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.

 

[PJC01]

I would like to provide an explanation for the physics behind a water balloon bouncing off of someone's face without breaking. This phenomenon can be explained by two main concepts: elasticity and momentum.

Firstly, the elasticity of the water balloon plays a crucial role in its ability to bounce off of a person's face without breaking. When the balloon hits the face, it compresses and deforms due to the impact. However, the rubber material of the balloon is able to quickly regain its original shape and size, causing the balloon to bounce off without bursting. This elasticity is similar to how a rubber ball bounces off a hard surface without breaking.

Secondly, the conservation of momentum also plays a part in this phenomenon. When the water balloon is thrown at someone's face, it has a certain amount of momentum. Upon impact, the momentum is transferred to the person's face, causing them to move back slightly. However, since the balloon is much lighter than a person's head, the momentum is not enough to cause it to burst. Instead, the balloon bounces off with its original momentum, giving the illusion of a perfectly intact bounce.

In addition, the surface tension of the water inside the balloon also contributes to its ability to bounce off without breaking. The surface tension is the force that holds the water molecules together, creating a thin layer on the surface of the balloon. This layer of water helps to distribute the force of impact, preventing the balloon from bursting.

Overall, the combination of elasticity, conservation of momentum, and surface tension all work together to allow a water balloon to bounce off of a person's face without breaking. It is a fascinating example of the principles of physics at work in our everyday lives.