Fundamental Studies of Particle Growth and Structure During Powder Synthesis

Publication Reference: 
20-02
Author Last Name: 
Flagan
Authors: 
R C Flagan
Report Type: 
ARR
Research Area: 
Particle Formation
Publication Year: 
1991
Publication Month: 
11
Country: 
United States

Executive Summary

The primary objective of the Caltech program supported by IFPRI is to under- stand the dynamics of particles which do not coalesce immediately upon coagulation and, through that understanding, to guide the development of processes for production of particles with particular properties. Theoretical and experimental investigations of the dynamics of aggregate aerosols have been undertaken with IFPRI support. Following on our studies of the properties of agglomerate particles, we have developed models to describe the kinetics of agglomeration. The kinetics of agglomeration are determined by the mobilities of the agglomerate particles and by their collision cross sections, The lower density of agglomerates has competing effects on the coagulation kinetics as compared with dense particles of equal mass: (i) the aerodynamic drag on the particles is increased due to the larger size of the particle of the same mass; (ii) the low density tends to increase the collision cross section of the agglomerate over that of the dense sphere. We have used fractal scaling concepts to evaluate the relationship between these measures of particle size and the particle structure.

In this report, we first examine the scaling that determines the asymptotic form of the size distribution, the sacalled self-preseting particle size distribution. A dynamic exponent is defined that describes the rate of growth of the mean particle size. The dynamic exponent is shown to pass through a minimum in the transition regime, behavior that has not previously been described. Essential features of this asymptotic solution are observed experimentally, although direct comparison between experiment and theory is complicated by the transition between coalescing coagulation and agglomeration that took place in our experiments at about the same particle size as the transition between the free molecular and continuum size regimes.

A more complete description of aerosol agglomeration was made in numerical solutions to the coagulation equation using a modified sectional code. Comparison of the collision frequency functions of spheres and agglomerate particles reveals that the predominant effect is the increased collision cross section of the agglomerate, leading to a dramatic increase in the collision frequency of agglomerates as compared to dense particles of the same mass. Simulations that assume that particles coalesce completely up to a limiting size and do not coalesce at all beyond that size exhibit a broadening of the size distribution at that transitional size. Experimental evidence of that broadening is provided. The comparisons at this point can be considered only provisional in as much as direct experimental validation of the collision frequency function is still lacking. This experimental evaluation is the key objective of the research under this program for the coming year.