Particles, drops and bubbles moving in an otherwise quiescent visco-plastic material are totally engulfed by bounded yielded regions, surrounded by un-yielded domains. Theoretical predictions fail to agree on the shape and extent of such regions, partly because of over-simplified rheological models and partly due to the regularization used in the numerical codes.
We report on a series of studies aimed to identify the shape of the yielded region surrounding drops or particles, isolated and interacting, slowly moving in a transparent Carbopol solution. The experiments involve direct photography and PIV and PTV techniques. Yielded region boundaries are established and the flow fields within them are reported. The results are compared to theoretical predictions. It is shown that even small amount of elastic and thixotropic effects are reflected in the boundary shape as deviation from the theoretically predicted fore-and-aft symmetry and the appearance of wakes. The flow of a heterogeneous visco-plastic material, such as Carbopol gel, near a solid boundary may results with an apparent interfacial slip.
