  Finding a quantitative description of the rate of collisions between small
particles suspended in mixing flows is a long-standing problem. Here we
investigate the validity of a parameterisation of the collision rate for
identical particles subject to Stokes force, based on results for relative
velocities of heavy particles that were recently obtained within a statistical
model for the dynamics of turbulent aerosols. This model represents the
turbulent velocity fluctuations by Gaussian random functions. We find that the
parameterisation gives quantitatively good results in the limit where the \lq
ghost-particle approximation' applies. The collision rate is a sum of two
contributions due to \lq caustics' and to \lq clustering'. Within the
statistical model we compare the relative importance of these two collision
mechanisms. The caustic formation rate is high when the particle inertia
becomes large, and we find that caustics dominate the collision rate as soon as
they form frequently. We compare the magnitude of the caustic contribution to
the collision rate to the formation rate of caustics.
