Measurement of near-contact motion of cloud-sized droplets

The growth of cloud droplets from about 15 to 30 microns in radius is not well-understood. Turbulence drives collisions for droplets in this size range, but experiments to date have been unable to reliably measure the motions of particles when the separations between them are close to the size of the particles. In this near-contact regime, the effect of one droplet on the motions of the other due to hydrodynamic interactions cannot be ignored. We present measurements of the motions of droplet pairs in quiescent air with both colliding and nearly-colliding trajectories of approximately 25-micron water droplets when the distance to contact is less than the average radius of the droplets and initial impact parameter is between 0 and 6. We show that the behavior of the droplets is sensitive to the size ratio between the pair. We also show that droplet rotation induced by interactions between the droplets can lift both off their previous streamlines, a potential new mechanism to drive collisions in systems of three or more droplets. The data were acquired in part in collaboration with A. Dubey and B. Mehlig at the University of Gothenburg.