Abstract
The bubble motion in non-Newtonian fluids plays a major role in many industrial processes. However, the knowledge in this domain is still very deficient. The present study provides new insights into the flow of non-Newtonian fluids around bubbles, by means of both a modern technique — particle image velocimetry (PIV) and a classical method — birefringence visualisation. The complete flow field around bubbles rising in polyacrylamide (PAAm) solutions shows for the first time the coexistence of three distinct zones: a central downward flow behind the bubble (negative wake), a conical upward flow surrounding the negative wake zone, and an upward flow zone in front of the bubble. This flow configuration is totally different from that in a Newtonian fluid. The birefringence visualisation reveals a butterfly-like spatial distribution of shear stresses between the conical upward flow and the negative wake. These stresses display a temporarily reduced local fluid viscosity which is responsible for the interactions and coalescence between bubbles rising in such media.
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