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NASA Science News for June 9, 2003

Strange Foam

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. 

June 9, 2003: 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 soda or beer. "Yet the physics
of foam is poorly understood," says Durian. 

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.


An unexplored realm

"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." 

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.


Measuring with light

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-colored) light. As a laser beam passes through the foam,
bubble membranes in motion cause a slight Doppler effect, shifting
the frequency--and hence the color--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. 

Credits & Contacts
Author: Patrick L. Barry, Dr. Tony Phillips 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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