French scientists have
studied how the transition from liquid to gas and back again slows
down in a weightless environment and how an artificial form of
gravity can be simulated using high-speed vibration of the sample.
This work has implications for work in space, where fluids don't
behave the way they do on the ground.
Past studies have shown that
vibrating an astronaut's legs and feet helps to prevent muscle decay
or bone decalcification. Daniel Beysens, a researcher at the
Commissariat a l'Energie Atomique (CEA, email@example.com) and his
colleagues study this problem at the much more basic level of
individual bubbles and droplets, and what happens to them when you
add or subtract the effects of gravity.
Movement between liquid and vapor states is aided by buoyancy:
bubbles rise and droplets fall. But without gravity these actions
cease and liquids condense only by the haphazard and slower process
of collision between droplets or bubbles. In the new experiment a
20-cubic-millimeter sample of liquid/gaseous hydrogen was levitated
in a strong magnetic field; the field grabs onto the magnetic
moments of the H2 molecules, helping to suspend them. This
essentially creates an artificial weightlessness (only about 1% of
Earth's gravity remains) and allows one to see how capillary
forces and "wetting" (the process by which a liquid layer builds up
on a surface) are dominant in a freefall environment. Then some of
the effects of gravity are artificially added back in, this time in
the form of high-speed but low-amplitude vibrations.
cause motion in the fluid, which induces effects that resemble
gravity. Bubbles and droplets go "up" and "down" again when the
vibration is turned on.
As far as simulating gravity, vibrations
seem to work. (
Beysens et al.,
Physical Review Letters, 15 July 2005)