Heat flow studies for large temperature gradients by molecular dynamics simulation

Response of glassy liquids to thermal gradients

The Soret effect, i.e., the flow of matter caused by a temperature gradient, is studied in a glass-forming binary Lennard-Jones (LJ) mixture, using nonequilibrium molecular dynamics computer simulation. The transport processes associated with this effect are thermal diffusion and interdiffusion. While interdiffusion processes exhibit a drastic slowing down when approaching the glass transition, thermal diffusion appears to be a fast process even in the glass. We show that the Soret effect becomes more pronounced in the vicinity of the glass transition, due to the decoupling between thermal diffusion and interdiffusion as well as the chemical ordering in the considered LJ mixture. This is reflected in the occurrence of large concentration gradients, nonlinear concentration profiles, and long-lived nonstationary structures.

Thermal Conductivity in Nanoscale Lennard-Jones Systems: Size Effects in the Fluid and Solid Phases

We simulate fluid and solid Lennard-Jones argon systems via non-equilibrium molecular dynamics and study how the system behaves under an imposed temperature gradient until it reaches the stationary state, from where the thermal conductivity is calculated. We show that transient pressure waves propagate in the system giving rise to a density oscillation pattern. Based on the damping of this pattern we estimate the time needed to reach the stationary state. We also show that thermal conductivity is size independent in the fluid phase, while it increases until the Casimir limit in the solid phase.

Pressure tensor and heat flux vector for inhomogeneous nonequilibrium fluids under the influence of three-body forces

We present a derivation of the pressure tensor and heat flux vector for inhomogeneous fluids under the influence of three-body forces. The derivation is based on the method of planes formalism of Todd, Evans, and Daivis [Phys. Rev. E 52, 1627 (1995); 51, 4362 (1995)]. Our derivation is validated against nonequilibrium molecular dynamics simulations of a confined fluid acted upon by a two-body Barker-Fisher-Watts force coupled with the Axilrod-Teller three-body force. Our method of planes calculations agree perfectly with the equivalent mesoscopic route of integrating the momentum and energy continuity equations directly from the simulation data. Our calculations reveal that three-body forces have an important consequence for the isotropic pressure, but have negligible influence on the shear stress (hence viscosity) and heat flux vector (hence thermal conductivity) for a confined simple fluid.

Temperature of nonequilibrium steady-state systems

We determined the operational temperatures of nonequilibrium-molecular-dynamics (NEMD) systems by the recently developed thermometer [A. Baranyai, Phys. Rev. E 61, R3306 (2000)] and compared these values to the dynamic temperatures [H. H. Rough, Phys. Rev. Lett. 78, 772 (1997)] of the same systems. NEMD models use a synthetic thermostat, a numerical feedback procedure to remove the dissipative heat instantaneously. A consequence of this feedback is a splitting of the dynamic temperature. The kinetic part is different from the configurational part because the energy is removed through the momentum subspace of the system. In addition to this, these temperature values also vary with respect to the direction of the external perturbation. In the case of planar Couette flow and color flow, the isotropic operational temperatures of dense liquids are always closer to the configurational than to the kinetic temperatures. We show that the observed split and the pronounced directional dependence of the dynamic temperature is an artifact caused by the instantaneous feedback of NEMD models. Since relaxation of a preset difference between the kinetic and the configurational temperature is an order of magnitude faster than the relaxation of the heat flux vector, for models with realistic thermostas such a split must be very small. We argue that in real systems, even far from equilibrium, the operational temperature and both terms of the dynamic temperature must be practically identical and isotropic.