MODELING OF SOLID PARTICLE FORMATION DURING SOLUTION AEROSOL THERMOLYSIS - THE EVAPORATION STAGE

The evaporation state of solution aerosol thermolysis (SAT) was modele d to study the effect of various parameters on solid particle formatio n by solute precipitation. A comparison of the characteristic time con stants for various processes demonstrated that droplet shrinkage and s olute diffusion are the slowest processes, and that the fast processes , i.e., vapor diffusion and heat conduction in the gas phase and the l iquid phase, can be assumed to have reached steady state. Differential equations for these faster processes were thus simplified and were so lved numerically along with a modified solute diffusion equation, usin g an explicit first-order finite difference scheme. The computations w ere done until the solute concentration at the droplet surface reached the critical supersaturation. Then, if the solute concentration at th e droplet center is higher than the equilibrium saturation, volume pre cipitation is proposed to occur. Solutes with a large difference betwe en critical supersaturation and equilibrium saturation were observed t o favor volume precipitation. High initial concentrations and low ambi ent temperatures were demonstrated to favor volume precipitation. Perc olation theory was invoked to provide insights about the space filling capacity of the precipitated solids, and a second criterion, the perc olation criterion, for solid particle formation was proposed; the solu te concentration at the droplet center should be high enough so that t he volume fraction of the precipitated solids is higher than the criti cal volume fraction. Volume precipitation only ensures that there are precipitated solids at the droplet center. The percolation criterion e nsures that there is a sufficient volume of precipitated solids at the droplet center to form a coherent three-dimensional network. For soli d particle formation by SAT, both the volume precipitation and the per colation criteria must be satisfied.