Dynamic driving eliminates volume fraction inhomogeneity and apparent yield stress in flowing dense non-Brownian suspensions

Dense suspensions of non-Brownian hard spheres are often characterized as yield-stress fluids despite having no intrinsic time or force scales that could lead to such rheology. One mechanism for the apparent yield stress is particle migration, which produces (or is caused by) inhomogeneous flow conditions and leads to local regions where the solids' content approaches or exceeds the limit of flowability. In such a scenario, one does not induce flow by exceeding a yield stress, but instead by exploring the only remaining control parameter, namely the flow history. We demonstrate using particle-based simulation that this apparent local yield stress behavior does indeed emerge in a model dense suspension of non-Brownian hard spheres and that it can be eliminated by imposing a time-varying flow field.