The paper addresses the question if there exists a finite stopping time for an unforced motion of a yield stress fluid with free surface. A variational inequality formulation is deduced for the problem of yield stress fluid dynamics with a free surface. The free surface is assumed to evolve with a normal velocity of the flow. We also consider capillary forces acting along the free surface. Based on the variational inequality formulation an energy equality is obtained, where kinetic and free energy rate of change is in a balance with the internal energy viscoplastic dissipation and the work of external forces. Further, the paper considers free small-amplitude oscillations of a droplet of Herschel-Bulkley fluid under the action of surface tension forces. Under certain assumptions it is shown that the finite stopping time T f of oscillations exists once the yield stress parameter is positive and the flow index α satisfies α ≥ 1. Results of several numerical experiments illustrate the analysis, reveal the dependence of T f on problem parameters and suggest an instantaneous transition of the whole drop from yielding state to the rigid one.

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