Radiative energy transfer between closely spaced objects (spacing much smaller than the nominal wavelength of thermal radiation, about 10 microns at 300 K) can exceed the predictions of Planck's theory of blackbody radiation due to tunneling of electromagnetic waves from one object to another. Thermal radiative transfer is due to the transport of energy via electromagnetic waves generated by the temperature induced fluctuations of electric charges. The relation between the temperature and the fluctuations of electric charges is given by the fluctuation dissipation theorem. The propagation of the resultant electric and magnetic fields is determined by the dyadic Green's function of the vector Helmholtz equation. Thus, radiative transfer between two objects, including near field effects, can be determined using the fluctuation dissipation theorem and the dyadic Green's function. The fluctuations of the electromagnetic field also give rise to the phenomenon of dispersion forces between objects, such as van der Waals or Casimir forces. While dispersion forces as well as near field radiation transfer phenomena have similar origins, and dispersion forces are relatively well understood, the same cannot be said of near field radiative transfer, where experimental research has not kept pace with theoretical progress. To bridge the gap between experimental and theoretical progress, this project aims to theoretically and experimentally investigate near field effects on thermal radiative transfer in two relevant geometries, namely 1) between two spheres, and 2) between a sphere and a flat surface.
Intellectual Merit: The scarcity of reliable experimental data involving near field radiative transfer is due to the difficulty of performing experiments in the two configurations which have been analyzed theoretically; between two parallel surfaces and between a flat surface and a sphere that is small enough that it can be approximated as a point dipole. The theoretical work in this research will focus on determining near field radiative transfer between two microspheres, without making the assumption that the spheres are point dipoles. In addition to yielding predictions associated with geometries that can be considered experimentally, this will pave the way for proposing an asymptotic theory for near field radiative transfer between a sphere and a flat surface. So far, this has not been possible with out making ad hoc assumptions. Complementing the theoretical work, we propose a novel and ultrasensitive technique of measuring heat transfer between a sphere and a flat surface, and between two spheres using a bimaterial cantilever as a temperature sensor. Similar configurations are commonly used in force spectroscopy, but they have never been used for the measurement of near field radiative transfer. These configurations will, for the first time, allow for a quantitative comparison between experimental and theoretical results. Hence, this research will bring together both theory and experiment in near field radiative transfer.
Broader Impacts: Direct conversion of waste heat into useful electricity is possible by thermophotovoltaic energy conversion, where photons from a thermal source are converted to electron hole pairs in a photovoltaic cell. Understanding nanoscale effects on thermal radiation is important for increasing the power density and efficiency of thermophotovoltaic conversion. Also, measurement of near field radiative transfer has applications for another related area; measurement of dispersion forces. This project presents an opportunity to introduce undergraduate and K12 students, especially underrepresented minorities and women, to cutting edge research through multiple avenues within Columbia University, such as Summer REU programs through Columbia MRSEC and NSEC, K12 programs through Columbia University Center for Technology, Innovation, and Community Engagement (CTICE) and the Double Discovery Center. The goal for the outreach effort is to ensure that the K12 and summer REU students gain from involvement with this project an appreciation and passion for research in engineering and mathematical and physical sciences. The experiments will provide undergraduate students who are acquainted with heat transfer an opportunity to not only investigate some of the novel features of nanoscale thermal transport, but also to engage in numerical simulations necessary to explain some of the experimental results.
