Mezcal is a classic Mexican spirit distilled from the agave plant. It can take your margaritas to the next level, but a new understanding of a traditional distillation technique used by mezcal producers could also provide insight into producing biofoams and better understanding lava flows.
When a mezcal producer wants to test the their product, they squirt a thin jet of the spirit into a vessel and observe the bubbles that result. If the bubbles pop quickly, that's not good; there is likely something wrong with the alcohol content. But if the bubbles survive 30 seconds or longer, they have a well-balanced spirit. And while this technique has been used for many years, exactly why it works has long been a mystery.
Bubbles on the surface of a liquid pop when the bubble "membrane" thins and becomes unstable. This happens to soap bubbles when gravity causes the fluid in the bubble to drain and thin. However, when a group of fluid mechanists studied mezcal bubbles, they found fluid flowed up, into the bubble.
Alcohol has a lower surface tension than water, which means it evaporates and spreads more readily. So, when a mezcal bubble forms, the alcohol begins evaporating faster than the water. This increases the surface tension in the bubble compared to the rest of the liquid, and causes more high-alcohol fluid to flow up, into the bubble, thus slowing the thinning process.
The specifics involve viscosity and density of the fluid as well, but the researchers found that the effect was largely dependent on alcohol content. There was a narrow range of alcohol contents that maximized bubble lifetime; deviations above and below 50% drastically reduced how long bubbles lasted.
The physics behind this process is useful for distilling alcohol, but the principals are transferable to all sorts of materials, like lava, biological materials, and soaps, just to name a few. The research team hopes this information will also be useful for better understanding some environmental, biological, and industrial processes.