There's nothing worse than being half asleep and trying to navigate your house with a cup of coffee in hand that's sloshing around inside the cup and spilling over the sides, burning your fingers.
However, there is a way to avoid such a mess: go for a latte instead.
It turns out that the foamy milk on top of a latte keeps the coffee from sloshing around so much in the cup, thereby preventing spillage.
A research team from Princeton University recently realized this and determined to find out why after they noticed a similar occurrence with beer. So they did what scientists do: they set up an experiment to further study how a foamy head stops the sloshing.
"While I was studying for my Ph.D. in the south of France, we were in a pub, and we noticed that when we were carrying a pint of Guinness, which is a very foamy beer, the sloshing almost didn't happen at all," says Alban Sauret, a researcher at the French National Center for Scientific Research (CNRS).
The team built a mechanism that tested how well foam dampens sloshing. They filled a rectangular glass container with water, glycerol (to increase the water's viscosity) and a dishwashing detergent (which is stable enough to prevent bubbles from disappearing quickly when they form). Then they injected constant air into the bottom of the container via a needle, which created bubbles on top of the fluid.
They jolted and rocked the container and recorded their results with a high-speed camera. In doing so, they found that five layers of foam at the top of a liquid made the wave height in the liquid ten times less than the wave height of it without foam. However, anything over five layers hardly made a difference.
The team theorizes that because of the foam's friction against the sides of the container, it absorbs the energy of the sloshing liquid. However, the foam in the middle that is not touching the container's sides has little effect.
So now you know the best way to avoid spillage for your coffee and why beer doesn't slosh around so much when it has a foamy top. However, this research has other applications, particularly with transporting fluids that are more dangerous, such as liquefied gas and oil. The team hopes to apply what they've learned to safely transporting large containers of such liquids.
"The potential applications are much bigger than just beer," says Sauret.
