The Shapes of Dancing Ejecta

Splashing of impacting drops produces a myriad of secondary spray droplets, which generate aerosols during rain on the ocean and can cause health hazards during the spraying of pesticides. Determining the size and number of the finest splashed droplets is therefore of practical interest. Herein we use a novel experimental facility with a 25-meter-tall vacuum tube, to study drop impacts at velocities as high as 22 m/s, where we reach parameter regimes not studied before. The ejecta sheet emerges horizontally at a velocity an order of magnitude larger than the impact velocity. We explain how convoluted shapes are generated by the interplay between inertia, air-resistance, viscous stress, and surface tension. In particular, we identify the key role of Bernoulli pressure and confined toroidal shapes when the sheet approaches the pool or drop surfaces. We also estimate the evolution of the sheet thickness, showing it reaching sub-micron dimensions, primarily owing to the azimuthal stretching. The ejecta evolution depends strongly on the initial angle of the ejection, which causes the jet to bend down towards the pool, or upwards towards the drop. We find that the direction of this bending is determined by the viscosity ratio of the pool to drop liquids.