Critical point research is recognized as a major item in the
field of microgravity fluid physics research. Especially for
experiments on a one-component fluid system the relevance of
microgravity is easily demonstrated. When a system approaches
its critical point it shows a strong increase of density
fluctuations in the sample as well as a strong increase in the
overall isothermal compressibility. The strongly increasing
density dictates an experimental requirement of a rather large
sample of uniform density, since its dimensions should exceed
the characteristic correlation length (5) by some orders of
magnitude; the strongly increasing compressibility leads to a
strong compression of a near critical fluid in a gravity field
under its own weight and hence to a strong vertical density
gradient. This makes it impossible to obtain uniform samples in
a 1-g gravity field close to the critical point.
Over the past years, a number of critical-point research groups
proposed and developed microgravity experiments for a one
component system. Two of these were carried out during the
Spacelab D1 mission, with remarkable results; they show neither
a homogeneous density distribution, nor the expected
non-analytic critical behaviour. These results have been
explained as being primarily due to strongly enhanced thermal
relaxation times. However, other mechanisms may also be of
importance. For instance the influence of density gradients
induced at the put constraints on the design of sample cells
for future studies; a motive for the present study.
Objectives
The objective of our experiment, was to put a sealed sample of
critical density through a number of near critical states by
slow or stepwise change of temperature and to observe its
response to the change in the acceleration field. The lay-out
of the apparatus is shown in Figs. 4.14 and 4.15. The SF fluid
samples, with T = 45C, are contained in glass tubes with a
diameter of 5.35 mm and about 7 cm long.
Experiment procedure
Due to divergence of relaxation times in a fluid sample when
the critical point is approached the study of critical
phenomena typically requires long waiting time (in the order of
hours) before the actual measurement is performed. However, in
studying relaxation mechanisms, the quenching technique has
proved to be very useful. In these experiments the sample is
equilibrated in the one-phase region and then "quenched" to
two-phase "conditions" by suddenly changing the temperature or
the pressure. The ensuing relaxation phenomena can then be
observed until two-phase equilibrium is attained.
Some information - be it qualitative - is already obtained in
the first seconds after the quench. This has led to the
assumption that some information on the preferred density
distribution in microgravity can be obtained by "
gravity-quenching", i.e. bringing a sample in equilibrium under
normal gravity conditions and then abruptly put it in
microgravity environment, for instance in a parabolic airplane
flight.
Results
The experiment was flown on the ESA KC-135 campaign in Houston.
A video camera recorded clear pictures of the g-quench
behaviour of the sample in the temperature range of Tc + 250 mK
to Tc - 2400 mK. The data rveal the following general behaviour.
At low temperatures (Tc -T>500 mK) the liquid spreads over the
glass surface, directly after the onset of the microgravity
phase.
This surface layer seems rather sensitive to residual gravity
and vibrations. Closer to Tc the fluid layer also develops, but
at a slower rate and with much turbulence. At high temperature (
T-Tc >200 mK) there is no significant change of density
distribution during the microgravity phase; there is an
indication of redistribution during the high-g phase.
Interesting phenomena appear for T-Tc 30 mK. At this
temperature the overall density difference in the sample at 1g
is a few percent, which is not shown by the laser beam since it
propagates parallel to the density gradient. Beam bending and
reflection is observed, a few seconds after the onset of the
microgravity phase indicating the build up of a layer of
unexpected high density along the tube walls. This layer seems
to move around with the residual g-jitter, but regular wave
patterns can also be observed. The delay time for the onset of
this effect is quite different for various parabolas. The
temperature stability however, does not allow to correlate this
time to the value of T-Tc .
Although not all results of the campaign have been completely
analyzed yet, we observed that the reaction of the sample to
the disappearance of the gravity field strongly depends on the
distance from the critical point, even in the one-phase region.
Also the influence of the walls is quite apparent. The
arrangement chosen for the experiment seems well suited to
obtain better information. Therefore we propose to repeat the
experiment in a future flight with minor modifications to
further improve the equipment.
Pictures
Exchangeable thermostat core with sample cell
Layout of the critical-point experiment
Extracted from a prototype of the Dutch Microgravity Compact Disc
