  Turbulence facilitates collisions between particles suspended in a turbulent
flow. Two effects have been proposed which can enhance the collision rate at
high turbulence intensities: 'preferential concentration' (a clustering
phenomenon) and the 'sling effect' (arising from the formation of caustic folds
in the phase-space of the suspended particles). We have determined numerically
the collision rate of small heavy particles as a function of their size and
densities. The dependence on particle densities reveals that the enhancement by
turbulence of the collision rate of particles with significant inertia is due
almost entirely to the sling effect.
