The Science of Washing Up - Foam Explored

Image courtesy NASA. The gravity-driven flow of liquid toward the bottom of a foam prevents scientists from experimenting near the "critical point" here on the ground. Notice how the walls of the bubbles near the bottom of the image - where the liquid content is greater - are more rounded, while the bubbles higher up have straighter, more angular sides.

Over roughly the last 10 years, Durian's research group at UCLA along with others have been developing ways to use beams of light to measure the size, wetness, and movement of bubbles in a foam. These techniques are central to the FOAM experiment.

In one method, called "diffuse-transmission spectroscopy," the scientists shine the beam through the foam and measure how much of the light reaches the point on the other side. In a foam with only a few, very large bubbles, most of the light will pass straight through with little interference; in a foam of many, tiny bubbles, the light will get scattered by the bubble membranes. Measuring how much light reaches the far side lets the scientists quantify the average bubble size.

The motion of the bubbles can also be detected using monochromatic (single-coloured) light. As a laser beam passes through the foam, bubble membranes in motion cause a slight Doppler effect, shifting the frequency - and hence the colour of the light. Watching these ever so slight shifts in the light's frequency tells researchers how fast the bubbles are moving and in what direction. This technique is called "diffusing-wave spectroscopy."

Onboard the ISS, a simple water-based foam will be formed within the FOAM apparatus. Durian and colleagues, who will be able to remotely control the experiment from the ground, will select the ratio of liquid-to-gas so the foam is near its critical point. Then they'll shine a laser beam through the foam to explore its properties as the foam is twisted and deformed by mechanical plates.

"The goal," says Durian, "is to discover how the internal structure of the foam changes as its elastic character vanishes." The data will be fundamental. They're bound to interest anyone who wants to spray a foam around a corner or into a fire ... or anyone who wants to craft a physical theory of foam.

And best of all, perhaps, it's something to think about the next time you're doing the dishes.