Bubbles and beer bottles: an explosive combination

Inside Science News Service

March 18, 2009

Bubbles and beer bottles: an explosive combination

High-speed camera reveals physics principle behind bar trick

By Devin Powell
ISNS Contributor

(Pittsburgh, PA) -- You might think that smashing beer bottles is an uncivilized activity best left to cowboys in spaghetti Westerns and drunken revelers on St. Patrick's Day. But at this year's meeting of the American Physics Society in Pittsburgh, a group of scientists demonstrated how to break a bottle with a bare hand. The secret to the trick, they say, is the explosive power of bubbles.

The experiment started at a party at New York University two years ago, where mathematician Sunny Jung was drinking the beverage of the night – Corona beer, which comes in a clear glass bottle. He and his colleagues at the Massachusetts Institute of Technology, New York University, and Kent State University tried a trick featured on Youtube: fill a glass bottle with water, leaving a small space of air at the top. Holding the neck in one hand, give the bottle a sharp jerk upwards and smack the opening smartly with the palm of the other hand. If done properly, the bottom of the bottle will shatter violently.

One explanation for the trick is that the pressure change in the bottle caused by the hand strike is responsible for the explosion. But when the group of scientists repeated the stunt with degassed water – ultra-pure liquid that lacks bubbles – the bottle remained intact, even though the hand struck with the same force. Bubbles in the liquid were critical in triggering an explosion.

Bubbles have already been blamed for other scenarios of liquid destruction. When water moves quickly and changes pressure, bubbles form and collapse, releasing miniature shock waves – a phenomena called cavitation. When many bubbles collapse simultaneously, the combined force of cavitation can be tremendous. A predator called the pistol shrimp, for example, shoots bubbles at its prey using a quick snap of its special claw. As fish move through water, they generate bubbles that explode and mark their scales with tiny pits; the same thing happens to the steel propellers of boats and submarines.

To see if cavitation explains the beer bottle trick, the team hooked up a high-speed camera and a microphone and repeated the experiment in the lab. They noticed that when the beer bottle struck the hand, all of the liquid rapidly rushed upwards. A thousand tiny new bubbles appeared throughout the liquid as the pressure in the moving water dropped.

As the liquid rushed back downward, the bubbles clumped together at the bottom and imploded. At that moment, the microphone picked up a high-pitch noise. “It sounds like gravel being sucked through a pump,” says team member Jake Fontana. Eighty milliseconds after this “signature sound” of cavitation, the bottle erupted.

“The force of all of these collapsing bubbles becomes concentrated into a small area” says Fontana. By figuring out how fast the bottle must be jerked upwards to break, he calculated a rough estimate of the pressure generated at the bottom edge – about 1,000 pounds per square inch, the pressure a scuba diver would feel half a mile below the ocean's surface.

Christopher Brennen, who studies cavitation at the California Institute of Technology, agrees with the explanation. Based on his own calculations, he suspects that the team's rough calculation may have underestimated the actual pressure generated by the powerful bubbles.

The trick does not work in other glass containers such as test tubes. The beer bottle, with its flat bottom and narrow neck, seems to be the perfect shape for concentrating the bubbles. If you want to try this yourself, the group suggests putting on a pair of protective gloves, buying a cheap brand of beer with a thin bottle, and doing the whole thing over a bucket to catch the spectacular mess it makes.

This story is provided for media use by the Inside Science News Service, which is supported by the American Institute of Physics, a not-for-profit publisher of scientific journals. Please credit ISNS. Contact: Jim Dawson, news editor, at jdawson@xxxxxxx