January 11, 2005
When a drop of liquid falls and hits a dry flat surface, you might expect a splash. Indeed, the striking images of crown-like splashes that form when droplets hit a liquid surface have become familiar icons since the advent of high-speed photography with Worthington, and one would hardly expect to find any surprises remaining in such an ordinary phenomenon. However, Lei Xu, Wendy Zhang and Sidney Nagel at the University of Chicago MRSEC have unearthed a spectacular result: removing the ambient gas completely suppresses the splash.
The photographs below show some frames from a movie that Lei Xu has taken of a drop of alcohol falling in air as it hits a smooth dry glass substrate. In frame #2, the drop develops a corona which in frame #3 ruptures into many droplets. This is what we would have expected any drop to do when it hits a hard surface – it should splash!
However, if the experimentalists evacuate the container, the splashing becomes suppressed as shown in the next set of images. The only difference between these two experiments is that this second one is at the very modest vacuum of 223 Torr (comparable to the pressure found on the top of Mt. Everest) rather than at atmospheric pressure.
When the experimentalists released the drop from a higher initial height, the drop splashed again even at 223 Torr. However, when a slightly stronger vacuum was pulled, the splash at this new height was again suppressed. By making a series of measurements for the onset pressure for splashing at different heights, they obtained a splashing/non-splashing phase diagram. From this data it appears as if the splashing can always be suppressed no matter from what initial height the liquid was dropped if the pressure is low enough. Thus, splashing would be completely absent on the moon! In other words, high-speed impact onto a solid substrate does not guarantee that a splash will form. Instead, the formation of a splash depends crucially on the presence of an ambient gas. This fact has been overlooked until the present study.
Moreover, the curve of (initial height) vs. (onset pressure for splashing) is not monotonic. The doubly reentrant shape of the curve was found for a variety of different ambient gases – SF6 (sulfur hexa-fluoride), krypton, air and helium. The experimentalists also showed that the critical pressure scaled with the square-root of the molecular weight of the gas. By varying the type of liquid used they found that the drop splashes more easily when the liquid viscosity is increased. None of these results are intuitive. However, all these experimental results at velocities above the reentrant behavior can be accounted for by a simple model which compares two stresses: the destabilizing stress associated with compressible effects in the outside gas, and the stabilizing stress due to the surface tension of the liquid. Splashing is suppressed when the stabilizing stresses outweigh the destabilizing ones.
These findings can be expected to have implications across a range of applications. In ink-jet printing, the splash formed upon impact is detrimental because it limits the resolution of the printing. In combustion or atomization, the formation of small droplets is desired. The discovery that splash formation can be suppressed or enhanced by the ambient pressure offers a simple and efficient way to tune the drop production in all these applications.
This work has been listed by the American Institute of Physics as one of the 10 top stories of 2005. For a link see: http://www.aip.org/pnu/2005/split/757-1.html
- "Drop Splashing on a Dry Smooth Surface," Lei Xu, Wendy W. Zhang, and Sidney R. Nagel, Phys. Rev. Lett. 94, 184505 (2005).