Taming the Physics of Splashing

Splashing is one of those facts of everyday physics that none of us will really ever have to ponder outside of getting hosed by a passing car on a rainy day. Yet, in some cases, the propensity of a liquid to go everywhere when impacted can be quite dire. Consider, for example, a hospital environment, in which a mild splash involving a dangerous pathogen might result in the spread of infection, or a laboratory or industrial environment, in which a slash may spread toxic chemicals.

Physicist Robert Style and colleagues at the University of Oxford are interested in taming the physics of splashing. Style leads an interdisciplinary group that focuses, generally, on the mechanics of soft materials. Relevant questions include: How do droplets wet surfaces? Why do soft materials seem to become stiffer when you punch holes in them? How do potholes form? How and why do the mechanics of splashing change as the materials being splashed upon become softer?

This last question leads us to a paper published this week in thePhysical Review Letters. Here, Style describes experiments in which soft silicon gels of variable stiffness were bombarded with droplets of ethanol and then imaged using shadowgraphy, an optical method of visualizing transparent substances by the shadows they cast as light passes through them. Splashing is a complex process, but the researchers suspected that they could craft materials to be splash-free.

What the imaging revealed is that when a droplet hits a hard surface, it immediately pancakes and spreads outward. It's the outer ring of this pancake that ultimately becomes the splash as it thins and becomes a fine spray of tinier drops. On a softer material, things start to look different—rather than atomizing into a spray, the pancake stays together and less splashing occurs until there's none at all.

Image: Style et al

So, it's possible to tune a surface to the point that it becomes splash-free or very nearly so. It's just a matter of finding the right softness for the right impact force. "Splashing is reduced or even eliminated," Style and his team write. "Droplets on the softest substrates need over 70 percent more kinetic energy to splash than they do on rigid substrates. We show that this is due to energy losses caused by deformations of soft substrates during the first few microseconds of impact."

Simply, there's a threshold that an impacting droplet reaches as it pancakes outward where its kinetic energy reaches zero. If this occurs before the expanding sheet starts to come apart into microdroplets (and, thus, splash), then the splash is averted. This seems intuitive enough, but apparently no one's actually gone out and done the experimental footwork to confirm such splash-quenching until now.

"Finally," Style and co. conclude, "our work gives insight into the wide variety of processes involving impact on soft substrates, with examples ranging from maintaining hygiene, to pesticide delivery on plants, optimizing inkjet printing on soft or biological materials, and circuit printing of soft electronics."

from Taming the Physics of Splashing