Cloaking: particles in a yield-stress fluid

Single particles in a yield stress fluid move only when the driving body force exceeds a critical value depending on the yield stress and particle shape. The critical limit is not unique and here we show that for the same critical limit we may have different shaped particles that are cloaked inside the same unyielded envelope. The critical limit (or critical plastic drag coefficient) is related to the unyielded envelope rather than the particle shape. We show how to calculate the unyielded envelope directly. We then use the theory of perfect plasticity to construct an estimate of the critical plastic drag coefficient that appears to coincide closely with that computed from the limiting viscoplastic flows.