Critical Viscosity of Xenon

Get Preliminary Results of the CVX experiment.


Viscosity originates in the interactions between fluid molecules. These interactions are so complicated that, except for low density gases, no fluid's viscosity can be calculated accurately from theory. Progress in understanding viscosity has been made by studying moderately dense gase and, more recently, fluids near the critical point. A pure fluid's critical point occurs at the highest temperature (Tc) where liquid and vapor can coexist in the same container. One of the unusual behaviors near this unique temperature and pressure is an increase in viscosity. Although the critical temperature and viscosity can differ greatly among fluids, the relative size of the viscosity increase is the same for all pure fluids. Modern theories predict this universal behavior and relate the increase in viscosity to the spontaneous fluctuations in density that occur near the critical point. By measuring the viscosity of a small sample of xenon, the CVX experiment will test these theories with great precision.

Xenon was chosen because it is a simple fluid, its other critical properties have been well measured, and it has a critical temperature which is conveniently just below room temperature. CVX will measure the predicted 40% viscosity increase as the sample is cooled to within 0.0006 degrees of Tc. Measurements this close to Tc cannot be made on Earth because normal gravity causes the xenon's density to vary with height. Near Tc, the density at the bottom of a 1-millimeter-high sample is 8% higher than the density at the top of the sample. This so-called "stratification" is greatest exactly at Tc, the temperature of most significance. In contrast, CVX's microgravity sample will be uniform to within 0.3%.

In addition to advancing fundamental science, CVX's development fostered several technical innovations. For example, CVX's viscometer is the first to be calibrated by exploiting a hydrodynamic similarity which relates viscosity to frequency. Also, CVX's programmable voltage divider is an innovative circuit which fits on a single electronic card while maintaining voltage stability to within one part per million.


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Data provided by Irene Bibyk

This Page Last Updated 2/4/97 by Ted Fabian Submit Feedback!!