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<γ̇τ<700, where γ̇τ is the shear rate scaled by the relaxation time τ 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 μm<x₀<430 μm and frequencies 1 Hz<ω/2π<5 Hz. To separate shear thinning from other nonlinearities, we computed the ratio of the viscous force on the screen at γ̇τ to the force at γ̇τ≈0: C_γ≡F(x₀,ωτ,γ̇τ)/F(x₀,ωτ,0). At low frequencies, (ωτ)²<γ̇τ, C_γ depends only on γ̇τ, as predicted by dynamic critical scaling. At high frequencies, (ωτ)²>γ̇τ, C_γ depends also on both x₀ and ω. The data were compared with numerical calculations based on the Carreau-Yasuda relation for complex fluids: η(γ̇)/η(0)=[1+A_γ|γ̇τ|]^{−x_η/(3+x_η)}, where x_η=0.069 is the critical exponent for viscosity and mode-coupling theory predicts A_γ=0.121. For xenon we find A_γ=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.