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The Science of Washing Up - Foam Explored

The physics underlying common everyday foams is poorly understood. An experiment scheduled to fly on the International Space Station will help fill in the gaps.

by Patrick L Barry and Dr Tony Phillips

Put some dish-washing soap in the kitchen sink and fill it with water. You'll create a truly bizarre substance.

Despite being made almost completely of air, the sudsy foam in that sink somehow behaves like a springy solid. Strange.

Douglas Durian, a professor of physics at UCLA suggests the following: "Take some shaving cream and put it in your hand. Touch it. Run your fingers through it. Ask yourself, is it a solid, a liquid, or a gas?"

Ordinary aqueous foams, like shaving cream or the suds in a dishwasher's sink, are mostly gas (95%) and a little bit of liquid (5%). The gas subdivides the liquid into a matrix of tiny bubbles. Good foams usually contain complex molecules that toughen the walls of the bubbles. Milk fat, for instance, serves this purpose in whipped cream. The way the bubbles stick together or slip past one another determines how the foam behaves.

Many of us are so accustomed to foams that we hardly notice how odd they are. Foams are on our legs or faces when we shave, on our dishes as we wash them, atop our glasses of beer. "Yet the physics of foam is poorly understood," says Durian.

Courtesy Junction of Occupational Therapy Function

A little soap and water is all it takes to make your own science experiment.

Much of what is known comes from trial and error. No theory currently exists for predicting exactly how stiff or oozy a foam will be based on its traits like the size of its bubbles or the amount of liquid it contains. And the precise stiffness of a foam is crucial for many uses. Just imagine: a fire-retardant foam that must flow quickly through the valve of the extinguisher and then cling tightly where it lands; or a counter-biological weapons agent that expands to fill cracks and crevasses and kills microbes hiding there.

Durian would like to take the guesswork out of foams by learning more about their fundamental physics. That's the goal of an experiment he and colleagues are designing for the International Space Station (ISS). It's called FOAM, short for Foam Optics and Mechanics.

"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.

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.


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First Science 2014