Drop impact, splashing and free-surface singularities

4 problems

• How does the surrounding gas cause an impacting drop to splash?

  Contested

  A drop hitting a smooth dry surface in air throws off a thin sheet that breaks into droplets, but in thin enough air the same impact does not splash at all. How the air produces the splash is not agreed.

• Does air-bubble pinch-off in water reach a universal self-similar state?

  Partially resolved

  When an air bubble detaches underwater, its neck thins to zero width in a fraction of a millisecond. Theory predicts that the final collapse forgets how it started, but experiments show that small initial asymmetries persist to the end.

• Size of the smallest jet drops from bursting bubbles

  Contested

  When a bubble bursts at a liquid surface, a thin jet shoots up and breaks into tiny droplets that carry sea salt and microbes into the air. How small the first droplet can be, as a function of liquid properties and bubble size, is disputed.

• Scaling of the splash-threshold gas pressure with viscosity and gas

  Open

  The air pressure below which splashing stops depends on the liquid and the gas. A single rule that covers thin and thick liquids and different gases has not been found.