Fine Structure of the Spreading Pattern of a Freely Falling Droplet in a Fluid at Rest

High-resolving photo and videorecording was used to visualize the fine structure of the flow pattern upon the coalescence of a droplet, \(0.39 < D < 0.43\) cm in diameter, falling at a velocity \(3.1 < U < 3.9\) m/s with water in a reservoir. The spreading of droplets of water, water solution of ink, and copper and iron sulphates is studied. For all pairs of substances, in addition to cavities, crowns, and sheets with shaped outer edges, capillary waves radiated by the annular region of fluid coalescence and thin jetlets at the cavity bottom and the crown walls were observable in the splash formation regime. The jetlets penetrate the sheet and emerge from the tooth tips in the form of spikes. Sequences of small droplets fly out from the spike tips, whose positions change with time. The fluid acceleration is favored by the conversion of the available potential energy upon the annihilation of the free surfaces of the coalescing fluids.