The viscosity of xenon has been measured along ten isochores and the viscosity of ethane has been measured along five isochores in the region of the gas-liquid critical point. The viscometer was a specially cut quartz cylinder, which was excited into a torsionally oscillating mode upon application of an alternating voltage. Thecylinder, oscillating in this manner, generated a viscous wave when immersed in a fluid. The viscous wave caused an impedance loading on the quartz which changed its resonant resistance and frequency. This change in resonant properties can be related to the viscosity-density product of the fluid.
The resonant properties of the quartz crystal were measured by connecting the crystal to the unknown arm of a Wheatstone bridge circuit, modified so as to measure parallel resistance and capacitance. The crystal was driven by a very stable frequency generator, and the circuit was tuned using an oscilloscope as a null detector.
The crystal was enclosed in a high pressure stainless steel cell and placed in a water thermostat which controlled temperature to ± 0.001°C. The temperature was measured with a calibrated platinum resistance thermometer. The density was determined by a gravimetric technique.
The viscosities exhibit an "anomalous" increase as the critical temperature is approached along an isochore of close to critical density. The anomaly is consistent with a logarithmic divergence of the form:
∆ƞ = A log ɛ + ʙ
where ∆ƞ is the anomalous viscosity, ɛ is the reduced temperature difference from critical, and A and ʙ are constants. The possibility of an exponential divergence or a cusp-type finite limit for viscosity is not precluded, however.
The critical temperatures and densities have been determined by visual observations. For xenon, Tc = 16.627 ± 0.005°C, ρc = 1.11 ± 0.01 g/cm3 ; for ethane, Tc = 32.218 ± 0.005°C, ρc = 0.2055 ± 0.002 g/cm3. The temperatures are stated with respect to the 1968 International Practical Temperature Scale.
