Preliminarily collision-coalescences between a large water drop and water droplets under free fall were observed. Movements of the droplets due to the effect of the large water drop were also observed. The main experiment was performed by the use of a large water drop and droplets under free fall at terminal velocity, and it was proved that collision-coalescences occurred between them under the condition similar to natural rainfall. The size of the water drop and the size range of the droplets used were 6.2 mm and 80 microns to 200 microns in diameter respectively. Droplets which collided with a drop as they fell in front of the drop were all captured by the drop and no repulsion was observed between them. It was observed that smaller droplets behind a large drop were not captured by the latter because of the wake effect. Furthermore, collisions or coalescences between droplets were not observed within the large drop's wake.

Experimental examination of interactions between small water droplets, such as wake interactions, collision-coalescence phenomena, and the dependence of these interactions on the electrical charge and field, has awaited the development of a dependable source of multisize droplets. A simple droplet generator has been developed to produce controlled pairs of equal and unequal size droplets. The generator and its applicability to droplet interaction studies are described.

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