Bubbles determine the amount of alcohol in mescal

Mescal is a Mexican spirit, distilled from fermented agave juice, and it holds distinctive significance in the country’s rural villages. The traditional (and still most common) way to determine the alcohol content in mescal is to splash a jet of the liquor in a small container, as shown here, and visually inspect the bubbles that form. The splashing creates air cavities that rise to the surface as bubbles. If the alcohol concentration is close to 55%, the bubbles remain stable for up to 30 seconds. But if it’s higher or lower than that value, they rapidly burst within a few seconds.

A new study led by Brown University’s Roberto Zenit explains the fluid dynamics behind that peak in bubble longevity and confirms that the visual-inspection method for gauging alcohol content is indeed reliable—at least for mescal. (Experiments conducted in 1990 revealed a similar peak in whiskey.) Earlier work attributed the longevity to changes in surface tension, but the precise mechanism remained unclear.

Zenit and his colleagues conducted experiments and ran numerical simulations of bubble lifetimes—in mescal and in other water–ethanol mixtures—as a function of ethanol concentration. As part of the experiments, they measured the time it takes bubbles to drain enough of their film to burst. That drainage time results from the balance of viscous forces and either gravitational or surface-tension forces on the bubble. A dimensionless parameter known as the Bond number, Bo = ρgD²/σ, determines the relative importance of gravitational and surface-tension effects. In the equation, ρ is the liquid density, g is the gravitational acceleration, D is the bubble diameter, and σ is the surface tension.

The measurements revealed that changes in the mixtures’ viscosity and surface tension, and the appearance of gradients in surface tension due to the presence of surfactants, were key influences on the lifetimes of the bubbles. Indeed, maximum lifetime occurs at the same alcohol concentration at which viscosity peaks. Moreover, as the Bond number changes, so does the drainage mechanism in the bubbles. When Bo << 1, the surface tension dominates, the film is uniform, and the bubble lifetimes increase. But when Bo >> 1, gravity dominates and the opposite trend occurs: Their lifetimes decrease with Bo, but the gradients in surface tension cause the bubbles to thin nonuniformly until they burst. The critical value between the two regimes, where Bo is unity, corresponds to the bubbles’ maximum lifetime, a result confirmed by the group’s experiments. Zenit hopes that the team’s dimensionless analysis of bubble longevity and film drainage will provide insight into other natural and industrial problems, such as biofoams, ocean froth, and volcanic flows (see the article by Zenit and Javier Rodríguez-Rodríguez, Physics Today, November 2018, page 44 ). (G. Rage et al., Sci. Rep. 10, 11014, 2020 .)