Shear viscosity in a gluon gas

The relation of the shear viscosity coefficient to the recently introduced transport rate is derived within relativistic kinetic theory. We calculate the shear viscosity over entropy ratio eta/s for a gluon gas, which involves elastic gg-->gg perturbative QCD (PQCD) scatterings as well as inelastic gg<-->ggg PQCD bremsstrahlung. For alpha_{s}=0.3 we find eta/s=0.13 and for alpha_{s}=0.6, eta/s=0.076. The small eta/s values, which suggest strongly coupled systems, are due to the gluon bremsstrahlung incorporated.     

Chiral restoration effects on the shear viscosity of a pion gas

In calorimetric neutrino mass experiments, where the shape of a beta decay spectrum has to be precisely measured, the understanding of the detector response function is a fundamental issue. In the MIBETA neutrino mass experiment, the X-ray lines measured with external sources did not have Gaussian shapes, but exhibited a pronounced shoulder towards lower energies. If this shoulder were a general feature of the detector response function, it would distort the beta decay spectrum and thus mimic a non-zero neutrino mass. An investigation was performed to understand the origin of the shoulder and its potential influence on the beta spectrum. First, the peaks were fitted with an analytic function in order to determine quantitatively the amount of events contributing to the shoulder, also depending on the energy of the calibration X-rays. In a second step, Monte Carlo simulations were performed to reproduce the experimental spectrum and to understand the origin of its shape. We conclude that at least part of the observed shoulder can be attributed to a surface effect.     

π-π scattering in a hot pion gas at URHIC-conditions

Pion-pion scattering in a hot pion environment is considered. In medium effects considerably diminish the pion — pion scattering rates in the σ — and ρ — meson channels at temperatures and chemical potential of the pion gas close tom _π     

Thermodynamics of a pion gas

The thermodynamics is presented for a gas of uncharged pions coupled to a static nucleon background through a classical Yukawa interaction. By using invariant phase space in the grand canonical ensemble it is possible to write a general analytical form for the thermodynamical functions.     

The ratio of viscosity to entropy density in a pion gas satisfies the KSS holographic bound

We evaluate the ratio of shear viscosity to entropy density in a pion gas employing the Uehling–Uehlenbeck equation and experimental phase-shifts parameterized by means of the SU(2) inverse amplitude method. We find that the ratio for this monocomponent gas stays well above the KSS 1/(4π) bound. We find similar results with other sets of phase-shifts and conclude that the bound is nowhere violated. PACS 05.20.Dd; 51.20.+d     

Transport coefficients of a massive pion gas

We review our main results concerning the transport coefficients of a light meson gas,in particular we focus on the case of a massive pion gas.Leading order results according to the chiral power-counting are presented for the DC electrical conductivity,thermal conductivity,shear viscosity,and bulk viscosity.We also comment on the possible correlation between the bulk viscosity and the trace anomaly in QCD,as well as the relation between unitarity and a minimum of the quotient η/s near the phase transition.     

Transport coefficients of a massive pion gas

We review our main results concerning the transport coefficients of a light meson gas,in particular we focus on the case of a massive pion gas.Leading order results according to the chiral power-counting are presented for the DC electrical conductivity,thermal conductivity,shear viscosity,and bulk viscosity.We also comment on the possible correlation between the bulk viscosity and the trace anomaly in QCD,as well as the relation between unitarity and a minimum of the quotient η/s near the phase transition.     

Phase transition in hot pion matter

The equation of state for the pion gas is analyzed within the third virial approximation. The second virial coefficient is found from the ππ-scattering data, while the third one is considered as a free parameter. The proposed model leads to a first-order phase transition from the pion gas to a more dense phase at the temperature T_pt<136 MeV. Due to relatively low temperature this phase transition cannot be related to the deconfinement. This suggests that a new phase of hadron matter — ‘hot pion liquid’ — may exist.     

Pion dispersion relation in hot pion matter

We study the modification of the pion propagation properties when it is embedded in hot pionic matter. We calculate the real part and the imaginary part of the pion self-energy in a scheme, analogous to the Brückner-Hartree-Fock approximation used in nuclear physics, which allows to incorporate in a fully consistent way unitarity and Bose correlations at finite temperature. We discuss the consequences of the medium effects on the pion transverse momentum spectrum measured in ultrarelativistic heavy ion collisions and the so-called soft pion puzzle.     

Shear viscosity of pseudo hard-spheres

We present molecular dynamics simulations of pseudo hard sphere fluid (generalized WCA potential with exponents (50, 49) proposed by Jover et al. [J. Chem. Phys 137, (2012)] using GROMACS package. The equation of state and radial distribution functions at contact are obtained from simulations and compared to the available theory of true hard spheres (HS) and available data on pseudo hard spheres. The comparison shows agreements with data by Jover et al. and the Carnahan–Starling equation of HS. The shear viscosity is obtained from the simulations and compared to the Enskog expression and previous HS simulations. It is demonstrated that the PHS potential reproduces the HS shear viscosity accurately.     

Shear instability in fluids with a density-dependent viscosity

We present a shear instability, which can be triggered in compressible fluids with density-dependent viscosity at shear rates above critical. The instability mechanism is generic: It is based on density-dependent viscosity, compressibility, as well as flow two-(three-)dimensionality that provides coupling between streamwise and transversal velocity components and density variations. The only factor stabilizing the instability is fluid elasticity. The corresponding eigenvalue problem for a plane Couette flow is solved analytically in the limiting cases of large and small wave numbers.     

Shear viscosity of a binary mixture of simple liquids

Russian Physics Journal -     

Shear viscosity for inelastic hard spheres

The hydrodynamic equations of the Enskog theory for inelastic hard spheres is considered as a model for rapid flow granular fluids at finite densities. A detailed analysis of the shear viscosity of the granular fluid has been done using homogenous cooling state (HCS) and uniform shear flow (USF) models. It is found that shear viscosity is sensitive to the coefficient of restitution α and pair correlation function at contact. The collisional part of the Newtonian shear viscosity is found to be dominant than its kinetic part.     

Shear viscosity of polymer and surfactant monolayers

The surface shear viscosity of monolayers formed at the surface of water by adsorbed polyethyl- eneoxyde and by stearic acid is measured as a function of the surface pressure of the monolayer using a new surface viscometer. The principle of the viscometer is the measurement of the drag force on a circular disk undergoing a uniform translation at the water surface: a hydrodynamic model based on the lubrication approximation allows a calculation of the surface viscosities from the absolute measurement of the drag forces. Received: 26 August 1999     

Shear viscosity and entropy of quark matter

We consider the shear viscosity of a system of quarks and its ratio to the entropy density above the critical temperature for deconfinement. Expressions for both quantities are derived in the quasi-particle approximation and calculations are carried out for different modeling of the quark self-energy, also allowing for a temperature dependence of the effective mass and width. Beyond the temperature dependence, the behaviour of the viscosity and the entropy density is discussed in terms of the strength of the coupling and of the main characteristics of the quark self-energy. A comparison with existing approaches is also discussed.     

The shear viscosity of a trapped Bose-condensed gas

By obtaining Kubo formula type and using nonequilibrium Green’s functions, we calculate the shear viscosity of a trapped Bose-condensed gas below and above the Bose-Einstein condensation temperature (T_BEC). The contributions of the interactions between condensate and noncondensate atoms and between noncondensate atoms take into account to the viscous relaxation time, by evaluating second order self-energies in Beliaev approximation.     

Entropy production and viscosity of a dilute gas

It is known that the viscosity of a dilute gas can be derived by using kinetic theory. We present here a new derivation by using two entropy production principles: the steepest entropy ascent (SEA) principle and the maximum entropy production (MEP) principle. The known result is reproduced in a similar form.     

The volume viscosity of a massive neutrino gas

The volume viscosity of a gas of neutrinos with mass m is found to be equal to $\text{2}{}^{\mathrm{?6}}\text{3}{}^{\mathrm{?2}}\text{π}{}^{\mathrm{?1}}\text{G}{}^{\mathrm{?2}}\text{?}{}^2\text{c}{}^3\text{(kT)}{}^{\mathrm{?5}}\text{(mc}{}^2\text{)}{}^4\text{×}\text{log}\text{(}\text{kT}\text{mc}{}^2\text{)}$ for the case kT ? mc². Here G is the weak interaction constant.     

Nuclear viscosity of hot rotating 240Cf

In a detailed investigation of giant dipole resonance (GDR) γ-ray yield from ²⁴⁰Cf populated in the ³²S + ²⁰⁸Pb reaction, the absolute γ-ray/fission multiplicities are extracted over a wide range of excitation energy and angular momentum. The enhanced yield of GDR decay γ rays has been analyzed within the framework of a modified statistical model containing the nuclear viscosity as a free parameter. The extracted nuclear dissipation coefficient is found to be independent of the temperature. Large constant dissipation during the saddle-to-scission path provides good fits to the γ-ray spectra.