Shear Flow and Relaxation of Soft Granular Particles at Controlled Volumes

We study fluid-immersed hydrogel spheres in response to steady shearing or cyclic cessations. Particles are slippery, deformable, and density-matched with interstitial fluid between geometrically roughed cones, allowing a survey of phenomena over a wide range of volume fractions. Measured flow curves reveal a nontrivial trend towards the dynamic yield stress, calling for an extension of the existing paradigms in theories for granular-fluid flows. We observe the timescales of relaxation upon shear cessations and point out their possible connection to the flow curves and to the characteristics of hydrogel. All assessments are accompanied by evidence from simultaneous refraction-index-matched imaging that captures the particle rearrangements inside the soft packing.