Often the most familiar phenomena evade a clear physical description. That a liquid droplet makes a splash when it hits a smooth dry surface would strike most people as fairly intuitive. In fact, its ubiquity belies an intriguing complexity that we are still struggling to understand. Michelle Driscoll and Sidney Nagel have now written the latest chapter in the story (Phys. Rev. Lett. 107, 154502; 2011).

Credit: © 2011 APS

The realization that splashing is linked to a thin fluid sheet emerging from the point of impact came nearly half a century ago. But it wasn't until ultrafast imaging techniques enabled isolation of the events leading to sheet ejection that the investigation really gained momentum. Temporally resolved observations prompted theoretical studies that pointed towards the existence of an air layer between drop and surface that might cushion the impact, inducing the ejection of the thin liquid sheet. Driscoll and Nagel argue otherwise, with the results of a high-speed interference imaging study that provides strong evidence to suggest that no such layer persists as the droplet spreads.

This is not to say that the pair downplay the importance of the liquid–gas interface — quite the contrary. Although it came as no surprise to learn that formation of the sheet depends on the physical properties of the fluid and substrate, the most striking development in recent years was their group's finding in 2005 that the surrounding gas is crucial for splashing. In fact, they found that splashing can be completely suppressed by decreasing the air pressure around the droplet, supporting the notion that air dynamics at the liquid–gas interface fulfils a splash-stabilizing role.

The threshold pressure at which this occurs depends on the viscosity of the liquid, which also seems to affect the mechanism: splashing of high-viscosity liquids is significantly delayed, compared with their less-viscous counterparts. This time separation between impact and splash ultimately enabled Driscoll and Nagel's investigation of air-layer formation. Using a syringe pump to produce water and glycerol droplets, they measured the thickness of the air layer using interferometric high-speed imaging, and saw that an initial air bubble created on impact quickly vanished, leaving no trace of an air layer by the time the sheet was ejected.

The finding suggests that splashing originates at the edge of the droplet, and puts us one step closer to controlling a mechanism with widespread implications for applications, from automotive to agricultural.