Two-dimensional colloidal aggregation mediated by the range of repulsive interactions

Abstract

We study the effect of the interaction's range on the structural and kinetic properties of a computer-simulated two-dimensional aggregating colloidal system. For this purpose, we considered that the particles of the system interact through a repulsive Yukawa potential which depends on two parameters: the value of the interaction potential between particles in contact V-0 and the range of the interaction kappa d. We observed that the increase of the interaction range or V-0 provokes the arrangement of the small aggregates in linear structures. The repulsive interactions have also a strong influence on the kinetic behavior of the coagulation process. Indeed, they induce the formation of three different time-separated aggregation regimes. In the first regime (at early states) the aggregation is dominated by the range of the repulsive forces, and the cluster-cluster repulsion increases with the cluster size. The second regime (at intermediate times) is reached when the average cluster size is larger than the interaction range. Here, the cluster-cluster repulsions do not grow anymore with the cluster size, so the probability of overcoming the repulsive barrier is the same for all clusters. This corresponds with the so-called reaction-limited-cluster-aggregation regime, where more than one collision between the clusters is needed to form a bond. The third aggregation regime is found at long aggregation times. In this region the coagulation is mainly determined by the diffusion time and the kinetics becomes diffusion controlled. A physical interpretation for the transition between chain structures and the typical fractals aggregates from the point of view of the range of the interactions is discussed. Moreover, a method has been developed in order to obtain the effect of the interactions with a non-negligible range over the aggregation rates directly from the simulations. The relation between these different regions with the parameters of the interaction potential V-0 and kappa d is analyzed.