The Science of Washing Up - Foam Explored

"One way to understand the basic physics of any material is to explore its 'critical point' - the threshold where the material changes phases, for instance, from a solid to a liquid," says Durian. "Exploring the critical point of foams is what FOAM will do."

Courtesy Dan Sandler A foam near the "critical point" would have perfectly round air bubbles

Foams, which can act like solids, are part gas and part liquid. What does it mean for such a substance to change phases?

Durian explains: The critical point of a foam occurs when the liquid content is so high (roughly 37% by volume) that the air bubbles are completely spherical and only touch each other at one point, like steel ball bearings piled together in a jar. That's when the foam ceases to act like a semi-solid stack of bubbles and begins acting instead like bubbles floating freely inside a flowing liquid - a "phase change" of sorts.

"It's impossible to explore the critical point of a foam on the ground, but in space we can study it quite well," Durian says.

Gravity causes the liquid in a foam to ooze downward, especially when the foam is relatively wet as it would be near the critical point. Here on Earth the critical point can't be reached because the liquid quickly pools at the bottom of the container, leaving a foam with odd flat-sided bubbles and only about 5% liquid content floating on top.

"In orbit, drainage of the foam is virtually absent, so we can bring a foam to the critical point and then explore it at our leisure," Durian says.

How do you explore a foam? You can't touch it, obviously, or you'll pop the bubbles and change the foam. Somehow, the researchers need a way to measure the traits of a foam without disturbing it.

The answer, says Durian, is light.