The marvelously stabilizing power of beer foam

A group of mechanical engineers walk into a bar. It sounds like the start of a bad joke, but it was actually the start of a cool scientific experiment.

The engineers order a few beers and one of them accidentally bumps the table. Their beers slosh around — but the beer with the most foam on top doesn't spill. Maybe it's just the way engineers see the world — or maybe it was the booze — but a few fundamental questions bubbled up.

Why did the beer with the foam not spill like the others? Why is it easier to carry a beer with foam than, say, a glass of wine? Why is a cappuccino less likely than a regular cup of coffee to slosh over the edge of the cup when you set it down?

In short: What are the fluid physics behind foam? A team of researchers from the Complex Fluids Group at Princeton University wanted to know, and they didn't cut any scientific corners to find out.

“When I first heard about this, I thought, ‘Okay, I'll go film this and they'll have pint glasses everywhere,’” says Luke Groskin, Science Friday’s video producer. “No. This is rigorous, real science. … They wanted to know what is actually going on with the foam that's preventing the sloshing.”

Using a special container and a high-speed camera, both mounted on a small table, the Princeton team recorded precise measurements of the waves inside the container while they adjusted the speed and amplitude of the jostling table, the amount and the density of the bubbles, the number of layers of bubbles and more.

“What they're doing is trying to isolate all the different variables so that they can study the actual effect of [the foam],” Groskin says.

What they found is that the friction created by the foam reduces the waves that move through a liquid when it is jostled. When the bubbles are touching one another, or the sides of the glass, while also in contact with the liquid surface of the beer, the friction they cause dampens the “harmonic oscillations” of the liquid.

That results in what we laypeople call less "sloshing around." But the researchers made it clear that this isn’t about your pint, Groskin says: “They're not just playing around with beer in the lab. They have a higher goal.”

When an oil tanker, a truck or rail tanker carries explosive fluid inside it, sloshing can be dangerous. If a tanker truck is making a sharp turn, or a rail car takes a curve too quickly, the fluid sloshing around makes the container more unstable and could cause an accident or damage to the container itself. 

The researchers wonder if putting foam on top of the fluid could prevent accidents or lessen damage. Finding an answer to that question is their long-term goal, Groskin says.

But for now, they're sticking to beer, and they have made a significant discovery: "Apparently, the magic number is five layers of foam. That applies to your everyday beer,” Groskin says. "Everything past five was just extra. Everything below and you weren't getting the full foamy effect."

Guinness was the winner in the study. The researchers studied four different commercial beers and found that Guinness performed the best because it had the finest foam and the smallest bubbles.

The researchers eventually want to examine all the different dimensions of foam. “They’re going to keep scaling up and scaling up,” says Groskin. “Hopefully they'll be able to eventually put this in a real world situation.”

And maybe you could win a few bets at your local pub.

This story is based on an interview that aired on PRI's Science Friday with Ira Flatow