Guillaume Polidori, Philippe Jeandet, Gérard Liger-Belair. American Scientist. Volume 97, Issue 4. Jul/Aug 2009.
Legend has it that the Benedictine monk Dom Pierre Pérignon discovered the Champagne method for making sparkling wines more than 300 years ago. As it happens, a paper presented to the Royal Society in London described the Champagne production method in 1662, six years before Pérignon ever set foot in a monastery. In fact, Pérignon was first tasked with keeping bubbles out of wine, as the effervescence was seen as vulgar at the time. But then tastes changed and fizz became fashionable, so Pérignon’s mandate was reversed; he went on to develop many advances in Champagne production, including ways to increase carbonation. In any case, the process was not regularly used in the Champagne region of France to produce sparking wine until the 19th century. Since that time, Champagne has remained the wine of celebration, undoubtedly because of its bubbling behavior.
But what is the exact role of the bubbles? Is it just aesthetics? Do they contribute to only one aspect, or to many aspects, of the subjective final taste? We have been rigorously analyzing Champagne for more than a decade, using the physics of fluids in the service of wine in general and Champagne-tasting science in particular.
The Champagne Method
Fine sparkling wines and Champagne result from a two-step fermentation process. After completion of the first alcoholic fermentation, some flat Champagne wine (called base wine) is bottled with a mixture of yeast and sugar. Consequently, a second fermentation starts inside the bottle as the yeast consumes the sugar, producing alcohol and a large amount of carbon dioxide (CO2). This is why Champagne has a high concentration of CO2 dissolved in it—about 10 grams per liter of fluid—and the finished Champagne wine can be under as much as five or six atmospheres of pressure.
As the bottle is opened, the gas gushes out in the form of tiny CO2 bubbles. In order for the liquid to regain equilibrium once the cork is removed, it must release about five liters of CO2 from a 0.75 liter bottle, or about six times its own volume. About 80 percent of this CO2 is simply outgassed by direct diffusion, but the remaining 20 percent still equates to about 20 million bubbles per glass (a typical flute holds about 0.1 liter). For Champagne connoisseurs, smaller bubble size is also a measure of quality.
For consumers and winemakers as well, the role usually ascribed to bubbles in Champagne tasting is to awaken the sight sense. Indeed, the image of Champagne is intrinsically linked to the bubbles that look like “chains of pearls” in the glass and create a cushion of foam on the surface. But beyond this visual aspect, the informed consumer recognizes effervescence as one of the main ways that flavor is imparted, because bursting CO2 bubbles propel the aroma of sparkling wine into the drinker’s nose and mouth.
One cannot understand the bubbling and aromatic exhalation events in Champagne tasting, however, without studying the flow-mixing mechanisms inside the glass. Indeed, a key assumption is that a link of causality may exist between flow structures created in the wine due to bubble motion and the process of flavor exhalation. But the consequences of the bubble behavior on the dynamics of the Champagne inside the glass and the Corpropelling process are still unknown. Quantifying the exhalation of flavors and aromas seems a considerable challenge, something that is difficult to control experimentally, but this constitutes the aim of our current work.
The Birth of Bubbles
The first step is to elucidate how bubbles themselves come into being. Generally speaking, two methods exist, and sometimes coexist, to generate bubble chains in Champagne glasses. Natural effervescence depends on a random condition: the presence of tiny cellulose fibers deposited from the air or left over after wiping the glass with a towel, which cling to the glass due to electrostatic forces. These fibers are made of closely packed microfibrils, themselves consisting of long polymer chains composed mainly of glucose. Each fiber, about 100 micrometers long, develops an internal gas pocket as the glass is filled. Capillary action tries to pull the fluid inside the micro-channel of the fiber, but if the fiber is completely submerged before it can be filled, it will hold onto its trapped air. Such gas trapping is aided when the fibers are long and thin, and when the liquid has a low surface tension and high viscosity. Champagne has a surface tension about 30 percent less than that of water, and a viscosity about 50 percent higher.
These microfiber gas pockets act as nucleation sites for the formation of bubbles. To aggregate, CO2 has to push through liquid molecules held together by van der Waals forces, which it would not have enough energy to do on its own. The gas pockets lower the energy barrier to bubble formation (as long as they are above a critical size of 2 micrometers in radius, because below that size the gas pressure inside the bubble is too high to permit CO2 to diffuse inside). It should be noted that irregularities in the glass surface itself cannot act as nucleation sites—such imperfections are far too small, unless larger microscratches are purposely made.
Once a bubble grows to a size of 10 to 50 micrometers, it is buoyant enough to detach from the fiber, and another one forms like clockwork; an average of 30 bubbles per second are released from each fiber. The bubbles expand from further diffusion of CO2 into them as they rise, which increases their buoyancy and accelerates their speed of ascent. They usually max out at less than a millimeter in diameter over the course of their one- to five-second travel time up the length of a flute.
Because natural nucleation is very random and not easily controllable, another way to generate bubbles is to use a mechanical process that is perfectly reproducible from one filling to the next. Glassmakers use a laser to engrave artificial nucleation sites at the bottom of the glass; such modified glasses are commonly used by Champagne houses during tastings. To make the effervescence pattern pleasing to the eye, artisans use no fewer than 20 impacts to create a ring shape, which produces a regular column of rising bubbles.
Fizz and Flow
The displacement of an object in a quiescent fluid induces the motion of fluid layers in its vicinity. Champagne bubbles are no exception to this rule, acting like objects in motion, no matter whether the method used to produce them was random or manufactured. Viscous effects make the lower part of a bubble a low-pressure area, which attracts fluid molecules around it and drags some fluid to the top surface, although the bubbles move about 10 times faster than the fluid.
Consequently, bubbles and their neighboring liquid move as concurrent upward flows along the center line of the glass. Because the bubble generation from nucleation sites is continuous, and because a glass of Champagne is a confined vessel, this constant upward ascent of the fluid ineluctably induces a rotational flow as well.
To get a precise idea of the role bubbles play in the fluid motion, we observed a Champagne flute with single nucleation site at the bottom. A bubble’s geometric evolution is well studied in carbonated beverages. For example, we know that the bubble growth rate during vertical ascent reliably leads to an average diameter of about 500 micrometers for a 10-centimeter migration length in a flute. In fact, for such a liquid supersaturated with dissolved CO2 gas molecules, empirical relationships reveal the bubble diameter to be proportional to the cube root of the vertical displacement.
Another property of bubbles is that they can act as either rigid or flexible spheres as they rise, depending on the content of the fluid they are in, and rigid spheres experience more drag than flexible ones. Champagne bubbles do not act as rigid spheres, whereas bubbles in other fizzy fluids, such as beer, do. Beer contains a lot of proteins, which coat the outside of the bubbles as they ascend, preventing their deformation. Beer is also less carbonated than Champagne, so bubbles in it do not grow as quickly, making it easier for proteins to completely encircle them. But Champagne is a relatively low-protein fluid, so there are fewer surfactants to stick to the bubbles and slow them down as they ascend. In addition, Champagne’s high carbonation makes bubbles grow rapidly on their upwards trip, creating ever more untainted surface area, in effect cleaning themselves of surfactants faster than new molecules can fill in the space. However, some surfactants are necessary to keep bubbles in linear streams—with none, fluid flows would jostle the bubbles out of their orderly lines.
We carried out filling experiments at room temperature to avoid condensation on the glass surface, and allowed the filled glass to settle for a minute or so before taking measurements. Our visualization is based on a laser tomography technique, where a laser sheet 2 millimeters wide crosses the center line of the flute, imaging just this twodimensional section of the glass using long-exposure photography. We seeded the Champagne with Rilsan particles as tracers of fluid motion. These polymer particles are quasi-spherical in shape, with diameters ranging from 75 to 150 micrometers, and have a density (1.060) close to that of Champagne (0.998). The particles are neutrally buoyant and do not affect bubble production, but they are very reflective of laser light. It is amazing to see the amount of fluid that can be set in motion by viscous effects. In our resulting images, a white central line corresponds to the bubble train path during the exposure time of the camera, and the fluid motion is characterized by a swirling vortex that is symmetrical on both sides of the bubble chain. We were able to reveal the same vertical structures with fluorescent dye.
The vortex-pair in the planar view of our image can be extrapolated to show a three-dimensional annular flow around the center line of bubbles. This means that a single fixed nuclear site on the glass surface can set the entire surrounding fluid into a small-scale ring vortex. But what really happens in normal Champagne-tasting conditions, with multiple nucleation sites? Is the entire volume of the Champagne affected? Are there different mixing flow patterns according to the method of effervescence? To answer these questions, we investigated two cases: one where only random nucleation sites are present and another where only controlled effervescence occurs.
As we mentioned previously, random effervescence is mainly due to the presence of cellulose fibers deposited on Champagne glasses. The number and distribution of sites is unpredictable. Indeed, most bubble-generating sites are found freely floating within the Champagne after pouring. Because they move about in swooping patterns and produce off-shooting bubble paths that don’t go straight up, we call these particles fliers. Our recent estimation of the dynamics of these fliers has shown them to be neutrally buoyant on average with regard to the surrounding fluid. In quiescent Champagne, the vertical velocity of a flier can be either positive or negative, depending on its buoyancy parameters and the gas-pocket volume it contains. After rough calculations, we found the free vertical velocity of fliers to range between -0.19 and 0.13 millimeters per second. These values are negligible compared to the fluid velocity, so fliers can make rather good fluid-motion markers.
Because of their high buoyancy, natural bubble nucleation sites can end up being prisoners of the motion they themselves initiated. Time-lapse images of fliers look something like claw scratches, with each lighted filament corresponding to a bubble trajectory. These visualizations are a powerful tool for giving a precise idea of the bubble-emission frequency and wavelength. For example, linear motion in the laser-lighted plane results in a flier print made from the combination of the vertical ascendant motion of bubbles and the linear oblique velocity of a flier. When the flier describes a complex curvilinear travel path, the visualization yields a spectacular result looking like an abstract art painting.
Random effervescence causes bubbles released from fliers to form complex fluidflow patterns with multiple unsteady cells that evolve over time. For example, an image of the top corner of one glass shows that no less that three eddies occupy a small area, leading to small-scale but vigorous mixing and circulation processes. The cells change in size and location over time according to an arbitrary scheme. Purely chaotic behavior characterizes the flow in random effervescence.
Champagne-tasting science involves a number of very subjective judgments, often difficult to quantify. For example, there is an inherent compromise between the visual aspects of bubbly behavior and olfactory stimulation, as these two qualities appear to be at odds. Too much nucleation will excite the sense of sight but cause the carbonation to quickly fizzle out, making for unpleasant tasting. On the contrary, poor nucleation will produce fewer bubbles in the glass, but more bubbles and aromas in the taster’s nose and mouth, consequently enhancing the senses of smell and taste at the expense of sight. From the many experiments we have conducted with controlled effervescence, it seems that an ideal number of about 20 nucleation sites best satisfies this dilemma.
Our laser visualizations of fluid flow have shown that a flute with an engraved circular crown reaches a steady state of fluid motion about 30 seconds after the glass is poured. The vortices do not swirl around and change shape, in contrast to those created in unetched glasses. The bubbles are highly reflective, allowing one to clearly observe the formation of a rising gas column along the vertical glass axis from the treated bottom up to the free surface of the beverage. Consequently, the driving force it imparts to the surrounding fluid generates two large counter-rotating vortices in the vertical lighted section. These cells are located outside the rising bubbles, close to the wall of the flute. Because this gas column acts like a continuous swirling-motion generator within the glass, the flow structure exhibits a quasisteady two-dimensional behavior with a geometry that is symmetrical around the center line of the glass. It clearly appears in the case of an engraved flute that the whole domain of the liquid is homogeneously mixed.
To complete our observations, we also studied the flow in an engraved traditional Champagne coupe, which is much wider but shallower than the flute. As in the flute, the rising CO2 bubble column causes the main fluid to move inside the coupe. However, two distinctive steady-flow patterns, instead of one, appear in a glass of this shape. Like the flute, the coupe clearly exhibits a single swirling ring, whose cross section appears as two counter-rotating vortices close to the glass axis. What strongly differs from the motion in the flute is that this recirculation flow region does not occupy the whole volume of the glass. The periphery of the coupe is instead characterized by a zone of no motion. Thus, for a wide-rimmed glass, only about half of the liquid bulk participates in the Champagnemixing process. Nevertheless, in an engraved glass of either shape, the presence of a ring vortex is not timedependent; it still forms in the coupe, despite the ascent time being about a third of that in the flute.
High-speed photography can also capture the end of a bubble’s lifespan (see “The End of a Bubble” on page 299). Most bubbles burst at the free surface during their migration from the center toward the edge of the vessel, whatever the glass shape. Only the top of the bubble emerges from the liquid, like an iceberg. As the fluid drains from the bubble top over about 10 to 100 microseconds, it reaches a thickness of less than 100 nanometers and ruptures. The inrushing sides of the collapsing bubble meet at the bottom of the cavity and cause it to eject a jet of liquid, which breaks up into droplets. The jet can travel at as much as a few meters per second and reach up to a few centimeters above the surface. A laser sheet in the symmetry plane of the glass highlights the projection of hundreds of Champagne droplets induced by such bursts. With a long enough exposure time, a digital still image gives one the feeling of visualizing a splendid droplet fog in motion above the Champagne surface.
As time increases after pouring, surfactant levels at the surface of the wine increase; these interlock in the liquid layer over the bubble caps, strengthening the surface tension and reducing the liquid velocity of the film so it does not drain away as rapidly, which extends the bubble lifespan. The wine develops a long-lasting collar of foam at the periphery of the flute. Even minute amounts of oils will instantly rupture bubble caps, however, so it is aesthetically vital to keep such substances (from snacks or lipsticks, for instance) apart from Champagne.
The most significant difference in flows between widened glasses and elongated ones is the size of the recirculation region. Further, a causal relationship clearly exists between the radial migration extent of the bubbles and the size of the vortical flow below the surface: Faster flow below the surface propels the bubbles farther towards the edge. There exists also a strong relationship between the aroma that emits from the Champagne surface and the presence of numerous droplets issued from bursting bubbles.
The bubble’s kinetic energy at the moment of collision with the liquid surface has a profound influence on bubble radial velocity. In the case of a widened glass, the short ascent distance precludes kinetic energy sufficient to make a bubble reach the edge of the glass before it bursts. The limited liquid-swirling motion and the short lifetime of the bubbles mean that their surface motion is confined in a limited radial area of the free surface. In a coupe glass, only about half of the surface area participates in both the mixing process below the liquid surface and the olfactory droplet production above the fluid. However, in the case of a flute, once bubbles have reached the surface, their kinetic energy level is sufficient to let them reach the glass edge, and the whole liquid surface is involved in the aroma exhalation process.
The convettive cells below the liquid surface in a flute carry the bubbles that have emerged from the center of the surface toward the glass edge over a distance of about 2.5 centimeters, whereas in the case of the Champagne coupe, the distance is only about 1 centimeter.
Our work shows that the emission of aromas from a Champagne glass cannot be decoupled from what happens below the free surface, in particular the flow-mixing patterns. The classical engraved, slender and elongated Champagne glass mixes the whole domain of the liquid phase homogeneously, whereas in the engraved Champagne coupe, the recirculating flow region does not occupy the whole volume in the glass. Instead, a zone of no motion inhibits the formation of the desirable collar of foam at the glass edge.
We hope that our analysis of bubble-induced flow patterns and other objective elements of Champagne behavior will be just the beginning of the scientific study of the olfactory behavior of Champagne and sparkling wines in a glass. One area that merits further study is the release of aromas from bubbles. Droplets from bursting bubbles commonly contain much higher concentrations of aromatic compounds than those found in the bulk of the liquid. This is largely because bubbles attract surfactant molecules as they ascend, the same surfactants that can cause them to have increased drag. In Champagne these molecules include flavor-active volatile thiols, as well as other alcohols, aldehydes and organic acids.
Engraved glasses, particularly flutes, have much more vigorous mixing than non-engraved ones, so one would expect the etched glasses to release more CO2 bubbles and flavor compounds. But this may not be all good, because too many bubbles can irritate a taster’s nose, affecting the evaluation of the aroma that the winemaker is trying to achieve. We hope that there may be comparison testing of the same Champagnes from plain and etched glasses in the near future. The good news is that glassmakers are eager to experiment with various glass shapes, and engraving shapes and locations, in order to achieve the perfect glass of Champagne.
The End of a Bubble
A Champagne bubble’s life comes to an end when it bursts at the liquid surface, but how it pops depends on how long the wine has been fizzing. Immediately after pouring, sparkling wines form a layer of foam at the top surface, and bubbles in this foam collapse in avalanche fashion-the bursting of one induces its neighbors to pop as well, producing clusters of disintegration events.
After a few seconds, the Champagne surface loses its foamy head and settles into a raft of close-packed bubbles, where each bubble has six neighbors in a single layer (top photo, right). Most of the bubble is actually below the liquid surface—only the top, or bubble cap, pokes through, much like an iceberg in the ocean. The fluid of the bubble cap begins to drain away, and after about 10 to 100 microseconds, it reaches a critical thickness of less than 100 nanometers. At this point the membrane is so unstable that any disturbance in temperature or vibration will cause it to rupture.
Bubble bursts happen too fast to see, but high-speed photography shows that a bubble collapse leaves a temporary indentation in the fluid surface, forming a flower-like structure with the surrounding bubbles (second photo, left). The sides of the former bubble suddenly experience positive pressure, whereas the bottom of the cavity becomes a zone of negative pressure, so the sides rush down towards the bottom in order to equalize the imbalance in tension.
This sudden, dramatic increase in surface tension in the area of the former bubble has a remarkable effect on its neighbors. Paradoxically, even though the bubble has burst upwards, surrounding bubbles are not blown up but sucked down into the hollow left by the disintegrated bubble cap. The shear stress is so great that it deforms the adjacent bubbles into elongated shapes (third photo, right). The stretching significantly increases the surface area in the surrounding bubble caps, and they also absorb the energy released by the collapse of the central bubble, much as a tiny air bag would do. They store this energy in the thin liquid film of their bubble caps, a process which eventually leads to higher stresses around these bubble flowers than would be found around single collapsing bubbles. However, despite these violent perturbations, the neighboring bubbles are not induced by the bursting of their central member to collapse in a chain reaction, unlike what happens in the foam stage, largely due to the viscosity of Champagne.
The final stage of bubble collapse happens when the inrushing sides of a burst bubble collide at the bottom of the cavity with such force that they push upwards a jet of fluid at a speed of as much as a few meters per second (last photo, left). The jet can reach up to a few centimeters above the surface. It then becomes unstable and breaks up into about five or more droplets, each on the order of 100 micrometers in diameter. Inertia and surface tension combine to give the drops an amazing variety of initial shapes and sizes, which then stabilize back into the expected quasispherical form. The entire process of a bubble’s collapse, from the first puncture in the bubble cap to the liquid jet breaking up into droplets, takes only about 100 microseconds.