Spanish scientists have discovered why beer transforms from liquid to foam when one bottle is bumped against another.
Scientists at Universidad Carlos III de Madrid (UC3M) and colleagues got the idea for the research when they observed that the foam of one beer spilled over when somebody jokingly hit the neck of one bottle against the base of another.
"We all began to propose hypotheses and theories about the cause of the phenomenon, but none of them convinced us, so we decided to take it to the laboratory to do research using controlled experiments in well-defined conditions to analyse which physical phenomena are behind the appearance of that foam," said Javier Rodriguez, a professor in UC3M's Department of Thermal and Fluids Engineering.
The study explained in detail what happens after a bottle receives an impact. First, expansion and compression waves appear.
These advance inside the liquid and cause the gas cavities (bubbles) to burst at the bottom of the bottle. Afterwards, small balls of foam are formed because the bubbles break into even smaller ones.
Finally, given that they weigh less than the liquid surrounding them, these bubbles move to the surface so rapidly that the final result is similar to an explosion.
"In fact, those clouds of foam are very much like the mushroom cloud caused by a nuclear explosion. In one second, almost all of the beer shoots out of the bottle," said Rodriguez.
The foam appears because, in carbonated beverages, there is more carbon dioxide (CO2) than the water (the main component) is able to maintain in the solution.
"Usually, the CO2 escapes very slowly. But the chain of events set off by the blow to the bottle multiplies the escape of gas by a very high factor: a bottle may lose in seconds the same amount of gas that it would normally lose in hours if we simply left the bottle open on the table," said Rodriguez.
In order to demonstrate the validity of their theory, the researchers came up with a system for studying the phenomenon in slow motion. First, they aimed at the base of the bottle with a high energy pulsed laser to cause a bubble to appear.
Then they hit the neck of the bottle and recorded everything with a high-speed camera that allowed them to obtain over 50,000 still photos per second.
Because of this, they were able to describe in detail the process that is behind this chain reaction: cavitation. This is a hydrodynamic affect that is similar to boiling (as far as the formation of bubbles is concerned), but which occurs when the pressure in a liquid drops.
Far from being just an interesting trick, the study may have very serious applications, researchers said.
"One of applications is the prediction of the quantity of gasses produced by the eruption of a volcano," said researcher Daniel Fuster, with the Institute Jean le Rond D'Alembert in France.
