Aggregation dynamics in systems of coalescing non-deformable droplets

Abstract

Emulsion stability simulations (ESS) are used to study the aggregation behavior of oil-in-water (O/W) emulsions composed of non-deformable drops. In the absence of a strong repulsive force (low surfactant concentration), the number of aggregates decreases along with the number of drops following the dynamics predicted by Smoluchowski for irreversible flocculation. During this initial stage, the drops coalesce as soon as they make contact and no stable flocs are formed. The polydispersity of the system increases while its interfacial area decreases. This behavior disfavors the formation of aggregates with the characteristic fractal dimensions exhibit by suspensions under diffusion limited cluster aggregation (DLCA) and reaction limited cluster aggregation (RLCA) regimes. Moreover, the increase of the surfactant surface excess due to the redistribution of surfactant molecules among the available interfaces progressively augments the repulsive force between the remaining drops. In the absence of surface deformation, coalescence stops once the total interfacial area of the emulsion can be stabilized with the available surfactant concentration. At this point, aggregation occurs. In this terminal stage the aggregation behavior is likely to be determined by the characteristics of the secondary minimum of the interaction potential. A high surfactant concentration in the initial system leads to very different results. It prevents coalescence making the drops behave as solid particles. In this case clusters with the fractal dimensions corresponding to the DLCA and RLCA regimes are observed.