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

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 +0.6 gamma(c,r) similar to phi((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 anisotropy coincides with a maximum in shear stress. Qualitative differences between the transient anisotropy of fractal gels comprised of spheres and rods support the conclusion that the microstructural origin of the anisotropy maximum in sphere aggregates is the free rotation of singly connected regions of the gel backbone. (C) 2005 The Society of Rheology.