Robert Berg,
National Institute of Standards and Technology,
Gaithersburg, MD 20899,
(301) 975-2466

Greg Zimmerli,
National Center for Microgravity Research,
NASA LeRC, MS 110-3,
21000 Brookpark Rd.,
Cleveland, OH 44135

 Greg Zimmerli

Laura Maynard/NASA Lewis Research Center
21000 Brookpark Rd.
Cleveland, OH 44135
216/433-8756

 TAS-01, STS-85, August 7, 1997

The objective of the experiment is to produce archival viscosity data on xenon that is closer to its liquid-vapor critical point than is possible in Earth’s gravity.

The heart of the viscometer is a nickel screen 7 mm wide and 19 mm long. An oscillating electric field causes the screen to oscillate between pairs of electrodes. The viscosity of the xenon fluid damps the oscillating screen motion. Thus, the viscosity can be derived from the ratio between the oscillator’s motion and the applied force.

Two Hitchhiker canisters.

In microgravity, CVX will accurately measure the viscosity of xenon as close as 0.0006 kelvin above Tc, 15 times closer than is possible on Earth. As the temperature difference (T–Tc) becomes small, the viscosity is predicted to increase toward infinity. To measure viscosity, an oscillating voltage is applied to the viscometer’s electrodes. The resulting movement of the oscillator is monitored by a sensitive capacitance bridge. Both the input and output waveforms are measured and stored on a hard disk drive. In normal operation, this measurement is repeated once every 64 seconds. The screen, the surrounding electrode assembly, and the xenon are contained in a thick-walled copper cell. Surrounding the copper cell is a thermostat consisting of three concentric aluminum cylinders. Each cylinder improves on the temperature control of the next inner cylinder, so that the xenon’s temperature is extremely stable and uniform. Temperature differences within the xenon are less than 0.2 microkelvin, less than one billionth of Tc. Close to Tc, CVX’s central thermometer measures temperature to a precision of 10 microkelvin. The entire flight instrument is contained in two Hitchhiker canisters: the experiment package and the avionics package. The experiment package contains the thermostat, the most sensitive electronics, and a battery pack to keep the xenon warm when the space shuttle descends to Earth. The avionics package contains an accelerometer to measure vibrations, four computers, and the hard disk drive to store the expected 100 megabytes of data. The two Hitchhiker canisters will be mounted in the space shuttle’s payload bay. CVX’s measurements will require nine days, far too long to do the experiment in a drop tower or on an airplane flown on a parabolic path. The long duration is due to the extremely slow thermal equilibration of the near-critical xenon. If the viscometer’s temperature were changed too rapidly, it would disturb the xenon’s density. Effects associated with equilibration near the critical point are particularly noticeable in microgravity. On Earth, they would be overwhelmed by buoyancy-driven convection. CVX’s timeline was designed by calculating the evolution of the xenon sample’s density.

 Close to the critical temperature TC, theory predicts: Viscosity increases according to (T-Tc)^(-y). The exponent y is universal for all fluids. The fluid becomes slightly elastic. CVX verified these predictions and found: The universal exponent has the value y = 0.043. Imaginary part of viscosity increases from 0 to 3 percent.

The CVX experiment successfully flew on the TAS-01 payload as part of the 11-day STS-85 mission. Launch occurred on time (Aug. 7th, 10:41am) and the mission was extended one day, which the CVX team used to collect additional data. The experiment performed exceptionally well, and data were collected for nearly the entire mission. The main objective of the CVX experiment was to measure the viscosity of xenon within 0.3% of the critical density and to within 0.6 mK of the critical temperature Tc, which is 30 times closer than can be measured on Earth. Preliminary analysis of the data suggests that accurate viscosity measurements were obtained from Tc + 3 K down to at least 1 mK from Tc, and possibly as close as 0.6 mK. The weak divergence of the viscosity was clearly seen in the microgravity environment , and it was approximately twice as large as the best measurements on Earth. The divergence is strongly masked in Earth's gravity due to stratification of the fluid density. Two separate temperature scans through the critical point, a fast scan and a slow scan, gave remarkably good agreement. This was somewhat surprising, since it was conjectured that the fast scan might introduce unacceptable density gradients in the fluid. The metal electrodes inside the sample cell may have aided in the thermal equilibration. Nevertheless, the slow, primary scan was necessary to collect viscosity data with sufficient signal to noise ratio. Approximately 1 mK above the critical temperature, the viscosity data begin to show some frequency dependence. This was at a slightly higher temperature than anticipated, and may be due to a combination of shear rate effects and the fluid’s long fluctuation lifetimes. Other surprises were found during the mission: The magnitude of the viscometer’s transfer function (used in calculating the viscosity) showed small oscillations having a period of about 45 minutes (twice per orbit). This may have been due to the flux of charged particles in the space environment since we also observed a large effect when passing through the western edge of the South Atlantic Anomaly (a region where the Earth's magnetic field is unusually low). The space environment may have also been responsible for noisy low frequency data (the low frequency transfer function measurements were 10 times noisier than on Earth), and for a communication problem with CVX - where the instrument stopped downlinking data. After a nerve wracking six hours, it was decided to cycle power to the instrument, which re-booted the communications processor. This worked, and there was no loss of science since it occurred during a planned 30-hour temperature soak.

 Microgravity

Fluids

 Critical fluids

 viscosity

2.8.5   General Keywords

Critical temperature, viscometry, xenon

Wherever possible, CVX’s design made use of existing conventional devices and techniques. Nevertheless, the constraints of the flight experiment fostered several technical innovations: • CVX’s viscometer was designed specifically for operation in the presence of normal space shuttle vibrations caused by motors, thruster firings, and astronaut movements. These vibrations cause random movements of the oscillating screen and thus add noise to the viscosity measurements. The screen’s low mass makes it comparatively insensitive to the shuttle’s movements, but its large area leads to the large viscous resistance required to measure the viscosity with precision. Because the oscillator feels viscous drag over a wide frequency range, it can be calibrated by exploiting a hydrodynamic similarity relating viscosity to frequency. The CVX oscillator is the first viscometer to be so calibrated. • Typical temperature measurement bridges used in other experiments contain an adjustable component, such as a ratio transformer. An innovation used in CVX was to replace the ratio transformer with an additional pair of matched resistors. Matching the reference resistor to the thermistor’s resistance at Tc allows high-precision thermometry near Tc without the complexity and bulk of a ratio transformer. • Adjusting the capacitance bridge used for viscometry to its most sensitive setting requires a programmable voltage divider. Commercial dividers were too large for CVX. Because CVX requires only eight-bit resolution, an innovative circuit that fits on a single electronic card and is rugged enough to survive launch vibrations was developed by the Electricity Division at NIST.

To measure viscosity, an oscillating voltage is applied to the viscometer’s electrodes. The resulting movement of the oscillator is monitored by a sensitive capacitance bridge. Both the input and output waveforms are measured and stored on a hard disk drive. In normal operation, this measurement is repeated once every 64 seconds. The viscosity can be derived from the ratio between the oscillator’s motion and the force applied to the oscillator.

 none

 none

Flight data stored on CD-ROM.

Moldover, M.R.; Sengers, J.V.; Gammon, R.W.; and Hocken, R.J.: Gravity Effects in Fluids Near the Gas-Liquid Critical Point. Reviews of Modern Physics, vol. 51, 1979, p. 79. Berg, R.F.; and Moldover, M.R.: Critical Exponent for the Viscosity of Carbon Dioxide and Xenon. Journal of Chemical Physics, vol. 93, 1990, p. 1926. Berg, R.F.: Hydrodynamic Similarity in an Oscillating-Body Viscometer. International Journal of Thermophysics, vol. 16, 1995, p. 1257.

Stored on CD-ROM with flight data.