Carrying a cup of joe can prove dangerous: Just a small jostle can send hot liquid flying. A latte comes with considerably less risk. Thank the bubbles.
Alban Sauret of the French scientific research center CNRS in Aubervilliers and colleagues report February 24 in Physics of Fluids that bubbles can calm sloshing liquid.
With a high-speed camera, the team recorded oscillations of a mixture of water, glycerol and dish soap inside a rectangular container after they jolted or rocked it. Pumping air into the liquid, the researchers created varied quantities of bubbles. Bubbles form in layers on top of a liquid, and five layers were enough to reduce sloshing as long as the bubbles touched the container’s sides. The researchers think that foam bubbles absorb the energy driving the liquid waves by creating friction at the sides of the container.
The bubbles account for waiters’ ability to carry impossibly full cups of latte or glasses of beer without spilling them.
SPILLAGE SCIENCE To work out why lattes are less likely to spill than regular coffee, scientists analyzed video clips of sloshing liquid mixtures, shown here at one-fourth their normal speed.
Credit: Video by Alban Sauret/Princeton University; Photo by Nick Olejniczaqk/Flickr (CC BY NC-2.0).