  The role of the spatial structure of a turbulent flow in enhancing particle
collision rates in suspensions is an open question. We show and quantify, as a
function of particle inertia, the correlation between the multiscale structures
of turbulence and particle collisions: Straining zones contribute predominantly
to rapid head-on collisions compared to vortical regions. We also discover the
importance of vortex-strain worm-rolls, which goes beyond ideas of preferential
concentration and may explain the rapid growth of aggregates in natural
processes, such as the initiation of rain in warm clouds.
