Reciprocity of thermal diffusion in time-modulated systems

TL;DR

This paper demonstrates that, unlike electromagnetic and acoustic waves, thermal diffusion generally remains reciprocal under time modulation due to fundamental physical constraints, challenging previous assumptions.

Contribution

The authors develop a theoretical framework showing that thermal reciprocity is preserved under time modulation, and experimentally confirm the continued symmetry in heat transfer.

Findings

Thermal reciprocity is maintained in time-modulated systems.

Time modulation cannot generally break thermal reciprocity.

Experimental results show reciprocal heat transfer in dynamic devices.

Abstract

The reciprocity principle governs the symmetry in transmission of electromagnetic and acoustic waves, as well as the diffusion of heat between two points in space, with important consequences for thermal management and energy harvesting. There has been significant recent interest in materials with time-modulated properties, which have been shown to efficiently break reciprocity for light, sound, and even charge diffusion. Quite surprisingly, here we show that, from a practical point of view, time modulation cannot generally be used to break reciprocity for thermal diffusion. We establish a theoretical framework to accurately describe the behavior of diffusive processes under time modulation, and prove that thermal reciprocity in dynamic materials is generally preserved by the continuity equation, unless some external bias or special material is considered. We then experimentally demonstrate reciprocal heat transfer in a time-modulated device. Our findings correct previous misconceptions regarding reciprocity breaking for thermal diffusion, revealing the generality of symmetry constraints in heat transfer, and clarifying its differences from other transport processes in what concerns the principles of reciprocity and microscopic reversibility.