Nonequilibrium dynamics and fluctuation-dissipation relation in a sheared fluid.

The nonequilibrium dynamics of a binary Lennard-Jones mixture in a simple shear flow is investigated by means of molecular dynamics simulations. The range of temperature T investigated covers both the liquid, supercooled, and glassy states, while the shear rate γ covers both the linear and nonlinear regimes of rheology. The results can be interpreted in the context of a nonequilibrium, schematic mode-coupling theory developed recently, which makes the theory applicable to a wide range of soft glassy materials. The behavior of the viscosity η(T, γ) is first investigated. In the nonlinear regime, strong shear-thinning is obtained, η∼γ[sup -α(T)], with α(T)...⅔ in the supercooled regime. Scaling properties of the intermediate scattering functions are studied. Standard "mode-coupling properties" of factorization and time superposition hold in this nonequilibrium situation. The fluctuation-dissipation relation is violated in the shear flow in a way very similar to that predicted theoretically, allowing for the definition of an effective temperature T[sub eff] for the slow modes of the fluid. Temperature and shear rate dependencies of T[sub eff] are studied using density fluctuations as an observable. The observable dependence of T[sub eff] is also investigated. Many different observables are found to lead to the same value of T[sub eff], suggesting several experimental procedures to access T[sub eff]. It is proposed that a tracer particle of large mass m[sub tr] may play the role of an "effective thermometer." When the Einstein frequency of the tracers becomes smaller than the inverse 〈m[sub tr]ν²[sub z]〉 = k[sub B]T[sub eff], relaxation time of the fluid, a nonequilibrium equipartition theorem holds with where ν[sub z] is the velocity in the direction transverse to the flow. This last result gives strong support to the thermodynamic interpretation of T[sub eff] and makes it experimentally... [ABSTRACT FROM AUTHOR]