Theoretical Prediction of Thermal Polarization

Abstract

We present a mean-field theory to explain the thermo-orientation effect in an off-center Stockmayer fluid. This effect is the underlying cause of thermally induced polarization and thermally induced monopoles, which have recently been predicted theoretically. Unlike previous theories that are based either on phenomenological equations or on scaling arguments, our approach does not require any fitting parameters. Given an equation of state and assuming local equilibrium, we construct an effective Hamiltonian for the computation of local Boltzmann averages. This simple theoretical treatment predicts molecular orientations that are in very good agreement with simulation results for the range of dipole strengths investigated. By decomposing the overall alignment into contributions from the temperature and density gradients, we shed further light on how the nonequilibrium result arises from the competition between the two gradients.

Microscopic analysis of thermo-orientation in systems of off-centre Lennard-Jones particles

“…We focus on the simplest model for thermo-orientation that was considered in Ref. 8. Consider a fluid of identical particles interacting by Lennard-Jones (LJ) interactions.…”

“…Following Ref. 8, our theory also includes an external body force of magnitude ext parallel to , acting on the particles' centres of mass. Our analysis concerns linear responses to and ext .…”

“…We assume that there are no particle currents in the steady state of the system, as in Ref. 8. In response to the temperature gradient and the external force, the system will develop a density gradient parallel to .…”

“…(12) is given here using slightly different notation from that used in Ref. 8; we explain these differences in Appendix A.…”

“…In the linear response regime (small ), it is still consistent to use the equilibrium equipartition formula ⟨ ⟩ = in Eq. (8). (Corrections to equilibrium equipartition enter Eq.…”