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Abstract

We compare the collision rates and the typical collisional velocities amongst droplets of diferent sizes in a poly-disperse suspension advected by two- and three-dimensional turbulent fows. We show that
the collision rate is enhanced in the transient phase for droplets for which the size-ratios between the
colliding pairs is large as well as obtain precise theoretical estimates of the dependence of the impact
velocity of particles-pairs on their relative sizes. These analytical results are validated against data
from our direct numerical simulations. Our results suggest that an explanation of the rapid growth
of droplets, e.g., in warm clouds, may well lie in the dynamics of particles in transient phases where
increased collision rates between large and small particles could result in runaway process. Our results
are also important to model coalescence or fragmentation (depending on the impact velocities) and will
be crucial, for example, in obtaining precise coalescence kernels in such systems.