Often spills your coffee? Just add some milk froth


Carrying a full cup of coffee from the kitchen to the dining room is no easy task for the sleepy-eyed. With the pace of the person, the liquid level of the coffee will also sway continuously, and it will overflow from the mouth of the cup if the hand shakes. However, the situation is different with a latte covered in foam. Not only do these foams provide thermal insulation and creative space for latte art, they also have excellent shock absorption properties—the findings of a recent study published in the journal Physics of Fluids. The findings have implications not only for people's breakfast drinks, but also for the safe transport of liquefied gas and rocket propellants, the researchers say.

Emilie Dressaire is an associate professor of aerospace and mechanical engineering at New York University. She noticed while drinking a latte at Starbucks that it was less likely to spill without the lid, which led her to think that the foam might act as a shock absorber for the liquid surface. And her colleagues found the same phenomenon in beer.

“I was doing my PhD in the South of France at the time. In a pub, we found a pint of Guinness (a very foamy beer) that almost never spilled while walking. “said Alban Sauret, a researcher at the French National Centre for Scientific Research.

The scientists took the findings from the cafes and taverns back to the lab, systematically observed the effects of foam through experimental setups, and modeled calculations based on the phenomenon. They made a thin rectangular container out of glass and filled it with a mixed solution of water, glycerin, and dish soap (glycerin helps stabilize the foam and adjust the viscosity of the liquid). The researchers then injected air at a constant rate with a needle attached to the bottom of the container, so that bubbles of uniform size were formed. "The foam formed by the detergent is very stable, which allows us to complete the experiment before the foam disappears," said François Boulogne, another member of the research team.


Foam layers of different thicknesses. Image credit: Eurekalert

The researchers tested two types of motion: quick side-to-side vibrations and regular back-and-forth rocking. They used high-speed cameras to record the fluctuations in the liquid level. The results show that the effect of foam stabilizing the liquid level is very significant, and only five layers of foam are enough to reduce the fluctuation height by an order of magnitude.

Click "read the original text" to view the video: the effect of foam on slowing down the sloshing of liquid level. The red arrows indicate the velocity field of the foam flow.

The research team believes that the friction between the foam and the container dissipates kinetic energy, which is the main reason why the foam reduces the sloshing of the liquid level. More than five layers of foam doesn't add much more shock absorption because the top layer of foam doesn't actually move. The research team also found that the bubbles that did not contact the walls of the container did not contribute much to reducing the sloshing of the liquid surface.

The shaking of the liquid level not only brings trouble to the person serving the beverage, but also affects the transportation safety of dangerous liquids such as gasoline and liquefied gas. The authors hope that their findings will provide a simple, low-cost way to control sloshing during liquid transport. "Potential applications go far beyond beer," said Soret.