The formation of large aggregates by Brownian growth and the onset of a runaway process

In an experiment onboard a microgravity sounding rocket, we observed the growth of agglomerates consisting of μm-sized silica spheres under well-defined conditions. After an initial short charge-induced growth phase, the charge-neutralized agglomerates grew due to Brownian-motion induced collisions, first in an orderly fashion and thereafter, due to the temporal increase in particle concentration, in a runaway process. The latter is characterized by the detachment of large oligarch agglomerates. We modeled the growth using a mean-field approximation as well as a Monte Carlo technique.