Effect of particle-size dynamics on properties of dense spongy-particle systems: Approach towards equilibrium

Open-porous deformable particles, often envisaged as sponges, are ubiquitous in biological and industrial systems (e.g. casein micelles in dairy products and microgels in cosmetics). The rich behavior of these suspensions is owing to the elasticity of the supporting network of the particle, and the viscosity of permeating solvent. Therefore, the rate-dependent size change of these particles depends on their structure, i.e. the permeability. This work aims at investigating the effect of the particle-size dynamics and the underlying particle structure, i.e. the particle permeability, on the transient and long-time behavior of suspensions of spongy particles in the absence of applied deformation, using the dynamic two-scale model developed by Hütter et al. [Faraday Discuss., 158, 407-424 (2012)]. In the high-density limit, the transient behavior is found to be accelerated by the particle-size dynamics, even at average size-changes as small as 1%. The accelerated dynamics is evidenced by (i) the higher short-time diffusion coefficient as compared to elastic-particle systems and (ii) the accelerated formation of the stable FCC crystal structure. Furthermore, after long times, the particle-size dynamics of spongy particles is shown to result in lower stationary values of the energy and normal stresses as compared to elastic-particle systems. This dependence of the long-time behavior of these systems on the permeability, that essentially is a transport coefficient and hence must not affect the equilibrium properties, confirms that full equilibration has not been reached.