Highly Anisotropic Vorticity Aligned Structures in the Rheology of Attractive Fractal Colloidal Systems

Chinedum Osuji and Ajay Negi. Chemical Engineering, Yale University, 302 Mason Laboratory, 9 Hillhouse Avenue, New Haven, CT 06511

Dilute dispersions of fractal particles in hydrocarbon solvents flocculate and form gels with typical scaling of elasticity with particle volume fraction. Surprisingly, these attractive systems display shear thickening at high shear rates involving the breakdown of dense particle clusters into smaller aggregates. Pre-shear within the high shear rate shear thickening regime leads to enhanced modulus gels where storage modulus scales as a power law with the pre-shear stress. We propose a simple scaling model that accounts for this in terms of stress controlled cluster number density. Shear-rate quenches from shear thickening flows into the quiescent state result in rapid gelation accompanied by slowly decaying internal stresses. Deformation of these mechanically quenched gels is highlighted by the transient formation of highly anisotropic vorticity aligned structures and the relaxation of residual internal stresses. We analyze the dynamics of these structures and of the relaxation of internal stresses in shear thickened gels. Similar vorticity aligned structures are present in a window of low shear rates in the steady response of the dispersions.

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