Criticality for shear-induced gelation of charge-stabilized colloids

Colloidal systems that are well stabilized by electrostatic repulsive forces can be activated by intense shear flow and transformed into solid-like gels, without adding any electrolyte. We have experimentally quantified the critical particle volume fraction for such a transition and found that it is a function of primary particle radius and shear rate. In particular, the values of the critical particle volume fraction obtained under different conditions can be represented through a single power-law function of the Breakage Number (Br), which is defined as the ratio between the shearing energy and the interparticle bonding energy. This finding indicates that, instead of shear rate or stress, the correct parameter quantifying the criticality for shear-induced gelation is Br. In addition, it is shown that the clusters formed in the shear aggregation process exhibit fractal scaling with fractal dimension equal to 2.4 +/- 0.04, independent of Br (i.e., of the shear stress, the particle size and the interparticle bonding energy). This is similar to the case of quiescent systems where the Brownian motion-induced aggregations, i.e., diffusion-limited and reaction-limited cluster aggregations, lead to clusters with fractal dimension equal to 1.8 +/- 0.05 and 2.1 +/- 0.05, respectively, which are independent of the particle type and size and the electrolyte concentration. Moreover, the ratio between the radius of gyration of clusters constructing the gel network and the primary particle radius at criticality decreases as Br increases, following a power-law scaling with exponent of -0.31, which is in good agreement with that for breakup of dense fractal clusters of the same fractal dimension in laminar flow.