How droplets grow rapidly from 10 to 50 μ m is an outstanding question in cloud physics. We
show theoretically and numerically that caustics, locations of multivalued droplet velocity, of
small droplets near a single planar steady vortex offer one route through this bottleneck.
Such a vortex serves as a simple model for the more complicated turbulence field existing in
clouds. Within a special radial distance rc from the vortex center, droplets closer to the vortex
can centrifugally overtake those farther out and coalesce. Small polydispersity increases rc …