Soft Granular Particles Sheared at Controlled Volumes: Flow Curves and Multiple Relaxations.

We study the responses of fluid-immersed soft hydrogel spheres to both steady shearing and cyclic cessations. The slippery, deformable particles along with the density-matched interstitial fluid are sandwiched between two opposing rough cones, allowing the rheological measurements for volume fractions ranging from 0.71 to 0.26. Steady-state flow curves and shear-to-normal stress ratios reveal nontrivial trends over the decrease of volume fraction, revealing a solid-to-fluid transition in conjunction with the changing Reynolds number as the system becomes a suspension. In addition, studies with cyclic shearing and cessations at high volume fractions, accompanied by refractive index-matched internal imaging, reveal the connection between stress relaxation and particle rearrangement. Our studies also include the long-time relaxation of the packing as well as that of a single particle. These results demonstrate a multitude of timescales involved in the flow of soft particles, offering test grounds for the extension of theories in granular-fluid flows.