Drop impacts play a key role in many industrial applications, from spray coating of surfaces, to splashing of fuel-droplets within combustion chambers. Splashing, or break-up during ink-jet printing, can cross-contaminate biological assays, or degrade the quality of ink-jet printed products. Crime scene studies of blood splatter can give vital clues for the police. Spreading of plant diseases between nearby leaves by splashing depends on the velocity and trajectory of secondary droplets. In this dissertation, I study the early dynamics of splashing and the dynamics of ejecta sheets under extreme impact conditions, using ultra-high-speed video imaging at up to 5 million fps. In the first part, I show the effect of the surface tension differences on the break-up of the Edgerton crown, I verify that individual droplets hit the crown wall and generated Marangoni holes, thereby causing the crown wall to rupture at multiple locations. In the second part, I investigate the splashing of a drop impacting onto a solid substrate with high impact velocity, I show that for sufficiently high Re, splashing can no longer be suppressed by only reducing the surrounding air pressure. Furthermore, I tracked the earliest splashed spray droplets to catch their maximum velocity. Surprisingly, the splashed droplets can travel at extremely high speed of up to 1 km/s, which is 50 times faster than the impact speed. The influence of viscosity on the lamellar spreading along the substrate was investigated. I find that the intact lamella, following the fine spray, spreads as R(t) ~〖 t〗^(1/3) , while the maximum spreading radius of the drop was shown to be a strong function of viscosity, scaling as β_max∝〖Re〗^0.175. The data did not show a strong effect of surface tension on β_max over a wide range. Therefore, I concluded that surface tension at this parameter space does not play a major role in both splashing nor spreading. In the third part, I study extreme splashing dynamics of the Ejecta sheet when a drop impacts on a thin liquid film with very large impact velocities using the same device, at up to ~ 22 m/s. For this purpose, we have constructed a novel experimental device consisting of a 26-m-tall vacuum tube. I investigate the interplay between viscosity, the surrounding ambient air pressure, and surface tension, on the ejecta shapes and break-up. I show how the bending of the ejecta sheet is primarily produced by air-resistance. This is supported by an analytical and numerical model to quantify the effect of the surrounding air pressure on the sheet bending and touch-down.

Drop impact is a canonical problem in fluid mechanics, with numerous applications in industrial as well as natural phenomena. The extremely simple initial configuration of the experiment can produce a very large variety of fast and complex dynamics. Scientific progress was made in parallel with major improvements in imaging and computational technologies. Most recently, high-speed imaging video cameras have opened the exploration of new phenomena occurring at the micro-second scale, and parallel computing allowed realistic direct numerical simulations of drop impacts. We combine these tools to bring a new understanding of two fundamental aspects of drop impacts: splashing and air entrapment. The early dynamics of a drop impacting on a liquid pool at high velocity produces an ejecta sheet, emerging horizontally in the neck between the drop and the pool. We show how the interaction of this thin liquid sheet with the air, the drop or the pool, can produce micro-droplets and bubble rings. Then we detail how the breakup of the air film stretched between the drop and the pool for lower impact velocities can produce a myriad of micro-bubbles.