STRUCTURE AND FORMATION OF A GEL OF COLLOIDAL DISKS

We have performed static scattering experiments on the transition in t ime from a fluidlike sol to a solidlike gel of a suspension of disk-sh aped charged colloidal particles. The combination of static light scat tering 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 ge lation. 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 pl ateau 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, whic h decays before gelation occurs. After application of shear on the sus pensions 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 formati on of the gel does not proceed via aggregation to form ever larger clu sters. Based on our findings we propose that reorientation of the char ged particles is the mechanism by which the gelation occurs. [S1063-65 1X(98)06702-6].