The rapid elimination of tiny gas bubbles from highly viscous and/or viscoelastic fluids remains a significant challenge. We have proposed a new and effective method (Pressure Oscillating Defoaming; “POD” for short) for increasing the bubble rising velocity in such fluids based on application of an oscillating pressure. A bubble undergoes alternating expansion and contraction under pressure-oscillation field, where the oscillating flow decreases quadratically with the distance from the surface, giving biaxial elongational flow and shear flow in the vicinity of the bubble surface. When two air bubbles are set vertically in a quartz cell filled with shear-thinning fluids, the lower-bubble rises faster than the upper bubble under pressure-oscillating field generated by pressing a rubber sheet at the bottom of the cell. However, this mechanism of the behavior has not been fully understood. By using liquid with a good photoelastic correlation and rheological property similar to the UCM model, we try measuring the stress in the vicinity of small bubbles under pressure-oscillating field. In the case of single bubble, the stress profile measured by the birefringence flow technique was good agreement with stress profile calculated by unsteady state finite element analysis. In this study, retardation profiles were measured near and between the bubbles. In the closer bubble case, the higher retardation profile was observed between the bubbles under pressure-oscillating field.

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