Kinetics of colloidal gelation and scaling of the gelation point

We have investigated in situ the colloidal gelation in the RLCA regime using a small-angle light scattering technique, avoiding any sample handling, in the particle volume fraction range of 0.02 to 0.08, and observed some specific gelation features never indicated previously. The measured structure evolutions clearly show that colloidal gelation follows two stages: (1) cluster growth and (2) cluster interconnection when crowded to form the gel. In Stage 1, the aggregation kinetics in all the cases follows the power-law scaling, as if it is in the DLCA regime. This indicates that crossover from RLCA to DLCA kinetics takes place along the gelation process. When an appropriate dimensionless time is defined to account for the role of particle concentration and stability, all the kinetic data in Stage 1 obtained at different particle sizes and concentrations and different salt concentrations collapse to form a single master curve. Thus, all the gelation systems follow the same aggregation kinetics and the formed clusters have the same structure. In Stage 2, the average clusters grow only slightly with time. This allows us to distinctly determine the gelation point by the transition from the aggregation kinetics before the gelation point to the slow cluster growth in the gel. It is found that at the gelation point, the volume fraction occupied by all the clusters is constant, independent of the particle volume fraction. This is strong evidence that for a given colloid, the same crowding condition has to be reached in order to start the percolation forming a gel. Interestingly, this crowding condition depends on the particle size. It is observed that the occupied volume fraction at the gelation point is substantially larger for a colloid with a smaller particle radius than for a colloid with a larger radius.