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Bottles, Bubbles, and Breakage

A firm slap of the hand will break a glass bottle filled with water. A new fluid dynamics video shows exactly why

“A popular party trick is to fill a glass bottle with water and hit the top of the bottle with an open hand, causing the bottom of the bottle to break open,” say David Jesse Daily and pals at Brigham Young University in Provo, Utah. 

The bottles break only when filled still water and not when filled with the fizzy stuff or when empty. The question is: why? 

Today, these guys reveal exactly how this process occurs in an interesting video for this year’s Gallery of Fluid Motion competition, an annual event run by  the Fluid Dynamics Division of the American Physical Society (hi-res download or low-res download).

These guys first measure the instant at which the bottle breaks and say this does not coincide with the instant when the bottle is struck.

Instead, they use high speed video footage to show that the impact from the hand causes the bottle to accelerate downwards and this creates a region of low pressure at the bottom of liquid.

If the acceleration is great enough,  the pressure drops below the vapour pressure and the fluid vapourises, forming bubbles. This is a well known process known as cavitation.

As soon as the pressure returns to normal, the bubbles collapse. But, interestingly, this collapse occurs at some ten times the rate of bubble formation. It also produces instantaneous forces that are much more concentrated than those that produced the acceleration. 

It is these forces that break the bottle.

Daily and co go on to show that this process does not work when the bottle is filled with carbonated water. The reason is simple. The impact of the hand causes bubbles to form as before but instead of collapsing, the bubbles fill with carbon dioxide and float away.

Similarly, the effect does not work with an empty bottle because the bubbles cannot form at all.

So now you know. 

And the moral of this story? Avoid grad student parties in Utah.

Ref: arxiv.org/abs/1210.3764: Catastrophic Cracking Courtesy of Quiescent Cavitation

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