Shear thinning near the critical point of xenon

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: 10(-3)<gamma tau<700 , where gamma 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 microm<x 0<430 microm and frequencies 1 Hz<omega/2pi<5 Hz . To separate shear thinning from other nonlinearities, we computed the ratio of the viscous force on the screen at gamma tau to the force at gamma tau approximately 0: C gamma triple bond F(x 0,omega tau,gamma tau)/F(x 0,omega tau,0) . At low frequencies, (omega tau)2<gamma tau , C gamma depends only on gamma tau , as predicted by dynamic critical scaling. At high frequencies, (omega tau)2>gamma 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)/eta(0)=[1+A gamma|gamma 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 Tc were independent of the cooling rate (both above and below Tc ) and these data were symmetric about Tc 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 Tc .