EXTERNAL THERMAL-GRADIENT DEPENDENCE OF THE SHEAR VISCOSITY OF HELIUMIN THE TRANSITION REGIME

The effective viscosity of helium in the transition regime has been me asured at different external thermal gradients with a rotatory cylinde r viscometer used as a damped oscillator. It was found that the viscos ity of helium does not vary monotonically with the gradient of tempera ture, showing an appreciable change at G = 6.9 +/- 0.8 K/cm for averag e temperatures between 293 and 327 K and a distance between the fixed and moving plates of 2.0 +/- 0.1 cm. Similar experiments held at const ant thermal gradients and variable pressure indicated that under those conditions, and within the range 1-150 mu mHg, the viscosity of heliu m presents two regions of rapid variation with the pressure whose posi tions depend on the magnitude of the thermal gradient. Some general ch aracteristics of the behavior found can be predicted in terms of a sim ple theory based on the Boltzmann equation under the relaxation time a pproximation, although the detailed variation of the shear viscosity w ith the thermal gradients awaits a thorough explaination. It is clear, however, that higher order terms in the collision time are required f or the theoretical consideration of this effect. The region of rapid v ariation of the shear viscosity at constant thermal gradient and varyi ng pressure can be theoretically reproduced assuming a molecular diame ter for helium of 3.1 +/- 0.3 Angstrom, while those at constant pressu re and variable thermal gradient can be reproduced for a diameter of 1 .9 +/- 0.3 Angstrom. The first of these values is close to the current ly accepted diameter of helium gas deduced from theoretical computatio ns and van der Waals excluded volume measurements. On the other hand, the second value is close to the shear viscosity and thermal conductiv ity measurements in the continuous regime.