Why do bottles have a neck

Bierstreich: How is the foam fountain created?

Beer was the fifth most consumed beverage in Germany in 2014. The mix of water, malt and hops not only has a lot to offer in terms of taste, there are also interesting physical phenomena to consider. Carbonic acid, which is a chemical reaction of carbon dioxide, is particularly important (CO₂) with water. It is not only responsible for the popular head of foam, but also for the explosive rise of beer foam that is created when a freshly opened beer bottle is hit.

A common party prank goes like this: if someone has just opened a bottle of beer, you hit the head of the other person's beer with your own beer bottle (not so hard that the bottle breaks, of course). The other person's beer suddenly starts foaming and splashes out of the bottle. This effect is so rapid that the victim can hardly react. But be careful: If your own beer has only just been opened, the same thing happens - and the trick backfires. But what exactly happens when you hit the bottle head? And is it really the blow or is it a quick hand movement?

Video: foaming over various drinks

Behavior of beverages when hit on the bottle

Carbonated drinks are under high pressure in filled bottles. If you open such a bottle, the pressure in the bottle is suddenly reduced and undissolved carbon dioxide leaves the liquid. As the pressure decreases, more CO₂ dissolves and forms the small, familiar gas bubbles in the liquid. If you hit the opening of a freshly opened beer bottle, the glass will vibrate. This oscillation causes a pressure fluctuation in the beer, which spreads downwards in the liquid and is reflected several times on the glass bottom and on the air in the bottle neck. As a result of this pressure wave, the small carbon dioxide bubbles are constantly deformed in less than a millisecond until they break down into several even smaller carbon dioxide bubbles. These smaller carbon dioxide bubbles form a small amount of beer foam that spreads through the liquid. This foam forms an optimal surface on which further carbon dioxide can settle until there is no more carbon dioxide in the immediate vicinity. The foam then experiences a buoyancy and rises to the top, followed by more carbon dioxide. As a result, the foam cloud accelerates upwards, which in turn increases the volume faster and the speed increases again. This leads to an explosion-like increase in the volume of the foam. However, since the bottle volume cannot expand, the well-known foam fountain finally occurs: The foam leaves the bottle through the only exit, the bottle neck. This phenomenon works not only for beer but also with cola and sparkling wine and even plastic bottles can be vibrated enough to create a foam fountain.

Cause: movement of the bottle

The frothing and splashing of carbonated beverages also often occurs after shaking unopened bottles. Shaking also creates a pressure fluctuation in the bottle. Whereby carbonic acid dissolves from the water in places with lower pressure and CO₂ rises in the form of bubbles. However, since the bottle is unopened, it cannot escape and the pressure inside the bottle rises to around three and a half bar. When the bottle is opened, the carbon dioxide escapes, taking liquid with it. The assumption that the bottle was moving could have something to do with the explosive foam rise, so it is obvious. However, a team led by Javier Rodriguez-Rodriguez from the University of Carlos III in Madrid was able to rule this out as a cause by shooting a beer bottle with a high-energy laser. This resulted in a pressure fluctuation only at a single point, where beer foam formed and caused the foam fountain effect described above due to the vibration of the bottle. Since the bottle was not moved in this case, the movement could be excluded as the cause of the foam formation.

Carbon dioxide emissions in nature

A sudden rise in carbon dioxide is also known in nature. For example, Lake Nyos in Cameroon lies above an extinct volcanic rock that is steadily emitting carbon dioxide. Since carbon dioxide is soluble in water, it accumulates in the water. However, the lake released an estimated 1.6 million tons of this enriched carbon dioxide in 1986. This rose to the surface of the water. Since CO₂ is heavier than air, it collected on the surface of the lake or earth and flowed into the nearby valleys. Since carbon dioxide is odorless, the carbon dioxide flowing into the nearby villages went unnoticed - 1,700 people and many animals were suffocated. So far, the cause of the gas evacuation has not been clarified. But the phenomenon described here confirms the assumption that the trigger for the release of the carbon dioxide could have been an earthquake.