Water Balloon to the face physics

[physstudent123]

There are many slow motion videos of water balloons dramatically bouncing off of someone's face perfectly intact, yet none seem to explain the physics behind what is happening.



Anyone care to explain some physics concepts involved?

 

[Simon Bridge]

Welcome to PF;
The water balloon striking an irregular surface involves fluid mechanics and the elastic properties of materials. Both these are extremely complicated subjects so it is not surprising that few people have tackled it. But basically, and extremely crudely, the balloon, if well made, won't burst as long as no part of it stretches past a critical limit. This can happen easily if there is a sharp edge in the way but for soft round shapes like the human face it is not so likely.

Since you want a water balloon to burst when it hits someone (you want them wet not KO'd) it follows you want to use the cheapest balloons you can find. These will have small imperfections in their construction where they will tear more easily.

 

[A.T.]

Obviously soft obstacles without pointy elements, that eventually bounce back a bit (like a head) are less likely to kill the balloon.

But there could be a shape factor to it: round vs. plane. To test this you could drop many balloons on a smooth, clean plane. And then, from the same height, on a bowling ball. I suspect that the balloons are "rolling off" at round obstacles like heads, so they are less likely to burst.

 

[Simon Bridge]

... or drop into a large bowl... good idea!
Maybe a series of youtube videos will result?

It's unlikely that we are the first to be thinking about this ... let's see... oh yes: I love the internet:
Bursting water balloons
The impact and rupture of water-filled balloons upon a flat, rigid surface is studied experimentally, for which three distinct stages of the flow are observed. Due to the impact, waves are formed on the balloon's surface for which the restoring force is tension in the latex. Immediately following rupture of the membrane, a shear instability created by the retraction of the balloon is observed. At later times, a larger-scale growth of the interfacial amplitude is observed, that may be regarded as a manifestation of a phenomenon known as the Richtmyer-Meshkov instability. This flow is closely related to the classical understanding of the Richtmyer-Meshkov instability for when there exists a density difference between the fluids inside and outside the balloon. Further, it is shown experimentally that this growth of the interface may also occur when there is no density difference across the balloon, a situation that does not arise for the standard Richtmyer-Meshkov instability.

http://www.colorado.edu/MCEN/flowvis/galleries/2010/Team-1/Reports/Decker_Terese.pdf (pdf - photographic study)

Physics of Water Balloons... a blow-by-blow of what happens and links to more information.