Effect of Volume Fraction on Transient Structural Behavior of Aerosol Particles Using Off-Lattice Kinetic Monte Carlo Simulation

Abstract

Fractal-like aggregates exhibit interesting properties that determine their physicochemical advantages, and thus, the control and prediction of aggregation is critical for many applications. An off-lattice kinetic Monte Carlo (KMC) simulation was performed to investigate the aggregate evolution from primary particles to three-dimensional fractal aggregates, at three different volume fractions. We have found that at low volume fraction, aggregation kinetics is slow, and aggregate morphology is widely open and stringy, with fractal dimension of (D-f) 1.8, in which the system is constantly preserved in the dilute regime. In denser volume fractions, however, the aggregation kinetics appears to be accelerated and aggregate morphology is more compact and less stringy due to the transition from dilute to dense regimes. Moreover, the volume fractions determine what kind of coagulation mechanism may occur to produce aggregates with different morphologies. At low volume fraction, coagulation is predominated by coagulation between aggregates in which the maximum probability of interpenetration event is only 18%. This suggests that aggregates at low volume fraction can maintain their self-similarity behavior. While at high volume fraction, coagulation is predominated by two subsequent coagulation mechanisms, namely, primary particle-aggregate and aggregate-aggregate interaction. The probability of interpenetration event increases up to 40%. In addition, the interpenetration process as well as the primary particle-aggregate coagulation, particularly in the dense regime, could produce superaggregates with a hybrid structure with a high fractal dimension at large size scales and a low fractal dimension at small scales. A detail mechanism for the formation of superaggregates was discussed.