Anomalous Shear Viscosity Research Articles

Shear Viscosity of Uniform Fermi Gases with Population Imbalance

The shear viscosity has been an important topic in ultracold Fermi gases, and it has served as a diagnostic of various theories. Due to the complicated phase structures of population-imbalanced (polarized) Fermi gases with tunable attraction, past works on the shear viscosity mainly focused on unpolarized Fermi gases. Here we investigate the shear viscosity of homogeneous, population-imbalanced Fermi superfluid at finite temperatures by a pairing fluctuation theory for thermodynamical quantities and a gauge-invariant linear response theory for transport coefficients. The Cooper pairs lead to the anomalous shear viscosity analogous to the shear viscosity. We derive an exact relation connecting certain thermodynamic quantities and transport coefficients at the mean-field level for polarized unitary Fermi superfluids. An approximate relation beyond mean-field is proposed and only exhibits mild deviations from our numerical results. In the unitary and Bose-Einstein condensation (BEC) regimes, the total shear viscosity increases with the polarization because the excess majority fermions cause gapless excitations acting like a normal fluid. Moreover, competition among the excess fermions, noncondensed pairs, and fermionic quasiparticles may lead to non-monotonic behavior of the ratio between the shear viscosity and relaxation time as the polarization increases.

Relation connecting thermodynamics and transport of atomic unitary Fermi superfluids

The shear viscosity has been shown to be equal to the product of pressure and relaxation time in normal scale-invariant fluids, but the presence of superfluidity at low temperatures can alter the relation. By using the mean-field BCS-Leggett theory with a gauge-invariant linear response theory for unitary Fermi superfluids, we present an explicit relation between thermodynamic quantities, including the pressure and chemical potential, and transport coefficients, including the shear viscosity, superfluid density, and anomalous shear viscosity from momentum transfer via Cooper pairs. The relation is modified when pairing fluctuations associated with noncondensed Cooper pairs are considered. Within a pairing fluctuation theory consistent with the BCS-Leggett ground state, we found an approximate relation for unitary Fermi superfluids. The exact mean-field relation and the approximate one with pairing flucutaions advance our understanding of relations between equilibrium and transport quantities in superfluids, and they help determine or constrain quantities which can be otherwise difficult to measure.

Anomalous shear viscosity–temperature behaviour of jojoba oil/LDH-stearate suspensions

Stearate intercalated layered double hydroxide was synthesized by reacting the corresponding carbonate form with a large excess of stearic acid. Thermal analysis, FTIR, XRD and SEM–EDS characterization indicated the formation of a highly crystalline bilayer-intercalated product. The composition of the inorganic portion showed high variability in the Al:Mg atom ratio. This suggests that the crystals comprised stacks of randomly interstratified layers that varied in compositions from one close to magnesium stearate to one similar to aluminium distearate. The rheology of the jojoba oil suspensions containing this material showed strong shear thinning behaviour and also an anomalous temperature dependence.

Multiplicative renormalization of the anomalous shear viscosity in classical liquids

The behaviour of the shear viscosity near the critical point in fluids is discussed. The values of the critical exponents obtained by analysing available experimental data from the view point of multiplicative renormalization of transport coefficients are shown to be in a very good agreement with the theoretical result.

Vertex corrections to the shear viscosity critical exponent

Mode-coupling equations for the anomalous shear viscosity which incorporate vertex corrections are used to calculate the critical exponent for this transport coefficient. These corrections are shown to have an important effect on the magnitude of this exponent.

Extended calculations of the anomalous Rayleigh linewidth and shear viscosity

In this article the results of a fairly extensive set of mode coupling calculations of both the anomalous Rayleigh linewidth and shear viscosity of xenon are presented. The trilinear kinetic equations for the order parameter fluctuations are presented and solved. Although the effects of explicit trilinear coupling on the Rayleigh linewidth are found to be small in agreement with earlier work, there are some quantitative differences. The major results of this paper are related to an examination of the approximation of neglecting diffusive modes when they occur in combination with viscous modes, and the study of non-Ornstein–Zernike corrections to the anomalous shear viscosity. Both effects are found to be large.

Multimode contributions to the anomalous shear viscosity

The results of a mode–mode coupling calculation of the anomalous shear viscosity are presented. By explicitly including trilinear variables in the generalized Langevin equation expressions for the anomalous wave vector and frequency dependent shear viscosity are obtained which incorporate both vertex and frequency corrections. Explicit calculations of the anomalous viscosity of xenon have been carried out. The vertex corrections are found to be important and give rise to an additional anomalous contribution which, close to the critical point, is about 20% of the contribution considered in previous calculations. The inclusion of this effect leads to a much closer agreement with the experimental results. In addition, results are presented for the wave vector and frequency dependence of the shear viscosity which are relevant for Rayleigh linewidth calculations.

Anomalous shear viscosity near a critical point in multicomponent systems

A derivation of the fluctuating force appearing in the nonlinear Navier-Stokes equation for a multicomponent fluid system neat a critical point is presented. We point out that this nonlocal fluctuating force leads to an anomalous shear viscosity at any kind of critical point, in particular also in pure fluids.

Anomalous shear viscosity of the isobutyric acid-water system near the critical point

The anomalous shear viscosity of the isobutyric acid-water binary liquid mixture at the critical concentration has been determined. A power law divergence with γ = 0.65 is obtained in the range 0.1°C < < T−Tc < 1.0°C.

Transport Coefficients in the Gas Critical Region

The free energy of a system with nonuniform density or composition is assumed to depend quadratically on the density or composition gradient. The contributions of these quadratic terms to the equations of motion are then deduced from a mechanical variational principle. The resultant equations of motion are used to discuss time-dependent correlation functions in the critical region, and to calculate the anomalous shear viscosity η′ and thermal conductivity λ′ of a gas in its critical region. For CO2, λ′ (theory) and λ′ (experiment) are in good agreement. However, η′ (theory) is very small for CO2 [38 (T—Tc)−1/2 μP], and comparison with experiment is inconclusive.