Microstructure-dependent viscosity in concentrated suspensions of soft spheres

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Abstract
Monodisperse colloidal suspensions of polymethylmethacrylate spheres swollen in benzyl alcohol have been rheologically examined under applied steady and oscillatory shear while simultaneously monitoring microstructure via light scattering. In concentrated samples, long-lived nonequilibrium microstructures can be induced, corresponding to random hexagonal-close-packed planes (hcp) stacked in the direction of the shear gradient. The direction of closest packing within each hcp plane can be oriented along either the vorticity or direction of flow. Creep and creep recovery measurements have been examined for each of these two orientations as a function of particle concentration and stress. Results indicate a strain-dependent dissipative process correlated with changes sample microstructure, while dynamic measurements of the storage modulus show no significant difference between microstructures. We argue that instantaneous viscosities can be measured and show how they are correlated with changes in particle microstructure. The observed elastic response in these suspensions will be shown to be due to local microstructure and particle deformation.