Structure and formation of a gel of colloidal disks

We have performed static scattering experiments on the transition in time from a fluidlike sol to a solidlike gel of a suspension of disk-shaped charged colloidal particles. The combination of static light scattering and small angle x-ray scattering probes more than three orders of magnitude in the scattering vector q. At the smallest q the static structure factor S(q) shows a q(-d) dependence in both the sol and eel state. The algebraic exponent d evolves from 2.8 to 2.1 during the gelation. We find that the sol is not comparable to a simple liquid but rather to a low-viscosity precursor of the gel. At intermediate q a plateau connects this regime to the form factor (F-2) over bar(q) of the colloidal disks, which is observed at the largest q. On the plateau a small peak related to nearest-neighbor correlations is observed, which decays before gelation occurs. After application of shear on the suspensions we have observed the rapid formation of nematiclike order of the colloidal disks. This order decays in time due to reorientation of the colloidal disks while the final gel state is reached. The formation of the gel does not proceed via aggregation to form ever larger clusters. Based on our findings we propose that reorientation of the charged particles is the mechanism by which the gelation occurs. [S1063-651X(98)06702-6].