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Publication Citation: Shear Thinning Near the Critical Point of Xenon

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Author(s): Robert F. Berg; Michael R. Moldover; M Yao; G A. Zimmerli;
Title: Shear Thinning Near the Critical Point of Xenon
Published: April 17, 2008
Abstract: We measured shear thinning, a viscosity decrease ordinarily associated with complex liquids, near the critical point of xenon. The data span the range of reduced shear rates: 0.001 < γτ < 700, where γτ is the shear rate scaled by the relaxation time τ of critical fluctuations. The measurements had a temperature resolution of 0.01 mK and they were conducted in microgravity aboard Space Shuttle Columbia to avoid the density stratification caused by Earth's gravity. The viscometer measured the drag on delicate nickel screen as it was driven through the xenon at amplitudes 3 υm < x0 < 430 υm and frequencies 1 Hz < f < 5 Hz. To separate shear thinning from other nonlinearities, we computed the ratio of the viscous force on the screen at γτ to the force at γτ ≅ 0: Cγ = F(x0,ωτ,γτ/F(x0,ωτ,0). At low frequencies ((ωτ)2<γτ), Cγ depends only on γτ, as predicted by dynamic critical scaling. At high frequencies ((ωτ)2>γτ), Cγ depends also on both x0 and ω. The data were compared with numerical calculations based on the Carreau-Yasuda relation for complex fluids: Η(γ)/Η(0) = [1 + Aγ|γτ|]–y/(3+y), where y=0.069 is the critical exponent for viscosity, and mode-coupling theory predicts Aγ=0.121. For xenon we find Aγ=0.137{plus or minus}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 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.
Citation: Physical Review E (Statistical, Nonlinear, and Soft Matter Physics)
Volume: 77
Pages: pp. 041116-1 - 041116-23
Keywords: non-Newtonian;normal stress;rheology;viscoelasticity;viscometry
Research Areas: Gas Metrology, Condensed Matter Physics, Physical Properties, Chemical Engineering & Processing, Standard Reference Data, Chemistry, Physics, Fluids, Thermodynamics, Chemical Physics, Critical Phenomena
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