Gérard Liger-Belair, University of Reims Champagne-Ardenne, France, celebrates what gives champagne its sparkle

Since the time of the Benedictine monk Dom Pierre Perignon (1638-1715) champagne is the wine of celebration. With its image inextricably linked to the elegance of its effervescence - the small bubbles it emits. 

In champagne and sparkling wines, carbon dioxide molecules form in excess during a unique second fermentation process. And once opened, champagne in a typical 0.75 litre bottle releases approximately five litres of CO₂. This equates to a huge 20 million bubbles formed per bottle. 

 

A standard bottle of champagne releases 20 million bubbles

 

Bubbles do not just appear as champagne is poured, the dissolved CO₂ molecules must be able to group together and push their way through the liquid molecules. Energetically this is not easy, and close inspection of glasses filled with champagne shows that most of the bubble nucleation (growth) sites are pre-existing gas cavities on the surface of the glass. These gas cavities are trapped inside cellulose fibres on the surface of the glass, that come from the surrounding air or from wiping the glass with a cloth before use. 

Recent calculations have linked the final bubbles' size with a combination of their growth rate, the speed with which they ascend and other parameters. The amount of dissolved carbon dioxide content is important: a reduction by a factor of two - which is approximately the factor between champagne and beer - decreases the average bubble size by about 40 %. This is the main reason why, contrary to popular belief, beer bubbles are significantly smaller than those in champagne. The bubble size is also strongly gravity and pressure dependent. On the Moon for example, where the gravity acceleration is only a sixth of that on Earth, bubbles would be about three times larger in volume. And if you could enjoy a glass of champagne on the top of Mount Everest, where the overall pressure is only about 30 per cent of the pressure at the sea level, bubbles would increase by a factor of almost four in volume. This is basically the same phenomenon that is responsible for gas embolism in divers who have breathed high pressure air under water, if they resurface too quickly.

Once formed, bubbles rise toward the liquid surface due to their own buoyancy. While rising, they continue to grow in size by continuously absorbing carbon dioxide molecules dissolved into the champagne. Bubbles therefore continuously accelerate along their way through the champagne. This continuous acceleration can be watched using high speed photographs, where the continuously increasing spacing between the successive bubbles in a given bubble train can be seen. Contrary to popular belief however, champagne bubbles rise at a relatively slow pace (0.5 kilometres per hour), comparable to that of a turtle. That said, when opening a bottle of champagne the release of CO₂ means an uncontrolled cork can reach speeds of 50-60 kilometres per hour!

Without bubbles champagne would be unrecognisable, sparkling wines and beers would be flat. However, the role of effervescence is suspected to go far beyond the sole aesthetical point of view. Bubbles bursting at the liquid surface radiate hundreds of tiny liquid jets which quickly break up into a multitude of tiny droplets every second. Those tiny droplets, ejected up to several centimetres above the liquid surface, partly evaporate, thus accelerating the transfer of volatile organic compounds and enhancing the flavour profile of the wine.

The close observation of bubbles collapsing at the surface of a glass filled with champagne reveals another unexpected and lovely phenomenon. A few seconds after pouring, and after the collapse of the foamy head, the surface of a champagne flute is covered with a layer of bubbles - a kind of bubble raft. Here each bubble is surrounded by approximately six neighbouring bubbles. These bubbles arrange themselves in a hexagonal pattern, resembling those in beeswax. Due to capillary forces, when a bubble bursts and leaves an open cavity at the champagne surface, adjacent bubbles are sucked towards this empty cavity and creating short-lived flower-shaped structures, invisible to the naked eye.

Who would have imagined that a flute of champagne is such a fantastic playground for a chemical physicist in love with microphotography, or for a champagne lover with the time and knowledge to reflect on what is happening right under his nose?

Read Gérard Liger-Belair's critical review 'Recent advances in the science of champagne bubbles' in issue 11, 2008 of Chemical Society Reviews.