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Physics

In the rising of bubbles

Brazilian and French researchers show that champagne bubbles in four distinct manners

The popping of a champagne precedes the fumaroles that spouts out of the bottle, inebriates the environment with its perfume of brioche, yeasts, white fruit and nuts and announces the next movement: pour the golden and nervous drink into a tall straight glass, the flûte to the French, whose rims are taken over by the eruption of small bubbles that advance towards the surface of the liquid. A few moments later, the curtain of foam that had agitated the upper part of the flute dissipates completely. But, on the inside walls of the glass, the bubbling continues, signature of the most famous and imitated wine, the carbon dioxide (CO2) bubbles, popularly known as carbonic gas, one of the two bi-products of sugar fermentation erstwhile present in the drink (the other is ethyl alcohol). Served in an adequate glass, and assuming that nobody ventures to drink it, some specialists guarantee that the champagne maintains its effervescence, though in a decreasing manner, for up to five hours – a resistance test that rarely should be done outside of the research laboratories. Indeed, it is exactly from the scientific area, not that of gastronomy, that the report of a recent discovery comes concerning the dynamics of the production of the drink’s gaseous pearls: Brazilian and French physicists have shown that the process of the formation of the carbonic gas spheres obeys a sequence of different rhythms of bubbling as a function of the passage of time. Indeed, they have unveiled the mathematics that is wrapped around the birth of champagne bubbles or almost this.

It might seem like idle chatter, but the finding is serious – and it required the analysis of approximately 16,000 gas bubbles, coming from one hundred bottles of champagne donated by the company Moët & Chandon, the largest producer of the drink. In spite of the representative sample that occurs in the tumultuous interior of the delicious liquid, this is a miniscule fraction of the estimated total of bubbles contained in a single bottle of 750 ml of bubbly, in the order of 20 million. The results of the study, whose experimental part was carried out in France and whose data interpretation was under the charge of the Brazilians, came out in the September issue of the scientific magazine Physical Review E, published by the American Physics Society. The researchers served champagne at 20°C in glasses (12 degrees Celsius above that recommended), filmed and photographed with an ultra-rapid camera the chain of bubbles that were born on certain parts of the glass for half an hour, and, finally, studied the experimental information in search of standards that could lie behind the genesis of the piles of effervescence. They found them. At the minimum, four distinct rhythms of bubbling, which the scientists called periods, were identified. “But in some cases, we perceived up to seven distinct periods”, comments the physicist Alberto Tufaile, from the University of São Paulo (USP), a specialist in chaotic systems in liquid mediums and one of the authors of the study. Anyhow, the formal report from the researchers took into account, for now, the four main standards of champagne bubbling.

Initially, just when the glass receives the liquid, the bubbles come about in pairs, they form in groups of two by two and in this manner ascend to the top; afterwards, they appear in a form that is more or less disordered, in line with a variable number of units, as if they were in a transition phase; next they originate in trios, in bands of three by three; and finally they sprout only one bubble at a time, in a movement as monotonous as the tick tock of a clock (see the photos as the side). The oscillations in the rhythm of effervescence never departs from this circular sequence of events: after the period in which it produces a sphere of carbonic gas all at once, the champagne comes back to generating gaseous bubbles in twosomes and then so on.

The duration time of each of these four bubbling phases can vary from a few seconds – shortly after the drink is poured into the glass, when the quantity of carbonic gas in the foam is still high and the phases that succeed in high velocity – to some minutes, as the measurement of the levels of CO2 in the liquid go on reducing. “After about 15 minutes from the moment the champagne was served into the glass, the quantity of carbon dioxide dissolved in the drink is very small to bring about more alterations in the standards of bubble formation”, says the physicist Gérard-Liger-Belair, from the University of Reims Champagne-Ardenne, another of the study’s authors and a specialist in champagne bubbles and other carbonic acid drinks. On the points of the glass capable of generating the piles of carbonic gas, the birthplaces of effervescence, shortly afterwards the phase of the production of a single generated bubble predominates.

When the difference between two successive standards or phases shortens to the increase of only one unit in the production rhythm of the object being analyzed, the physicists describe this phenomenon with the technical name of period-doubling route. Also it has been found in the movement of sea waves, in the complex responses of neurons and in electronic circuits, to site only a few examples, the period-doubling route indicates, at times, that the system is in the anti-room of chaos. In the case of champagne, one cannot as yet say if there is or is not chaos in the bubble formation process. “We need more data to arrive at this conclusion as well as longer experiments, in which we can control the temperature and the quantity of carbon dioxide dissolved in the champagne, among other parameters”, considers the physicist José Carlos Sartorelli, from USP, who participated in the analysis of the behavior of the bubbles in the frothy wine. Here the term chaos, which in popular terms is synonymous with disorder and confusion, is used in the sense adopted by physicists and mathematicians: to design dynamic non-linear systems that, although they remain functioning in an random manner, are ruled by some parameters and sensitive to certain predictabilities, above all at the initial moments when they function. Therefore, in a few words, chaotic systems can be extended and, to some extend, controlled.

To understand the dynamics that led to the alterations in the routine of bubble production in fluid mediums rich in gases, as is the case of the bubbly wine full of CO2 molecules, could well be useful for the control of various situations, many of which have no relationship with the world of fermented drinks. The excess of bubbles in liquids can unchain risk scenarios for animals and plants. In vascular plants, the transporting of nutrients could be interrupted due to the sudden appearance of gas bubbles in the xylem, the plant tissue that supplies water from the roots to the rest of the vegetal. “The main cause of embolism in human beings (the bursting of a blood vessel because of an abnormal mass of material coming from another part of the body) also involves the formation of bubbles starting from supersaturated liquids with dissolved gas”, says Tufaile. “A gaseous embolism can even occur with divers who return to the surface too quickly after they have been breathing the high pressure air contained in diving cylinders.” Collective tragedies can derive from instabilities brought about in liquid solutions that are made up of many gases. In August of 1986, a deep lake in the northwest of the Republic of Cameroon, named Nyos, which covers the mouth of an extinct volcano and for this reason received a huge quantity of carbon dioxide gas into its water, expelled a cloud of this gas that asphyxiated and killed 1,700 inhabitants on its surroundings.

The dirt in glasses
Once the different rhythms that impel the champagne bubble production was identified, what was still missing was to explain the factors that brought about the constant change of phases. In the end, why does the birthplace of the bubbles in the bottom of the glass stop producing gaseous spheres of threesomes and abruptly pass to originate those of one after one? Belair hurled himself into this mystery some years ago, but only now, with the help of the Brazilians, has he managed to formulate a hypothesis consistent with the phenomenon. And the explanation has to do with a discovery made this decade by the Frenchman himself, who upset many gourmets: the majority of the bubbly wine bubbles are born on the points of the glass wall to which very small impurities adhere, in general cylindrical fibers of cellulose of 100 micrometers, which arrive there from the air or are the by-products of poorly washed glasses. A very small piece of dirt, harmless to health, is the matrix of the noble bubbles of champagne. Up until then many people had believed that the bubbles sprung up exclusively from imperfections, scratches and prominences on the glasses – a belief that had led restaurants to scratch their very own flutes in the hope of serving more bubbly champagne to their clients. Such a gesture, say the scientists, is just as inefficient as placing the end of a teaspoon into the mouth of a bottle to stop the loss of gas from the drink.

Returning to the role of impurities in the genesis of effervescence, the cellulose micro-fibers are hollow inside and possess a negligible cavity in one of their extremities to where the dissolved CO2 in the champagne enters and leaves. “Because of pressure, temperature and other chemical parameters of the liquid, these gas bubbles function like a motor and dictate the rhythm of bubble production”, explained physicist Tufaile. When the interior of the fiber reaches its limit of gas storage via diffusion, spheres of carbon dioxide set themselves loose from the cavity. In a second, or at the maximum five, the bubbles hit the surface, not without before having increased their diameter, considering that they gain more gas during their ascension. And, grouped onto them rise the molecules that carry the aromas typical of champagne, which delight the consumers.

Legend and marketing
To understand and to control the process of the formation of the gaseous tales that give life to the sparkling wines is a challenge that man has been pursuing since the end of the 17th century, when, according to legend (and to the marketing producers), Don. Pierre Pérignon, a Benedict monk from the abbey at Hautvillers, a small locality in the region of Champagne-Ardenne, “invented”, without trying to, the first wine of this type, champagne. Although there is for now no scientific proof that the attributes of a sparkling wine guard some direct relationship with the characteristics of its bubbles, professional tasters interpret the existence of small, numerous and lasting small bubbles as a sign of excellence. “For sure it’s much more agreeable to look on the presence of many diminutive bubbles in champagne but there’s no connection between their size and the quality of the product”, clarifies Belair. The influence of the glass is very great in the effervescence of a sparkling wine. “I’ve already approved products of excellent quality that produce only a few bubbles and of relatively large diameter”, comments the enologist Mauro Celso Zanus, from Embrapa (Grape and Wine), from the town of Bento Gonçalves, in Rio Grande do Sul State. “For this reason it’s better to evaluate the quality of the sparkling wine by its finesse and the clarity of its aroma and taste on the pallet.”

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