Stress relaxation of dense spongy-particle systems

Spongy particles have a permeable structure that allows them to undergo rate-dependent volume changes as their elastic network takes up or expels the viscous suspending solvent. Their ability to be jammed well above random close-packing makes them particularly attractive in applications where tailoring the overall properties is a requirement such as pharmaceuticals and foods. In this work, we independently vary the particle modulus and the particle permeability to study their effect on the stress-relaxation behavior of jammed permeable-particle suspensions. The dynamic two-scale model developed by Hütter et al. [Faraday Discuss. 158, 407-424 (2012)], which explicitly accounts for the particle size dynamics, is used for this purpose. We perform flow-cessation simulations of dense permeable-particle systems subjected to different preshear deformations. The stress relaxation occurs on shorter time scales in the case of permeable particles compared to impermeable particles. In terms of particle dynamics, stress relaxation is found to be promoted primarily by the motion of the particles within the cages formed by the surrounding particles, rather than by cage escape. The stress-relaxation process is accelerated by the permeability of spongy particles, namely, due to the sustained volume change that was induced during preshear, which renders their cages less effective.