Krüger and Colleagues Models Contactless Heat Engine for Torque Generation

A new contactless heat engine powered by quantum and thermal electromagnetic fluctuations has been demonstrated, potentially paving the way for innovative nanoscale mechanical devices. Dhruv Shah and colleagues at MIT, in collaboration with the Israel Institute of Technology, Massachusetts Institute of Technology, Belarusian State University, Georg-August-Universit¨at G¨ottingen, and Middlebury College, reveal that two concentric cylinders, with the inner cylinder stably levitated due to Casimir forces, can generate sustained rotation through nonreciprocal electromagnetic interactions. Their research, grounded in the principles of Rytov fluctuational electrodynamics, demonstrates that breaking reciprocity allows for net torque generation from exchanged photons, each carrying both energy and angular momentum. The study meticulously computes the frictional torque opposing this rotation and estimates the engine’s efficiency, finding it is bounded by the Carnot limit, thus providing a comprehensive understanding of fluctuation-induced angular-momentum transfer.

Sustained rotation achieved via angular momentum transfer in a contactless thermal engine

A net torque of 8.7 × 10−14 N’m was achieved, representing a significant advancement over prior attempts which failed to produce sustained rotational motion. This breakthrough hinges on the transfer of angular momentum, a phenomenon previously unrealised within contactless heat engines. The engine’s cylindrical design incorporates nonreciprocal dielectric materials, functioning effectively as one-way mirrors for light, to break the symmetry of the system and convert thermal energy into rotational motion, entirely eliminating the need for physical contact or traditional bearings. The principle relies on the asymmetry introduced by these materials, allowing photons to propagate more readily in one direction than another, creating a directional force. This is analogous, in some respects, to the effect of a magnetic field on charged particles, but achieved through dielectric properties and electromagnetic fluctuations.

Modelling the electromagnetic interactions using Rytov fluctuational electrodynamics allowed the researchers to compute the frictional torque opposing rotation, enabling an estimation of engine efficiency bounded by the Carnot limit, the theoretical maximum efficiency for any heat engine operating between two thermal reservoirs. The engine’s core comprises two concentric cylinders maintained at differing temperatures. The inner cylinder is levitated due to repulsive Casimir forces, a quantum mechanical effect arising from the fluctuations of the electromagnetic vacuum energy. These Casimir forces, though weak, provide the stable levitation necessary for contactless operation. Each exchanged photon carries not only energy but also a discrete unit of angular momentum, denoted by the integer ‘n’, which contributes to the overall torque. The magnitude of this angular momentum is directly related to the photon’s polarisation and wavelength.

Rytov fluctuational electrodynamics modelling predicts potential for contactless engine efficiency

Contactless engines represent a compelling alternative to conventional mechanical systems, potentially overcoming issues of wear and tear, friction, and facilitating operation in extreme environments such as high vacuum or at cryogenic temperatures. Conventional mechanical systems invariably suffer from energy losses due to friction, whereas a truly contactless engine, if realised with high efficiency, could operate with minimal energy dissipation. Current modelling, however, relies on estimations of frictional torque, a factor that could significantly hinder practical performance and limit the overall efficiency of the device. Accurately accounting for all sources of friction, including those arising from the surrounding medium and the dielectric materials themselves, is crucial for realistic performance predictions. Achieving efficiencies predicted by the theoretical framework, bounded by the Carnot limit, necessitates careful material selection and meticulous design optimisation to minimise energy losses and maximise angular momentum transfer.

Although the team’s calculations do not yet demonstrate actual macroscopic power generation, this work constitutes a major step towards realising genuinely contactless engines. It demonstrates a viable pathway for converting heat into rotational motion without physical contact by skillfully manipulating electromagnetic fluctuations and Casimir forces. The core innovation lies in levitating a cylinder and inducing rotation without any mechanical parts, establishing a novel principle for energy conversion. The engine utilises materials that exhibit directional dependence in their interaction with light, the nonreciprocal dielectric materials, breaking symmetry and enabling the transfer of rotational force. Rytov fluctuational electrodynamics, a sophisticated theoretical framework, was essential for accurately modelling the complex electromagnetic interactions within the engine. This approach accounts for the random fluctuations of the electromagnetic field, which are crucial for understanding the energy transfer mechanisms. Further research will focus on scaling up the engine’s performance and exploring potential applications in micro- and nanoscale devices, such as sensors, actuators, and energy harvesters. The potential for creating self-sustaining, miniature machines powered solely by thermal gradients is a particularly exciting prospect. The team also intends to investigate different materials and geometries to further optimise the engine’s efficiency and torque output, potentially leading to a new generation of contactless technologies.

The researchers demonstrated a contactless heat engine powered by electromagnetic fluctuations between two concentric cylinders. This engine generates rotation without physical contact, utilising repulsive Casimir forces to levitate an inner cylinder within an outer one and nonreciprocal dielectric materials to induce movement. The study shows that heat can be converted into rotational motion via angular-momentum-resolved heat flux, with efficiency bounded by the Carnot limit. The authors intend to scale up performance and explore applications in micro- and nanoscale devices, optimising materials and geometries to improve efficiency and torque.