Internal motion of soft granular particles under circular shearing: Rate-dependent quaking and its spatial structure

Tightly packed granular particles under shear often exhibit intriguing intermittencies, specifically, sudden stress drops that we refer to as quaking. To probe the nature of this phenomenon, we prototype a circular shear cell that is capable of imposing a uniform and unlimited shear strain under quasistatic cyclic driving. Spherical PDMS (polydimethylsiloxane) particles, immersed in fluid, are driven in a fixed total volume at a wide range of shear rates, with particle trajectories captured in three-dimensional space via refraction-index-matched fluorescent tomography. Statistics on the magnitude of fluctuating displacements of individual particles show a distinct dependence on the shear rate. Particles move smoothly at high shear rates. At intermediate shear rates, quaking emerges with clusters of particles exhibiting relatively large displacements. At low shear rates, a cluster can span the entire system. and the cluster exhibits substructures in view of localized particle movements. Overall, we have confirmed that the quaking phenomena in the current setup are consistent with our previous work [] and that the dimensionless shear rate that we have defined [] is indeed a good parameter for unifying the transitions observed in different experimental geometries.