Acoustic waves and electromagnetic waves can transport heat between objects through their respective energy carriers: phonons and photons. At or near room temperature, the heat transfer between objects separated by a material medium occurs at a much higher rate when facilitated by phonons than by photons. However, phonons are generally thought to be ineffective at transporting heat between objects separated by a vacuum gap, because these energy carriers are vibrations in an atomic lattice and thus would require a material medium to propagate. Writing in Nature, Fong et al.1 report experimental evidence that phonons can travel across a vacuum gap and therefore induce heat transfer between vacuum-separated objects because of the effect of quantum fluctuations.
In simple terms, quantum fluctuations can be understood as being the source of an electromagnetic signal that a perfectly sensitive detector would detect in a vacuum, even when this vacuum is shielded from all possible internal and external sources of electromagnetic waves, such as charges and currents2. The fluctuations are a consequence of a law in quantum mechanics known as Heisenberg’s uncertainty principle3, which states that certain pairs of physical quantities cannot be determined at the same time with absolute precision. The presence of quantum fluctuations subtly influences surrounding matter, leading to several observable effects.
One of these effects, relevant to Fong and colleagues’ work, is the Casimir force4 — the force that two neutral atoms separated by a vacuum gap exert on each other. The Casimir force results when quantum fluctuations induce fluctuating charge densities in these atoms; the charge densities then interact through their electric fields. The force that sticks a gecko’s foot to a wall is an example of a macroscopic manifestation of the Casimir force. It arises from the combined interactions between fluctuating charge densities in all the atomic constituents of the two objects.
To understand how the Casimir force can induce phonon transfer between vacuum-separated objects, consider an object that is maintained at a particular temperature by being kept in contact with a heat source (Fig. 1). Thermal agitation of the object’s atoms, which can be thought of as being interconnected by elastic springs, gives rise to phonons. In the presence of these phonons, the surface of the object undulates over time. When a second object is brought close to the first one, it is subjected to a time-varying Casimir force owing to its interaction with the undulations of the first object’s surface. The second object’s surface is thus subjected to tugging that then gives rise to phonons in the object’s interior. Phonons are therefore transmitted from the first object to the second one.
Because phonons are heat carriers, when they are transported from one object to another across a vacuum gap, as a result of the Casimir force, they induce heat transfer if the second object is maintained at a lower temperature than that of the first one. This phenomenon of heat transport facilitated by the Casimir force has been predicted previously using theoretical models5–7. Fong et al. have now measured such a heat-transfer mode experimentally.
The authors used a technique called optical interferometry to observe the thermal agitation of atoms (Brownian motion) at the surface of a membrane. This membrane was kept in contact with a heat source held at a constant temperature. Measurements of thermal agitation can be related to, and therefore used as a gauge for, the temperature of the atoms at the membrane’s surface. Moreover, the difference in this temperature with and without Casimir interaction with another, closely juxtaposed membrane is directly proportional to the resulting heat transfer between the two interacting membranes. The authors used these features to estimate the amount of heat transmitted between the membranes for vacuum gaps of different sizes. They found that their measurements accurately conform to theoretical estimates of such heat transport.
Fong and colleagues’ work provides conclusive evidence that the Casimir force can induce heat transfer. However, the use of this method to transport heat between two objects is limited, because the Casimir force decreases rapidly in strength as the space between the objects is increased. It is only when the gap between two objects is of the order of a few nanometres that the Casimir force is strong enough for this heat-transfer mode to dominate over competing modes, such as photon tunnelling8.
The authors discovered a way to amplify the Casimir mode of heat transfer so that it remains dominant even when the gap between the membranes is in the range of hundreds of nanometres. The membranes were carefully designed in such a way that their dimensions and the temperatures at which they were maintained allowed them to vibrate with their maximum possible displacements — in other words, at their natural frequencies. Thus, applications that are devised to exploit this heat-transfer mode to dissipate heat (such as in a hard-disk drive, where the distance between the writing head and the storage disk is a few nanometres) would require such careful design to ensure that the mode is amplified. Achieving this would be a challenge for the future.
Nature 576, 216-217 (2019)