If a system is placed between a hot and a cold reservoir so that a temperature gradient is established, it is natural to expect that there will also be a density gradient, since systems are less dense at higher temperatures. It is perhaps less obvious that anisotropic molecules will also adopt, on average, a preferred net orientation. This effect, known as thermo-polarisation in polar liquids and thermo-orientation in non-polar ones, has been studied in depth in recent years, and can lead to other interesting thermoelectric effects such as thermally induced electric or magnetic monopoles.
Much of the literature in this field relies on phenomenological equations of non-equilibrium thermodynamics to rationalise the underlying behaviour. In our work (below), we have shown that we can use a simple statistical-mechanical treatment using mean-field theory to rationalise most of the underlying physics of the effect, and can gain excellent agreement with non-equilibrium molecular-dynamics simulations in many cases.
There are unfortunately limits to this very simple ‘local equipartition’ description however, and we have also explored, by analysing the equations of motion, how deviations from this description arise as the centre of mass of a molecule is displaced further from its interaction centre.