Van der Waals Force Assisted Heat Transfer

Abstract

Phonons (collective atomic vibrations in solids) are more effective in transporting heat than photons. This is the reason why the conduction mode of heat transport in nonmetals (mediated by phonons) is dominant compared to the radiation mode of heat transport (mediated by photons). However, since phonons are unable to traverse a vacuum gap (unlike photons), it is commonly believed that two bodies separated by a gap cannot exchange heat via phonons. Recently, a mechanism was proposed [J. B. Pendry, K. Sasihithlu, and R. V. Craster, Phys. Rev. B 94, 075414 (2016)] by which phonons can transport heat across a vacuum gap -through the Van der Waals interaction between two bodies with gap less than the wavelength of light. Such heat transfer mechanisms are highly relevant for heating (and cooling) of nanostructures; the heating of the flying heads in magnetic storage disks is a case in point. Here, the theoretical derivation for modelling phonon transmission is revisited and extended to the case of two bodies made of different materials separated by a vacuum gap. Magnitudes of phonon transmission, and hence the heat transfer, for commonly used materials in the micro-and nano-electromechanical industry are calculated and compared with the calculation of conduction heat transfer through air for small gaps as well as the heat transfer calculation due to photon exchange.

Photoinduced Tip–Sample Forces for Chemical Nanoimaging and Spectroscopy

“…, though would show a distinct distance dependence. Calculations showing that small displacements of a surface can result in significant changes in the van der Waals forces between nearby objects resulting in heat transfer across a vacuum gap are presented in refs and . Our results provide direct evidence that there are two or more long-range forces possible, because we separately observe an attractive force (Figure b) and a repulsive force with an absorptive spectral profile (Figure a), depending on sample material.…”

Contribution of the acoustic waves to near-field heat transfer

“…These discrepancies stimulate the search of the alternative channels of the heat transfer that can be activated in an extreme near-field. One of the obvious channel is related with phonon tunneling which stimulated active research of the phonon heat transfer in this region [27][28][29][30][31][32][33][34][35][36][37][38] . Most papers consider phonon tunneling mediated by the van der Waals interaction.…”

First-principles calculations of phonon transport across a vacuum gap

“…Prior theoretical [41][42][43][44][45][46][47][48][49][50][51][52][53] and experimental [40,54] works reported that phonon transport across vacuum gaps can dominate heat transfer in the extreme near field. Since this transport mechanism is mediated by lattice vibrations, an all-atom model is the most effective approach for incorporating the three-dimensional (3D) interatomic force interactions into phonon transport calculations.…”

“…where the superscript † denotes conjugate transpose, Γ j (ω, First-principles calculations of all interatomic force constants are performed with the open-source density functional theory (DFT) package ABINIT [67,68]. Prior works [48,49] have reported that phonon transport across vacuum gaps is dominated by van der Waals (vdW) interaction. As such, the impact of vdW interaction on the phonon heat transfer coefficient is also investigated hereafter.…”