Making a splash

In his "Milkdrop Coronet," strobe-photography pioneer Harold Edgerton famously captured the splash produced by a milk droplet falling into a saucer. But our understanding of the underlying physics remains poor. It's known that before a liquid droplet splashes upward from a surface, a thin sheet of liquid spreads out from the impact point. Four years ago experiments by Sidney Nagel and colleagues at the University of Chicago showed, surprisingly, that splashing on a dry surface can be suppressed by reducing the ambient air pressure. The researchers concluded that compressible effects in the air are responsible for the splashing (L. Xu, W. W. Zhang, S. R. Nagel, Phys. Rev. Lett. 94, 184505, 2005). Now Michael Brenner and coworkers at Harvard University have further looked into the air's role in how droplets splash on a dry surface. Taking into account the compressibility and viscosity of the gas and the surface tension of the liquid, they modeled the behavior of the approaching droplet as it reaches the surface. They find that instead of spreading out over the surface, the liquid spreads over a very thin film of air. When the droplet nears the surface, pressure builds beneath it and the bottom of the droplet deforms by flattening and then becoming dimpled. The droplet's bottom perimeter develops a kink that, still over a layer of air, moves out and creates capillary waves. The calculations don't, however, show any indications of splashing; the researchers suggest that other parameters, such as the droplet viscosity and thermal transfer, must become important after the initial spreading phase. (S. Mandre, M. Mani, M. P. Brenner, Phys. Rev. Lett., in press.) — Richard J. Fitzgerald