Dilution and resonance-enhanced repulsion in nonequilibrium fluctuation forces

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.