Using odds and ends from the space
station pantry, researchers have learned something new about fluid
by Dr Tony Phillips
Try this: In your kitchen at home, squirt
a stream of warm honey into a cup of water or tea, and watch what
happens. Sweet gooey rivulets, falling downward, twist themselves
into curly-cues, filaments, and spinning "smoke rings." It's mesmerizing.
But only for a split-second, then the honey splats into the bottom
of the cup.
Gravity is such a brute.
What you need is a
kitchen in space. Without gravity dragging everything down, spinning
rings of honey in water could hang suspended for hours. Honey-ribbons
would have more time to twist and turn, developing into â€¦ no one
"How fluids mix in
weightlessness is not well understood," explains chemistry professor
John Pojman of the University of Southern Mississippi. Here on Earth,
he says, the physics is dominated by gravity. Dense fluids sink
and light fluids rise; everything else is a side effect of that
In space, the pull
of gravity subsides and other, more subtle phenomena rule. Intermolecular
forces can hold films or globs of fluid together that, on Earth,
would be torn apart by their own weight. These delicate structures
can last for a long time, simply because they float rather than
crash into the floor of their container.
That's not to say weightless
fluids are still. On the contrary, in a container holding two different
fluids, like honey and water, scientists expect strange and complicated
currents to flow. "Tiny differences in fluid composition or temperature
can, in theory, induce stresses that cause convection," explains
Pojman. This effect, called "Korteweg stress," is unobservable on
Earth because buoyant motions overwhelm it. But in space it could
Honey and water in the author's kitchen.
How different is teatime
in orbit? Astronaut Don Pettit showed us in 2003 when he filmed
himself taking tea onboard the International Space Station (ISS).
Instead of sipping from a cup, Pettit used chopsticks to pluck grape-sized
blobs of tea from mid-air, grinning each time he popped one in his
mouth. Pojman remembers seeing the film. "I wanted to fly right
up there and start experimenting," he says.
Understanding how fluids
behave, singly or in mixtures, is important to the space program,
especially now that NASA plans to send people back to the Moon and
on to Mars.
"We're going to have
to manufacture things in space," explains Pojman, "and that means
dealing with fluids." As an example, he offers plastics - a key component
of habitats, radiation shields, rovers, etc. Plastics are usually
formed by combining dissimilar fluids or fluids and powders, then
heating the mixture. "If you've ever used BondoTM to
repair your car, you've done this yourself: you mix a resin together
with peroxide to create a sticky plastic substance," adds Pojman.
Mixing is also necessary
for certain types of medical space-research - "especially protein
crystal growth in microgravity," notes Pojman. When two fluids are
put together, do "Korteweg currents" flow? Do the fluids dissolve
evenly? Do they break apart into droplets? These details actually
make a difference.
Pojman himself couldn't
go to the ISS to investigate such questions, so he devised an experiment
that astronauts could do for him: the Miscible Fluids in Microgravity
Experiment or MFMG for short. "MFMG is a very simple experiment,"
he says. "It involves two syringes, a drinking straw, honey and
water. All of these things were already onboard the ISS."
One syringe is filled
with honey or a honey-water solution, the other with pure water.
The tips of the syringes are connected via a short tube (the straw).
When all is ready, an astronaut gently squirts a blob of honey into
the water, or vice versa, and films what happens. ISS science officer
Mike Foale did the experiment last week, and transmitted the video
"We've already learned
something new," says Pojman.
Image credit: NASA
injected into water during the MFMG experiment onboard
the International Space Station, March 2003.
There's a number in
fluid physics theory called "the square gradient parameter" or k.
It's proportional to the strength of intermolecular forces between
two different fluids, like honey and water. "How two fluids behave
when mixed in low-gravity is going to depend on k," says
Pojman. "We've never been able to measure k on Earth for
a pair of miscible (mixable) fluids. It's value could be anything!
But just from watching the video of MFMG we've got an upper limit
on k - it must be less than 10-8 Newtons."
He reached this conclusion
in the following way: If k were much greater than 10-8 Newtons, honey blobs injected into
water would quickly assume a spherical shape. But they didn't. The
blobs, squeezed into elongated shapes as they passed through the
nozzle of the syringe, remained elongated.
"The fact that we could
do this using only odds and ends onboard the space station is encouraging,"
says Pojman. A similar procedure could be used to set limits on,
or actually measure, k for many different pairs of fluids.
Some fluids are more
important than others. Pojman is most interested in monomers and
polymers that might be used in space manufacturing. Such fluids
are simpler, internally, than honey, so they might lend themselves
to "cleaner" measurements of fluid physics constants.
It's unlikely, though,
than any of those other fluids will be as much fun, or mesmerizing,
as honey. Who knows what new physics lies in its sweet spinning
"smoke rings" or gooey dancing ribbons? It's something to think
about the next time you're relaxing with a cup of tea â€¦ and you
reach for the honey.
note: The kitchen-science
experiment described in the opening passages of this story is best
done using a "honey bear" - a plastic honey-filled bear with a nozzle
on top, available in most supermarkets - microwaved for about 30
seconds. The warmed honey flows easily through the nozzle with a
viscosity only a little greater than water. Squirt the honey, gently,
into a transparent cup filled with cool tap water. You'll soon see
rings and a variety of other weird shapes.