Journal of Colloid and Interface Science, Vol.442, 75-81, 2015
Viscosity of soft spherical micro-hydrogel suspensions
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 (phi(0)) 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 (phi(rcp)) 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 phi(0)/phi(rcp) similar to 50% which indicates that their specific volume is decreasing with increasing phi(0). 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. (C) 2014 Elsevier Inc. All rights reserved.
Keywords:Suspension rheology;Maximum packing fraction;Soft spheres;Phase volume;Random close packing;Hydrogel particles;Microgels