In Time for the Holidays...

A glass of beer, deconstructed. (a) At the beer’s surface sits a head of foam. (b) A trail of bubbles rises from a nucleation site along the glass wall. The rising of bubbles from different nucleation sites induces a global circulation (sketched here with white arrows). (c) Cellulose fibers serve as nucleation sites; gas cavities inside the fiber are clearly visible. (d) A 3-mm-wide mushroom-shaped bubble plume arises from the implosion of a millimeter-sized bubble when a beer bottle is gently tapped. (e) Surface bubbles seen from below are nearly on edge. (Panels a and b courtesy of Rodrigo Viñas, TresArt Collective.)

Topics: Chemistry, Fluid Mechanics, Physics Humor

Carbonation can also occur by fermentation. When yeast eats simple sugars, it primarily excretes ethanol and CO₂. If the process occurs in a closed container, the pressure rises as the amount of CO₂ increases. In turn, as the pressure rises, the gas dissolves. Although beer making dates back thousands of years,³ 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.

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.⁴

Alcoholic or not, bubbly drinks are full of physics. Figure 1 illustrates 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 fluid mechanics of bubbly drinks, Physics Today
Roberto Zenit (zenit@unam.mx) is a professor and researcher at the National Autonomous University of Mexico in Mexico City.
Javier Rodríguez-Rodríguez is a fluid mechanics professor at the Carlos III University of Madrid in Spain.