Gelation and internal dynamics of colloidal rod aggregates.

The internal dynamics of fractal cluster gels of colloidal boehmite rods with aspect ratios r = 3.9, 8.6, and 30.1, and colloidal polystyrene spheres (r = 1) are reported. Increasing r decreases the minimum colloid volume fraction for gelation. The behavior of the dynamic structure factor of rod gels is consistent with the internal dynamics of a constrained Brownian fractal object. Colloidal boehmite gels display an abrupt transition from floppy to brittle dynamics at phi approximately 10(-4). Moreover, the fractal cluster size of rod gels is not the determinant of the relaxation time of density fluctuations as it is in spherical particle gels. Instead, the relative behavior of the magnitude and time scale of the constrained fluctuations suggests that the fractal rod network is viscously coupled only on local, rather than cluster, scales. We hypothesize that noncentral forces between the anisometric particles are responsible for this anomalous behavior.