Why champagne sparkles and cola bubbles
The gas bubbles in champagne bubble in rows - unlike those in cola & co. They seem to defy physics. A chemical effect explains the phenomenon.
The fine bubbles of champagne are not only an important part of the drinking experience, but also a physical puzzle. The tiny bubbles rise upwards in orderly rows - according to the rules of hydrodynamics, they should diverge in a funnel shape. This is what happens in drinks such as cola. There, the flowing water behind the bubbles creates vortices that push the bubbles rising behind them to the side. The fact that this does not happen in sparkling wine or champagne is due to a special chemical feature of these drinks, reports a research group led by Omer Atasi from the Université libre de Bruxelles. According to their analysis, now published in the journal "Physical Review Fluids", the fatty acids in these drinks attach themselves to the interface between water and gas and change the flow behaviour in such a way that the vortices keep the bubbles in place.
The dissolved carbonic acid in the drinks bubbles out as carbon dioxide on uneven vessel walls and creates sequences of countless fine bubbles that rise to the top. The water flows around the spherical cavity and creates two elongated vortices rotating in opposite directions behind the bubble, which influence the next bubble. In water or beverages such as cola, these vortices rotate in the opposite direction in successive bubbles. This drives the chain sideways apart. This is because each bubble exerts a force on the next bubble that is opposite to the force that it itself experiences through the bubble above it. This happens in most carbonated drinks - but not in champagne or sparkling wine.
As Atasi and her team report in their publication, there is a chemical reason for this. Drinks are not pure water, but contain a variety of chemical substances. Large quantities of so-called surface-active substances are responsible for the strange behaviour of the bubbles. These substances have one part of the molecule that is easily soluble in water and another that interacts poorly with water - in the case of champagne, these are fatty acids in particular. They therefore attach themselves to the bubble walls so that the water-soluble part remains in the liquid and the poorly soluble remainder protrudes into the gas.
The flowing liquid drives the attached molecules from the top to the bottom of the bubble, where they finally return to the liquid. As a result, the concentration of substances at the bottom of the bubble is higher than at the top, which has curious effects. For one thing, gas bubbles rise more slowly in champagne than in cola. The higher concentration at the bottom lowers the surface tension of the water compared to the top. As a result, a force suddenly acts on the liquid around the bubble in the direction of the higher surface tension, the so-called Marangoni effect. The water flows past the bubble more slowly and it takes longer to reach the surface.
This not only slows down the rise of the bubble, but also changes its properties. The more surface-active substances the liquid contains, the more the effect "blocks" the flow in the interface between gas and water. The air bubble behaves more and more like a sphere with a rigid wall instead of a cavity in the surrounding water - which in turn changes the vortices around the bubble, as Atasi's team shows in a computer simulation. According to this, most of the vortex formation now takes place above and next to the bubble instead of behind it. In particular, however, the vortices of successive bubbles are no longer mirror images of each other, but are orientated in the same way, so that the forces on all bubbles in the chain are the same.
This means that the bubbles in champagne remain in a neatly organised row, whereas in other drinks they are pressed apart in a funnel shape. However, beer is a special case among carbonated drinks, the team reports. In beer, it is mainly proteins that attach to the bubbles; these behave very differently to fatty acids, as they partially unfold when they attach to a bubble. Both theoretical considerations and observations of beers suggest that the bubbles could form straight or diverging bubble chains, depending on the circumstances. However, further thorough experiments with beer are needed to decipher these processes more precisely, according to the publication.
Spectrum of Science
We are partners of Spektrum der Wissenschaft and want to make well-founded information more accessible to you. Follow Spektrum der Wissenschaft if you like the articles.
[[small:]]
Cover image: Shutterstock / Dasha Petrenko
Experts from science and research report on the latest findings in their fields – competent, authentic and comprehensible.