Rheology of densely packed soft particles

We measure the rheological response of densely packed hydrogel particles in a density-matched solution over different volume fractions. The particles are sandwiched between two roughened cone-shaped surfaces. The top surface, driven by a motor, provides an oscillatory shear of variable strain amplitude. The irreversibility and the thixotropic loop observed in the stress-strain and stress-strain rate curves illustrate the transition from a nearly elastic response to a plastic flow. At large strain amplitudes and low oscillation frequencies, the system approaches the steady state and the intercept of stress extrapolated to zero strain rate becomes zero below a well-defined volume fraction. Internal imaging of immersed tracers provides clues behind these transitions.