Viscosity of soft spherical micro-hydrogel suspensions

• Maron–Pierce/Quemada hard sphere model predicts the viscosity of microgel suspensions up to random close packing (RCP).
• RCP is predicted independent of viscosity using particle size distribution.
• Particle elasticity does not directly influence suspension viscosity below RCP.
• Apparent dependence of viscosity on particle modulus is due to compression of the particles during sample preparation.

The rheology of soft particle suspensions is considered to be a function of particle micromechanics and phase volume. However, soft particles such as microgels present a challenge because they typically contain solvent in their polymeric network structure, and their specific volume can alter in response to mechanical forces and physiochemical effects. We investigate how particle elasticity affects the viscosity of microgel suspensions as a function of effective phase volume (ϕ0) using non-colloidal hydrogel spheres that, unlike many colloidal-scale microgels, are not highly responsive to physiochemical effects. In our unique approach, we compare the viscosity of microgel suspensions to a theoretical hard sphere viscosity model that defines the maximum packing fraction using the geometric random close packing fraction (ϕrcp) obtained from the measured particle size distribution. We discover that our harder microgels follow the hard sphere model up to random close packing, but softer microgels deviate around ϕ0/ϕrcp ∼ 50% which indicates that their specific volume is decreasing with increasing ϕ0. This effect arises because microgels at high phase volumes do not fully re-swell during their preparation. We conclude that particle elasticity does not directly affect the viscosity of soft sphere suspensions up to the random close packing fraction. We highlight a convenient method for analysing the viscosity of microgel suspensions with potential to be applied to a wide variety of soft sphere suspensions.

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