Dynamic light-scattering study of aggregating and gelling colloidal disks

We have studied the relaxation of the concentration fluctuations of gelling disk-like clay particles (Laponite) in water containing 5 mM NaCl. At a La ponite concentration of 10 g/l a gel is formed that does not flow when tilt ed. The comparison of dynamic light scattering and dynamic mechanical measu rements shows that the terminal relaxation time of concentration fluctuatio ns diverges not at the gel point, but only when the gel is sufficiently den se to inhibit motion over the length scale on the order of the inverse scat tering wave vector. The initial decay of the concentration fluctuations is due to cooperative diffusion and is insensitive to the gel formation. At a concentration of 1 g/L, large aggregates that slowly precipitate are formed . The scattering wave vector dependence of the decay of the concentration f luctuations shows that the aggregates are semi-flexible. (C) 2001 Academic Press.