Measurement of stress between and around 2 bubbles by the flow birefringence measurement under pressure-oscillating field and its application

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2D retardation distributions are measured by the high speed polarization camera near the air bubble under pressure-oscillating field. A CTAB/NaSal (a mixture of cetyl-trimethylammonium bromide and Sodium Salicylate) aqueous solution is used as a test fluid. Since it obeys stress-optic rule and the rheological property is similar to the UCM model, the flow stress in the vicinity of the small air bubble under pressure-oscillating field could be evaluated by the 2D optical anisotropy. Volume of the test bubbles is around 2 cubic mm. The results show alternative interesting patterns clearly over a period. The first one is strong retardation (i.e., larger birefringent phase difference) at the tail of the cusped bubble in the bubble compression phase due to the negative wake. And the other one is a weak retardation (i.e., relatively smaller birefringent phase difference) only near the bubble surface due to the elongational flow in the bubble expansion phase.