RADIATION FORCES ASSOCIATED WITH HEAT PROPAGATION IN NONISOTHERMAL SYSTEMS

The aim of this work is to show that, in the condensed phases, the flux of thermal energy couples with a flux of momentum. This follows from considerations of rational mechanics and can be also deduced as a corollary of a theorem established by Boltzmann, following a straightforward reasoning already developed by Ehrenfest in a somewhat simpler case than the one considered by us. When heat flux crosses the boundary between two adjoining media, the momentum flux changes, and "thermal radiation forces" should appear. Accordingly, the momentum-balance equation of continuous-field hydrodynamics should include, when applied to nonisothermal media, the momentum flux due to heat flow. When this is done, analytical expressions for thermal radiation forces are obtained with reference to various simple physical systems. The theoretical predictions lend themselves to direct experimental verification. Very good qualitative and quantitative agreement between theory and experiment has been found in various experimental systems, as described here, lending strong support to the proposed approach. Interestingly, experimental results obtained with macroscopic objects and with molecular or ionic particles both lead to analogous conclusions concerning the properties of thermal radiation forces, a circumstance that broadens the possible field of application of these concepts. Finally, it is suggested that the concept of momentum flux coupled to transport of thermal energy in the condensed phases can be fruitfully employed in the investigation of complex phenomena such as acoustic streaming and the Bernard-Rayleigh instabilities.