viscosity. The enhancement of bubble rising velocity estimated from the shear viscosity behaviour agreed well with the experimental data.