Abstract
About 30 years ago, Professor Alvin W. Nienow co-authored an experimental paper which reported slow motion of tracer particles, both close inside and outside of a cavern boundary formed in stirred Carbopol 940 microgel, however no theoretical explanation of the motion outside the cavern was given. Here, the motion of the particles outside the cavern was explained based on current knowledge, supported by rheological measurements and CFD modelling. According to current understanding of the yield stress fluids and the results of numerical simulation, the flow outside the cavern can result from presence of slip velocity of solid microgel at vessel wall and/or from periodic radial fluctuations of cavern boundaries during rotation.
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