Abstract
We measured shear thinning, a viscosity decrease ordinarily associated with complex liquids, near the critical point of xenon. The data span a wide range of reduced shear rate: , where is the shear rate scaled by the relaxation time of critical fluctuations. The measurements had a temperature resolution of 0.01 mK and were conducted in microgravity aboard the Space Shuttle Columbia to avoid the density stratification caused by Earth’s gravity. The viscometer measured the drag on a delicate nickel screen as it oscillated in the xenon at amplitudes and frequencies . To separate shear thinning from other nonlinearities, we computed the ratio of the viscous force on the screen at to the force at : . At low frequencies, , depends only on , as predicted by dynamic critical scaling. At high frequencies, , depends also on both and . The data were compared with numerical calculations based on the Carreau-Yasuda relation for complex fluids: , where is the critical exponent for viscosity and mode-coupling theory predicts . For xenon we find , in agreement with the mode coupling value. Remarkably, the xenon data close to the critical temperature were independent of the cooling rate (both above and below ) and these data were symmetric about to within a temperature scale factor. The scale factors for the magnitude of the oscillator’s response differed from those for the oscillator’s phase; this suggests that the surface tension of the two-phase domains affected the drag on the screen below .
14 More- Received 27 September 2007
DOI:https://doi.org/10.1103/PhysRevE.77.041116