  We present an analysis of the mean-field kinetics of Brownian coagulation of
droplets and polymers driven by input of monomers which aims to characterize
the long time behavior of the cluster size distribution as a function of the
inverse fractal dimension, $a$, of the aggregates. We find that two types of
long time behavior are possible. For $0\leq a < 1/2$ the size distribution
reaches a stationary state with a power law distribution of cluster sizes
having exponent 3/2. The amplitude of this stationary state is determined
exactly as a function of $a$. For $1/2 < a \leq 1$, the cluster size
distribution never reaches a stationary state. Instead a bimodal distribution
is formed in which a narrow population of small clusters near the monomer scale
is separated by a gap (where the cluster size distribution is effectively zero)
from a population of large clusters which continue to grow for all time by
absorbing small clusters. The marginal case, $a=1/2$, is difficult to analyze
definitively, but we argue that the cluster size distribution becomes
stationary and there is a logarithmic correction to the algebraic tail.
