Black coffee with some foam coffee is not easy to spill.

It is not easy for bleary-eyed people to bring a full cup of coffee from the kitchen to the restaurant. With the pace of the person, the liquid surface of the coffee will shake constantly, and a little shaking will overflow from the mouth of the cup. However, the situation of lattes covered with foam is different. Not only do these foams provide creative space for insulation and coffee flowers, they also have excellent shock absorbers, according to a recent study published in the Journal of fluid Physics (journal Physics of Fluids). The researchers say the finding makes sense not only for people's breakfast drinks, but also for the safe transportation of liquefied gas and rocket propellants.

Emily Drissel (Emilie Dressaire) is an associate professor of aerospace mechanical engineering at New York University. While drinking a latte at Starbucks, she noticed that the coffee was less likely to spill even without a lid, which made her think that foam might have a shock-absorbing effect on the liquid level. Her colleagues also found this phenomenon in beer.

"I was still studying for a PhD in the south of France at that time. In a pub, we found that a pint of Guinness beer (a very foamy beer) would hardly spill when walking. " Said Alban Sauret, a researcher now at the French National Centre for Scientific Research.

The scientists took the findings from cafes and taverns back to the laboratory, systematically observed the role of foam through experimental equipment, and modeled and calculated based on the phenomenon. They made a thin rectangular container out of glass, filled with a mixture of water, glycerin and detergent (glycerin can help stabilize the foam and adjust the viscosity of the liquid). The researchers then injected air at a uniform speed with a needle fixed to the bottom of the container so that bubbles of uniform size could be formed. "the foam formed by detergent is very stable, which allows us to complete the experiment before the foam disappears." Another member of the research team, Fran?ois? Boulogne) indicates.

Foam layers of different thicknesses. Picture from: Eurekalert

The researchers tested two modes of movement: rapid lateral vibration and regular back-and-forth shaking. They used high-speed cameras to record 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 is enough to reduce the fluctuation height by an order of magnitude.

Video playback is not supported. You can access the original text through the link at the bottom of the article.

Video: the effect of foam on slowing down liquid surface sloshing. The red arrow indicates the velocity field of the foam flow. The video is from the original paper.

The team believes that the friction between the foam and the container consumes kinetic energy, which is the main reason why foam reduces liquid surface sloshing. Bubbles with more than five layers do not significantly increase the damping effect, because the top foam does not actually move. The team also found that bubbles that do not come into contact with the container wall do not contribute much to reducing liquid surface sloshing.

Liquid level sloshing not only brings annoyance to people serving drinks, but also affects the transportation safety of dangerous liquids such as gasoline and liquefied gas. The authors hope that their research results will help people control the sloshing of liquids in a simple and low-cost way. "the potential applications go far beyond beer." Solet said. (editor: the window rains)