  A simple model of irreversible aggregation under differential sedimentation
of particles in a fluid is presented. The structure of the aggregates produced
by this process is found to feed back on the dynamics in such a way as to
stabilise both the exponents controlling the growth rate, and the fractal
dimension of the clusters produced at readily predictable values. The
aggregation of ice crystals to form snowflakes is considered as a potential
application of the model.
