Yielding of bentonite suspensions under oscillatory shear considering boundary slip and shear inhomogeneity

The rheology of bentonite suspensions under oscillatory shear flow was investigated using parallel-plate shearing geometries, considering both smooth and rough boundaries. With a smooth boundary, the oscillatory response consistently exhibited a two-step yielding phenomenon, which was identified as the onset of slip at the boundary followed by yielding in the bulk. With a rough boundary, a unique yielding transition was observed, accounting for the bulk yielding alone. Furthermore, considering the effect of shearing frequency, the yield stress for smooth boundary increased more remarkably with frequency compared to the condition using rough boundaries. The concentration dependence of the oscillatory characteristic stresses could be described by a Derjaguin–Landau–Verwey–Overbeek-based semi-empirical model adapted from the framework previously established for steady shear, indicating that interparticle interactions provide a physical basis for the observed concentration-dependent stress evolution, while empirical parameters account for the effects of particle arrangement and boundary conditions. These results provide a unified interpretation of slip-related and bulk-yield transitions in structured clay suspensions under oscillatory loading and clarify the coupled roles of boundary roughness, gap, and frequency.