What happens when a droplet of liquid lands on a glass surface? It's difficult to see with the naked eye, but the droplet splashes, right? Well, yes and no. It depends on the air pressure.

Physicists at the University of Chicago watched alcohol droplets fall onto glass slides within a sealed chamber, in which the air pressure could be controlled and adjusted. They recorded the droplets' falls at various pressures with a camera that takes 47,000 pictures a second.

They were surprised when the camera revealed that the higher the pressure, the more the splatter; the lower the pressure, the less the splatter or no splatter at all. The lightness or heaviness of the gas inside the chamber affects the splatter too. When helium, a lighter gas, was present, the droplet's splat was less than when heavier gasses were present.

Why this effect? Scientists think that the size of the splatter is largely a result of the resistance or lack thereof of the surrounding air as liquid rapidly expands outward against it. If the surrounding air is high in pressure or heavy, then it might resist being pushed outward. Thus, it pushes back at the droplet, lifting the edge of the liquid film upwards, creating a splash. If the pressure is low or the gas is light, then it may be unable to resist so strongly. Thus, it pushes back weakly or not at all, with little or no splash.

Who, besides physicists, cares about the effects of gas and air pressure on a splash? Engineers, that's who. These findings may help engineers to improve the quality of processes such as ink-jet printing and engine combustion.