From sedimentation to suspension: Critical strain as a predictor of particle resuspension thresholds

Viscous resuspension, the process by which sedimented particles are re-entrained into a fluid under flow, is central to numerous natural and industrial systems, including environmental contaminant transport, riverbed erosion, and biogeochemical cycling. Despite its importance, predicting when and how resuspension occurs remains challenging, particularly under oscillatory shear, where particle interactions are nonlinear, collective, and time-dependent. Here, we examine the resuspension dynamics of dense, non-Brownian suspensions under both steady and oscillatory shear using bulk rheometry and in situ rheo-microscopy over a broad range of particle volume fractions (ϕ=0.30–0.55). We show that resuspension cannot be described by the shear rate alone; rather, particle motion requires a critical stress condition (ShieldsNumber≈1), while the extent of resuspension is governed by the accumulated strain. This strain-driven evolution is mediated by effective interparticle collisions and collective particle motion. We develop a predictive model that captures the observed strain thresholds as a function of volume fraction, allowing for the construction of a state diagram delineating sedimentation, resuspension, and full suspension regimes. These findings indicate that, under the conditions investigated, strain governs the progression of resuspension once the stress threshold is exceeded and provide a framework for describing suspension behavior across steady and oscillatory flows. This framework offers mechanistic insight into particle resuspension and inform the understanding of related transport phenomena in more complex systems.