Thermal near-field radiative transfer between two spheres

TL;DR

This paper numerically analyzes near-field radiative transfer between two spheres of arbitrary sizes and gaps, revealing limitations of existing approximations and establishing a regime map for their validity.

Contribution

It extends theoretical analysis beyond existing approximations to arbitrary sphere sizes and gaps, providing a numerical framework and validity map.

Findings

Dipole approximation valid for small spheres

Proximity force approximation fails for large spheres

A regime map for approximation validity

Abstract

Radiative energy transfer between closely spaced bodies is known to be significantly larger than that predicted by classical radiative transfer because of tunneling due to evanescent waves. Theoretical analysis of near--field radiative transfer is mainly restricted to radiative transfer between two half--spaces or spheres treated in the dipole approximation (very small sphere) or proximity force approximation (radius of sphere much greater than the gap). Sphere--sphere or sphere--plane configurations beyond the dipole approximation or proximity force approximation have not been attempted. In this work, the radiative energy transfer between two adjacent non--overlapping spheres of arbitrary diameters and gaps is analyzed numerically. For spheres of small diameter (compared to the wavelength), the results coincide with the dipole approximation. We see that the proximity force approximation is not valid for spheres with diameters much larger than the gap, even though this approximation is well established for calculating forces. From the numerical results, a regime map is constructed based on two non--dimensional length scales for the validity of different approximations.