Nonequilibrium dynamics and fluctuation-dissipation relation in a sheared fluid

The nonequilibrium dynamics of a binary Lennard-Jones mixture in a simpleshear flow is investigated by means of molecular dynamics simulations. Therange of temperature investigated covers both the liquid, supercooled andglassy states, while the shear rate covers both the linear and nonlinearregimes of rheology. The results can be interpreted in the context of anonequilibrium, schematic mode-coupling theory developed recently, which makesthe theory applicable to a wide range of soft glassy materials. The behavior ofthe viscosity is first investigated. In the nonlinear regime, strongshear-thinning is obtained. Scaling properties of the intermediate scatteringfunctions are studied. Standard `mode-coupling properties' of factorization andtime-superposition hold in this nonequilibrium situation. Thefluctuation-dissipation relation is violated in the shear flow in a way verysimilar to that predicted theoretically, allowing for the definition of aneffective temperature Teff for the slow modes of the fluid. Temperature andshear rate dependencies of Teff are studied using density fluctuations as anobservable. The observable dependence of Teff is also investigated. Manydifferent observables are found to lead to the same value of Teff, suggestingseveral experimental procedures to access Teff. It is proposed that tracerparticle of large mass may play the role of an `effective thermometer'. Whenthe Einstein frequency of the tracers becomes smaller than the inverserelaxation time of the fluid, a nonequilibrium equipartition theorem holds.This last result gives strong support to the thermodynamic interpretation ofTeff and makes it experimentally accessible in a very direct way.Comment: Version accepted for publication in Journal of Chemical Physic