In most cases, the bubbles in a drink are the result of carbonation. The amount of carbon dioxide gas that dissolves in the liquid is proportional to pressure. And if the pressure is suddenly reduced, such as when a bottle of beer is opened, the gas quickly comes out of solution and forms bubbles that rise to the surface, only to burst after a brief instant or to aggregate into a frothy head of foam.

That’s just the start of what happens to carbonated drinks opened to air; many processes occur before the first refreshing sip. In this article we discuss the bubbles’ birth, motion, stability, and fascinating connections to a range of other phenomena that lie beyond the need for refreshment.

Perhaps the first question worth asking is, Why do we like bubbly drinks? A scientific answer has proven elusive. Carbonation, it turns out, triggers the same pain receptors in the deep brain that are activated by tasting spicy food.Curiously, when carbonated water is fed to other animals, such as mice, dogs, and horses, the animals refuse to drink it. But humans appear to enjoy the mildly irritating effects. Water, CO, and saliva enzymes react to produce small amounts of carbonic acid, the substance thought to be behind the tingly sensation. The bubbles themselves are known to alter a drink’s perceived flavor, at least in the case of soda: The smaller the bubbles, the faster they dissolve to produce carbonic acid.

Manufacturers usually adjust the gas content in their drinks to please the consumers’ palate; sweet drinks often contain more gas than savory ones do. To understand the bubbles’ influence on taste, researchers have tested the effect of their size distribution.The results were puzzling: The presence of bubbles was not required to experience the carbonation bite, and yet they did modulate the flavor. No one has a clear picture of how or why.

The bubbles also make noise. As resonant objects, they ring at particular frequencies that depend on their size and the mechanical properties of the liquid and gas. Indeed, researchers have tested how to “hear” the size distribution of the bubbles, which ring as they rise. (See the Quick Study by Kyle Spratt, Kevin Lee, and Preston Wilson, August 2018, page 66 .)

Alcoholic or not, bubbly drinks are full of physics. Figureillustrates the processes that occur when a carbonated drink is poured into a tall glass. If the liquid is poured shortly after the bottle is opened, the birth of bubbles is visible inside the liquid and on the surface of the glass. Streams of bubbles continuously form and induce convection that affects their production rate and motion. As they grow, the bubbles rise and eventually reach the surface. Once there, depending on the properties of the liquid, the bubbles either burst or float.

The presence of alcohol and other molecules during fermentation, such as proteins and enzymes, makes the physical description even more interesting. They affect the liquid’s surface tension, viscosity, density, and other properties, which in turn affect the formation, motion, and surface stability, or lifetime, of the bubbles. No less important is the bubbles’ ability to accelerate the absorption of alcohol in the body and thus the rapidity of intoxication.

Carbonation can also occur by fermentation. When yeast eats simple sugars, it primarily excretes ethanol and CO. If the process occurs in a closed container, the pressure rises as the amount of COincreases. In turn, as the pressure rises, the gas dissolves. Although beer making dates back thousands of years,it is unclear how bubbly beer could have been originally—old ceramic containers were most likely unsealed. Sparkling wine was discovered later—in the 17th century—and its carbonation comes from a secondary fermentation inside the bottle.

Carbonation can occur naturally or artificially. Better known for the discovery of oxygen, Joseph Priestley invented carbonation in 1772, when he discovered that air could be dissolved in water at high pressures. The original intention was to maintain potable water for consumption in ships. Even then, the most relevant result was the bubbly drink’s “distinct freshness,” as Priestley put it.

The birth and early life of bubbles Section: Choose Top of page ABSTRACT Origins The birth and early life ... << To burst or not to burst Jets and fizz Supplemental Materials References CITING ARTICLES

The behavior of bubbles in carbonated drinks is determined primarily by two physicochemical laws. The first one is Henry’s law: The concentration C s of dissolved gas at a liquid–gas interface is proportional to the gas’s partial pressure as k H p g , where k H is the so-called Henry’s constant (although its value actually depends on temperature). Usually termed the saturation concentration, C s has an important implication. If a liquid has been pressurized with CO 2 or some other soluble gas for sufficient time, it will contain more gas in solution than the liquid is able to store at a lower atmospheric pressure. The liquid is said to be supersaturated at ambient pressure.

Beer is typically bottled at a pressure of about 3 atm whereas champagne is usually bottled at 6 atm. Since Henry’s constant for CO 2 in water at 12 °C is 1.9 g/L · atm, a beer bottle contains between 5–6 g/L, whereas a champagne bottle contains about 11 g/L. Were those gases kept at standard pressure and temperature (25 °C at 1 atm), they would occupy 3 L and 5.6 L of CO 2 respectively. Compare those volumes with the gas content corresponding to saturation at ambient pressure, C s ≈ 1.9 g/L, which would occupy only 1 L.

The second law is Fick’s law of molecular diffusion. Analogous to Fourier’s law of heat conduction, Fick’s law states that in the presence of a concentration gradient, a mass flux is established that’s proportional to the gradient but in the opposite direction. The proportionality constant is the diffusivity D of CO 2 in water—about 2 × 10−9 m2/s.

When a bottle of liquid with pressurized CO 2 is opened at ambient pressure, Henry’s law dictates that any gas cavity inside it will immediately adopt the saturation concentration at ambient pressure. In response to the concentration gradient, molecular diffusion induces a net flux of gas toward the cavity that makes it grow as a bubble.

5 J. Chem. Phys. 18, 1505 (1950). 5. P. S. Epstein, M. S. Plesset,, 1505 (1950). https://doi.org/10.1063/1.1747520 d R d t = D Λ ζ 1 R + 1 π D t , where Λ is the Ostwald constant, the volume of liquid that can be saturated with a given volume of gas; ζ is the supersaturation level; and R is the bubble radius. The equation predicts that the size of a gas bubble grows by diffusion as the square root of time t. Gas cavities form as the glass is being filled and survive either by attaching to crevices on the glass, which pin and thus stabilize them, or by becoming trapped by impurities, such as cellulose fibers from barley or hops. 6 J. Agric. Food Chem. 53, 2788 (2005). 6. G. Liger-Belair,, 2788 (2005). https://doi.org/10.1021/jf048259e In the case of a spherical, isolated bubble, the two laws can be combined into the Epstein–Plesset equation,a building block for more elaborate models of gas–liquid mass exchange. If surface tension is neglected, the equation readswhere Λ is the Ostwald constant, the volume of liquid that can be saturated with a given volume of gas;is the supersaturation level; andis the bubble radius. The equation predicts that the size of a gas bubble grows by diffusion as the square root of time. Gas cavities form as the glass is being filled and survive either by attaching to crevices on the glass, which pin and thus stabilize them, or by becoming trapped by impurities, such as cellulose fibers from barley or hops.

When cavities become large enough for their buoyancy to detach them from such nucleation sites, they start to rise, usually leaving behind a smaller cavity to repeat the cycle. The repetition accounts for the trail of bubbles commonly observed in beer and champagne glasses. Once in the trail, new bubbles rise with a speed that increases with their radius. During that rise, the relative velocity between a bubble and the surrounding liquid enhances the mass transfer from the liquid phase to the gas phase, and the bubble actually grows faster than the predicted square root of time.

6 J. Agric. Food Chem. 53, 2788 (2005). 6. G. Liger-Belair,, 2788 (2005). https://doi.org/10.1021/jf048259e 2 2 diffuses into the water. For a small bubble of CO 2 in water, and thus for a small Reynolds number—the ratio of inertial to viscous forces—it can be shown that d R d t ~ Λ ζ ρ g D 2 μ 1 / 3 , where ρ is the density, g is the acceleration of gravity, and μ is the viscosity. In the equation dR/dt is independent of R and typically has a constant value of tenths of millimeters per second for bubbles found in drinks. That’s the reason bubbles inflate as they rise—not because of any change in hydrostatic pressure, which would only be about 1% of the ambient pressure in a glass 10 cm tall. Indeed, the radius of a bubble rising in a supersaturated liquid grows at a constant rate, independent of its size,as shown in figure. The upward motion is driven by the competition between buoyancy, which scales with the bubble’s volume, and viscous drag. The constant growth rate can be explained by the fact that the bubble motion is faster than the rate at which COdiffuses into the water. For a small bubble of COin water, and thus for a small Reynolds number—the ratio of inertial to viscous forces—it can be shown thatwhereis the density,is the acceleration of gravity, andis the viscosity. In the equationis independent ofand typically has a constant value of tenths of millimeters per second for bubbles found in drinks. That’s the reason bubbles inflate as they rise—not because of any change in hydrostatic pressure, which would only be about 1% of the ambient pressure in a glass 10 cm tall.

Similarly, a bubble sitting at a solid surface in a container grows more quickly if the liquid is flowing inside the container. That creates feedback between the bubble’s growth and the circulation induced in the glass after the drink is poured. More specifically, the flow near the glass walls promotes faster bubble growth. And as the bubbles rise, their momentum sustains the circulatory motion until a substantial part of the dissolved CO 2 is lost.

Eventually that chain of events degasses beer or sparkling wine left open to ambient conditions. By itself, diffusion would take an extremely long time to degas a carbonated drink—roughly H2/D, where H represents the height of the liquid. For a glass 10 cm tall, the calculation yields 5 × 106 seconds, or about 2 months.

2 bubbles actually take much less time to exsolve from a drink. Although the gas that bubbles transport to the surface is just 20% of the total volume, 6 J. Agric. Food Chem. 53, 2788 (2005). 6. G. Liger-Belair,, 2788 (2005). https://doi.org/10.1021/jf048259e 1 In 2005 Gérard Liger-Belair, a professor of chemical physics at the University of Reims Champagne-Ardenne, pointed out that CObubbles actually take much less time to exsolve from a drink. Although the gas that bubbles transport to the surface is just 20% of the total volume,the rising motion produces a global circulation that ultimately enhances the advection, as illustrated in figure

7 Phys. Rev. Lett. 113, 214501 (2014). 7. J. Rodríguez-Rodríguez, A. Casado-Chacón, D. Fuster,, 214501 (2014). https://doi.org/10.1103/PhysRevLett.113.214501 The degassing of a carbonated beverage can be further accelerated if the bottle is gently tapped. Four years ago, one of us (Rodríguez-Rodríguez) and coworkers showed that the impact creates a pressure wave that triggers the formation of dense bubble clouds, each consisting of about a million microbubbles.Those bubble clouds are much more effective than individual bubbles in generating a convective motion inside the bottle. Indeed, experiments have revealed the existence of bubbly plumes—akin to the mushroom clouds formed during an explosion—that promote mixing and thus degassing from a liquid.

Similar phenomena occur in chemical reactors when the products of a reaction are lighter, or warmer, than the reactants. They can likewise produce gas-driven eruptions known as limnic eruptions. The eruptions occur in lakes where the bottom becomes supersaturated with CO 2 because of geological or biological activity. Carbonated water is heavier than fresh water, which stabilizes the accumulation of substantial amounts of gas. For reasons not yet fully understood, some of the supersaturated water rises to a shallower depth where bubbles can nucleate and grow. The growth can then create a plume that, on reaching the free surface, establishes a bubble-laden conduit that continues degassing the bottom of the lake until the amount of dissolved CO 2 becomes too low to further support the plume. That sequence of events happened in 1986 in Cameroon’s Lake Nyos, when a limnic eruption suddenly released up to 300 000 tons of CO 2 . More than 1700 people and 3500 livestock suffocated in nearby towns and villages.

8 GSA Bull. 125, 664 (2013). 8. K. V. Cashman, R. S. J. Sparks,, 664 (2013). https://doi.org/10.1130/B30720.1 The importance of bubble growth and rise in a supersaturated liquid in geology goes beyond limnic eruptions. The sudden exsolution of volatile elements in magma, for instance, is known to strengthen the intensity of certain kinds of volcanic eruptions. As pointed out by volcanologists Katharine Cashman and Stephen Sparks, the mechanism is akin to the dramatic reaction that occurs when Mentos are dropped in a bottle of Diet Coke.