An interesting piece of research recently appeared at arXiv, and was mirrored by PhysicsWeb today. It concerns the behavior of fluid droplets falling on smooth surfaces at different atmospheric pressures and molecular weights. The research was done by L. Xu et al at the University of Chicago. Normally, when a droplet falls on a surface, primary and secondary splashes result, and the amplitudes of the splashes depend on the viscosity of the fluid. This research finds that the splash phenomenon is also dependent on the pressure and molecular weight of the gas the droplet falls through.
Decreasing pressure from top to bottom (courtesy: PhysicsWeb)It seems that the observed splashing can be totally suppressed, by reducing the pressure of the surrounding gas. In the limit, when the experiment is done at near vaccum (or below a threshold pressure), there is no splash! The threshold pressure is observed to be a function of the impact velocity, and scales with the viscosity of the fluid, and the molecular weight of the gas. Conjecture: The gas compresses the falling droplet, thereby increasing the chance of a splash. This is apparent in the above (high-speed-recorded) image, where the first row (droplet at 1 atm. pressure) is slightly deformed at the moment-of-impact, due to the pressure of the surrounding atmosphere. The team used three liquids (methanol, ethanol, 2-propanol) and four gases (helium, air, krypton, sulphur fluoride).

Questions
1. What happens during low-velocity/terminal velocity impacts, or zero-g?
2. Does the temperature of the gas affect the behavior?
3. How does the surface-air-fluid sandwiching at the moment-of-impact affect the splash (due to the lateral shock)?

The result at low pressures is quite counter-intuitive. Common sense suggests that a fluid droplet at vacuum should make a bigger splash! Now we know: when a tree falls down in a forest and no one is around, it makes no noise :-).