Special Issue: New Trends in Nanoscale Heat Transfers

The development of nanofabrication techniques during the last decade has opened new scientific insights for investigating the fundamental mechanisms that govern heat transport at the nanoscale. Besides, it paves the way to promising applications in the field of thermal management and energy conversion. It is in this context a seminar was organized in April 2016 in Bad Honnef (near Bonn in Germany), funded by WE-Heraeus-Stiftung, with the intention to bring together researchers studying energy transport at the nanoscale. This special issue is a compilation of contributions presented during this seminar. It covers recent developments in the field of nanoscale radiative heat transfer and heat conduction.

This focus issue starts with a contribution by Henkel giving an introduction to the fluctuational electrodynamics theory introduced by Rytov in the 1950s to deal with heat exchanges between systems having fluctuating sources, and inspecting the main limitations of this theoretical framework. Lapas et al. then discuss the possibility to extend the classical fluctuation-dissipation theorem to non-Markovian systems.

The contributions by Biehs and Ben-Abdallah as well as the contribution by Esquilvel-Sirvent apply the standard Rytov formalism to investigate the near-field heat exchanges between two multilayered hyperbolic metamaterials and between two random composite media. Here, it is assumed that the different bodies are at local thermal equilibrium so that the thermal state of each body can be described by one unique temperature. The following contribution by Ordonez-Miranda et al. is devoted to the study of radiative heat flux along a chain of largely elongated nanoparticles which is coupled at its two ends to thermal baths at different temperatures. Next, Messina et al. take one step further and combine the fluctuational electrodynamics theory with Fourier’s law to consider heat exchanges between two thermally inhomogeneous massive media separated by a small vacuum gap.

The two following articles are devoted to the control of the radiative exchanges: Ben-Abdallah and Biehs review different concepts for manipulating the near- and far-field radiative heat current in many-body systems in the same manner as the electric current in an electric circuit is controlled in devices such as diodes, transistors, memories and logic gates, using thermal photons instead of electrons. This thermotronics is also explored in the quantum regime in the contribution of Joulain et al., which reviews concepts for a diode and a transistor using two-level systems.

Besides energy exchanges, the fluctuational electrodynamics theory can be used to calculate momentum transfer between two bodies:

The contribution of Volokitin discusses radiative heat flux and Casimir friction in graphene structures. Sasihithlu et al. compare van der Waals force-assisted heat transfer with the conventional near-field heat transfer and conventional conduction. Finally, Termentzidis et al. explore the impact of amorphization on the thermal properties of membranes.

Of course this focus issue does not give an exhaustive picture on the state-of-the-art in this tremendous research field. There are still numerous exciting questions and aspects regarding radiative heat transfer and heat conduction at the nanoscale which have not been addressed at all here.

Nevertheless, we hope that this focus section will contribute to the development of this relatively young research field, and will inspire researchers to push forward the limits of knowledge and also to develop technological applications.