Phonons in liquids, Onsager’s reciprocal relations, and the heats of transport

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Abstract
We continue the study of liquids considered as composed by two interacting subsystems: a population of particles, weakly bound to the sites of a disordered lattice, and a ‘‘gas’’ of phonons. The interactions among the material particles and the wave packets are mediated by ‘‘thermal radiation forces,’’ a special class of inertial effects. When the liquid system is a solution, the application of a temperature or a concentration gradient breaks the symmetry of one of the two subsystems, producing an excess of phonon-solute collisions and performing work on the particles, at the expense of the thermal energy of the wave packets. The heats of transport of thermal diffusion, isothermal diffusion, and Soret equilibrium may be statistically calculated from the energy balance of these collisions. The expressions arrived at by this approach are coincident with the ones obtained by nonequilibrium thermodynamics, although for our derivation Onsager reciprocal relations are not required. It is also argued that, within the frame of the phonon theory, transport processes in liquids may be treated in general by means of the classical Boltzmann transport equation, applied to phonon-particle interactions rather than to particle-particle collisions.

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