Measured long-range repulsive Casimir-Lifshitz forces

Quantum fluctuations create intermolecular forces that pervade macroscopic bodies(1-3). At molecular separations of a few nanometres or less, these interactions are the familiar van der Waals forces(4). However, as recognized in the theories of Casimir, Polder and Lifshitz(5-7), at larger distances and between macroscopic condensed media they reveal retardation effects associated with the finite speed of light. Although these long- range forces exist within all matter, only attractive interactions have so far been measured between material bodies(8-11). Here we show experimentally that, in accord with theoretical prediction(12), the sign of the force can be changed from attractive to repulsive by suitable choice of interacting materials immersed in a fluid. The measured repulsive interaction is found to be weaker than the attractive. However, in both cases the magnitude of the force increases with decreasing surface separation. Repulsive Casimir - Lifshitz forces could allow quantum levitation of objects in a fluid and lead to a new class of switchable nanoscale devices with ultra-low static friction(13-15).