Among other results it was found that the dynamics of the In the volume fraction range (0.005,0.08), we have obtained aggregation also exhibited scaling behavior (8)(9)(10)(11)(12).
the temporal evolution of the structure factor S(q), in extensive It is currently believed that there are two limiting regimes numerical simulations of both diffusion-limited and reaction-limof colloid aggregation (13-16). Rapid, diffusion-limited ited colloid aggregation in three dimensions. We report the obsercolloid aggregation (DLCA) occurs when the aggregation vation of the scaling of this structure function in the diffusionis limited by the time taken for the clusters to encounter limited case, analogous to a spinodal decomposition type of scaleach other by diffusion. In this case each collision between ing. By comparing S(q) with the pair correlation function between diffusing clusters results in the formation of a bond. Slow, particles, we were able to identify the peak in the structure factor reaction-limited colloid aggregation (RLCA) occurs when as arising from the correlations between particles belonging to nearest-neighbor clusters. The exponents a and aЉ that relate the there is a substantial potential barrier between the particles. position and the height of the maximum in S(q) vs time, respec-In this other case only a small fraction of collisions between tively, were also obtained and shown to differ somewhat from the clusters results in the formation of a bond. For DLCA the spinodal decomposition exponents. We also found a terminal value of the fractal dimension is about 1.8, while the cluster shape for S(q) that corresponds to a close packing of the clusters size distribution function n s (t) takes the shape of a bell as after gelation. Moreover, this picture was shown to be valid in a a function of s, after a transient. In addition, the average concentration range larger than that suggested in recent expericluster size grows linearly with time. In the other case of ments. Although the S(q) for reaction-limited colloid aggregation RLCA the fractal dimension takes a value around 2.1, while does not show a pronounced peak for the earlier times, eventually there is an algebraic decay of n s (t), again after a transient, the peak stretches and becomes higher than that in the diffusiondefined by the exponent t (Å 1.5): n s (t) Ç s 0 t . Moreover, limited case. The S(q) curves, however, do not present the scaling there is now an exponential growth of the mean cluster size, shown for diffusion-limited aggregation.