Experiments were performed using the Critical Point Facility (CPF). The core of the CPF is a thermostat into which experimental cells may be inserted. The thermostat provides extremely precise the temperature stability of the order of 30 µK/hr with spatial gradients less than 10 µK/cm. The CPF is also equipped with optical and electronic interfaces which enable the stimulation and observation of the test fluid.
Experiments were performed in the temperature-range 2500 to 1 mK above the critical point where simultaneous density and temperature measurements are conducted during a number of transient heating runs and light, scattered off an incident beam, is measured at discrete angles between 22 and 90 degrees. An interferometry set-up is used to determine density changes in the fluid and trace the evolution of boundary layers following heat-pulses. Several high sensitivity (mK) temperature sensors (thermistors) are used to measure temperature changes of the test cell, one of which measures the temperature of the bulk of the fluid. Light scattering signals are collected using fiber-optic guides and transmitted to a photomultiplier tube.
Test cell
The test cell consisted of two interconnected cylindrical
chambers with a total volume of approximately 6 cm^3. The larger
chamber accommodated a mirror which formed a part of a
Twyman-Green interferometer (IF) system, while the smaller
chamber enabled light scattering (LS) measurements at discrete
angles between 22 and 90 degrees - referred to as the wide angle light
scattering (WALS) - and continuously over a range of 0 to 30 degrees -
small angle light scattering (SALS) - with the 0 angle serving for
turbidity measurement. Direct visualization (VIS) of the sample
in the smaller chamber was also available.
The CPF is a fully automated facility, however, it's telecommanding features proved to be absolutely essential for optimizing the operation of the experiment and frequent updates in the timeline were made during the course of the experiment.
The actual sequence of experimental steps was as follows. The sample was first heated to T-Tc 2500 mK (48 ºC) and time allowed for it to become homogeneous. It was then cooled down in steps to 1000, 300, 100, 50 and finally 15 mK above Tc. At Tc + 15 mK, a slow cooling ramp was initialized, ending a few mK below Tc when phase separation was confirmed. The sample was again homogenized at T-Tc 2500 mK and cooled down, in steps, to 2000, 1500 and 800 mK above Tc and then, in ramps, to 450, 150, 50, 30, 10, 5, 2 and 1 mK above Tc. Finally, the sample was heated slowly to Tc + 100 mK to check for hysteresis effects.
Following each change in temperature, various waiting periods were employed in an attempt to obtain, as close as possible, thermodynamic equilibrium. The evidence of the IF images shows that equilibrium was never reached but that with specific precautions a steady state could be achieved, i.e. at T-Tc = 50 mK after 5 hours.
Constant-current heating pulses were applied to the fluid by the plate heater after reaching a steady state at each set temperature. Pulse duration was varied between one and five minutes. A few pulses of elevated power (5.65 mW) but with duration 5 s were also employed using the thermistor in the fluid as the heat source. The power delivered to the system was varied between 0.04 and 0.18 mW.
Many quantitative analyses proved possible afterwards.
The experimentally determined adiabatic temperature rise displays a behaviour
in line with the theoretical predictions, supporting in this way the idea
of a crossover region in the adiabatic equilibration time scale, as
Tc is approached.
Thanks to the observed adiabatic expansion (AE), a new way for assessing
thermodynamic properties in the critical region is found,
based on the experimental determination of the adiabatic thermal expansion
coefficient. The figure clearly shows a discrepancy between our
measurements and the values obtained from literature demonstrating the
difficulty of producing reliable data in the near-critical region subject
to earth-gravitation.
The analysis for the determination of the thermal diffusivity of the
sample from the
interference patterns have provided preliminary results
The WALS measurements have been calibrated for stray light and
fiber efficiency and show a remarkably high consistency.
The discrepancy between theory and experiment close to Tc as displayed
originates from not incorporating the multiple scattering
and attenuation effects in the first.
As an initial result it proves the outstanding performance of the experimental
set-up as regards the WALS.
European operation centres participated
in the remote operations scenario for IML-2
In the remote operations room at NLR in Amsterdam, scientists supported by NLR support engineers received and analyzed experiment data, including digital interferometry image data (see above) from the observation systems in the Critical Point Facility. Quick-look results of the analysis were transferred to NASA's payload operations centre (MSFC). Using an elaborate communication infrastructure available the results could be discussed and communicated with the flight centre of NASA. In this way adjustement of experiment parameters was possible, thereby fully exploiting the precious experiment time. This scenario was realized as part of a demonstration programme anticipating remote operations for future space stations.
Operators and scientists in the remote operations room at NLR
(courtesy of Volkskrant)
The experiment of the Van der Waals-Zeeman Laboratory started 2 days and 4 hours after the launch of the Space Shuttle and lasted 56 hours. During these hours full remote support was available.
New experiments are being prepared as a follow-on using parabolic flights in intermediate steps. The remote operations experience was the basis for new activities to improve future remote operations scenario support, execution of experiments and post-processing.
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