A Thermal Force Drifting Particles Along a Temperature Gradient

In 1972, V. Gallina and M. Omini of the Polytechnic of Turin proposed a phenomenological model for the thermal diffusion in liquid metals, explaining the isotope separation as provoked by a thermal force which is arising when a temperature gradient is established in the material. Here, we discuss this thermal force and its statistical origin from the bulk. We will see that it can be considered as a force of the form F = − S grad T, that is as a thermal/entropic force obtained from the derivative of the Helmholtz free energy with respect to the volume.