Shear thinning near the critical point of xenon

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: 10(-3) (gamma) over dot tau < 700, where (gamma) over dot tau is the shear rate scaled by the relaxation time tau 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 3 mu m < x(0) < 430 mu m and frequencies 1 Hz < omega/2 pi < 5 Hz. To separate shear thinning from other nonlinearities, we computed the ratio of the viscous force on the screen at (gamma) over dot tau to the force at (gamma) over dot tau approximate to 0: C-gamma F(x(0), omega tau, (gamma) over dot tau)/F (x(0), omega tau, 0). At low frequencies, (omega tau)(2) < (gamma) over dot tau, C-gamma depends only on (gamma) over dot tau, as predicted by dynamic critical scaling. At high frequencies, (omega tau)(2) > (gamma) over dot tau, C gamma depends also on both x(0) and omega. The data were compared with numerical calculations based on the Carreau-Yasuda relation for complex fluids: eta((gamma) over dot)/ eta(0) = [1 + A(gamma)|(gamma) over dot tau|](-x eta/(3+x eta)), where x eta = 0.069 is the critical exponent for viscosity and mode-coupling theory predicts A(gamma)= 0.121. For xenon we find A(gamma) = 0.137 +/- 0.029, in agreement with the mode coupling value. Remarkably, the xenon data close to the critical temperature T-c were independent of the cooling rate (both above and below T-c) and these data were symmetric about T-c 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 T-c.