Scientists use slow-motion cameras to explain why beer foams over when hit

Sometimes, the mysteries of the scientific universe are best debated over a drink at the pub and sometimes that drink only introduces further mysteries.

The latter was certainly the case for a group of Spanish specialists in fluid mechanics who, on a night out in a local tavern, were bewildered by why beer in bottles foams up when the top of one bottle is tapped sharply by the base of another.

"We all began to propose hypotheses and theories about the cause of the phenomenon, but none of them convinced us,” Javier Rodríguez, a professor at Universidad Carlos III de Madrid’s (UC3M) Department of Thermal and Fluids Engineering told Phys.org.

“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.”

For people who have had this trick played on them before the question of ‘why?’ tends to be a soul-searching, ‘why is the universe so cruel?’ type inquiry, but Rodríguez and his team wanted more concrete results.

A first version of the theory was published in October last year, but now UC3M along with the Institute Jean le Rond D'Alembert and the Université Pierre et Marie Curie in France) have presented their findings as the American Physics Society’s most recent conference on fluid mechanics.

Using a special slow motion camera that records more than 50000 frames a second the scientists were able to break down the phenomena into three distinct stages. Firstly, the bottle is hit and the compression waves travel down through the liquid, hitting the bottom and creating gas cavities or bubbles.

These bubbles can’t take the pressure and begin to break down into even smaller pockets of gas, creating the foam. Finally, because this airy mixture weighs less than the liquid surrounding it, it begins to shoot up the neck of the bottle.

"In fact," said Rodríguez, "those clouds of foam are very much like the mushroom cloud caused by a nuclear explosion.”

This hydrodynamic affect is similar to boiling as far as the formation of bubbles is concerned, and the research could have serious applications in a number of areas, from creating longer lasting boat propellers (cavitation of this sort erode the blades over time) to predicting the behaviour of gases in volcanic eruptions.

"This is one of the great advantages of basic research," said Rodríguez. "You learn low-cost physics in the laboratory, with systems that are as simple as a bottle of beer, and this can later help you to understand and attempt to solve other important problems."

It seems there’s hope for the bar-stool scientist yet.

This study was originally published by arXiv, click here for more information