Dilution and resonance-enhanced repulsion in nonequilibrium fluctuation forces

Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. In equilibrium, forces induced by fluctuations of the electromagnetic field between electrically polarizable objects (microscopic or macroscopic) in vacuum are generically attractive. The force may, however, become repulsive for microscopic particles coupled to thermal baths with different temperatures. We demonstrate that this nonequilibrium repulsion can be realized also between macroscopic objects, as planar slabs, if they are kept at different temperatures. It is shown that repulsion can be enhanced by (i) tuning of material resonances in the thermal region and by (ii) reducing the dielectric contrast due to " dilution. " This can lead to stable equilibrium positions. We discuss the realization of these effects for aerogels, yielding repulsion down to submicron distances at realistic porosities.