Shear Viscosity of Hexane+Tetradecafluorohexane near the Upper Critical Endpoint

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 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 .

Neutron reflectivity studies of critical adsorption: behavior of the surface scaling function

Neutron reflectometry has been employed to examine the nature of the critical adsorption surface scaling function for a near-critical mixture of hexane-d14+perfluorohexane adsorbing to a solid substrate from the liquid one-phase region. The analysis method of Dietrich and Schack has been applied to examine the nature of the power-law part of the critical adsorption surface scaling function, which has been found to behave as m(z) approximately P0z(-mu) as the critical point is approached. Values of mu = 0.514+/-0.018 and P0 = 0.90+/-0.04 have been obtained. These values are consistent with theoretical expectations (mu(th) = 0.516+/-0.004; P0(th) = 0.94+/-0.05), the value determined from Monte Carlo simulations (P(MC)0 = 0.866, and other experimental determinations (P(ex)0 = 0.955=/-0.08).

Shear viscosity studies above and below the critical consolute point in a nitrobenzene-decane mixture

The shear viscosity has been studied in a nitrobenzene-decane critical mixture above the critical consolute temperature T(C), in the homogeneous phase, and below T(C), in coexisting phases. The form of background viscosity for coexisting phases has been postulated. The same value of the critical exponent straight phi has been obtained in the lower (L), upper (U), and homogeneous (H) phases. The pretransitional amplitudes (A(L,U)) in coexisting phases are approximately the same, whereas A(L,U)/A(H) approximately 0.965. In the homogeneous phase the possibility of the appearance of the quasinematic, field-induced structure of critical fluctuations has been discussed.