The effects of temperature and surfactants on secondary droplets generated by the impact of raindrops on a water surface*

The effects of temperature and surfactants on the secondary droplets generated by the impact of raindrops on a deep-water pool are studied experimentally in an artificial rain facility. A rain generator, consisting of an open-surface rectangular tank with an array of 360 hypodermic needles attached to its bottom, is mounted 2.2 m above a water pool. Secondary droplets are generated as water droplets from the rain generator impact the water surface of the pool. In this study, the temperature of  the water in the rain generator was varied from room temperature (about 24 ^oC) to 4 ^oC (where water has its highest density) and the concentration of surfactant, Triton X-100, in the water pool was varied from 0 to about  mol/l, corresponding to surface tension ranging from 73 to 30 mN/m. Both the diameters and trajectories of the droplets are measured with a cinematic digital in-line holographic technique set at various heights above the pool’s water surface. It is found that the diameters and velocities of the secondary droplets in the rain field change considerably with the temperature of the raindrops and the surface tension of the water in the pool. The effects of water properties on the dynamics of secondary droplets are explored.