Orientation and rupture of fractal colloidal gels during start-up of steady shear flow

Abstract

The transient structural evolution of polystyrene colloidal gels with fractal structure is quantified during start-up of steady shear flow by time-resolved small-angle light scattering and rheometry. Three distinct regimes are identified in the velocity-gradient plane: structural orientation, network breakup, and cluster densification. Structural anisotropy in the first regime is a universal function of applied strain. Flow cessation in this regime shows a lack of structural relaxation for Pe⪡1, where Pe is the Peclet number. In the second regime, the anisotropy attains a maximum value before monotonically decreasing. The volume fraction dependence of the critical strain for maximum anisotropy follows the scaling: 1+0.6γc,r∼ϕ(1−x)(3−D). Here x and D are the backbone and cluster fractal dimensions, respectively. This scaling agrees with the simple model of a gel network that ruptures after the cluster backbone is extended affinely to its full length. Rheological measurements demonstrate that the maximum an...