‘Casimir–Lifshitz’ force

Space is not completely empty; the vacuum teams with quantum mechanical energy fluctuations able to generate an attractive force between objects that are very close to each other. This ‘Casimir–Lifshitz’ force can cause static friction or ‘stiction’ in nanomachines, which must be strongly reduced. Until now only attractive interactions have been reported but in theory, if vacuum is replaced by certain media, Casimir–Lifshitz forces should become repulsive. This has now been confirmed experimentally. Repulsion, weaker than the attractive force, was measured in a carefully chosen system of interacting materials immersed in fluid. The magnitude of both forces increases as separation decreases. The repulsive forces could conceivably allow quantum levitation of objects in a fluid and lead to new types of switchable nanoscale devices with ultra-low static friction. Levitation depends only on the dielectric properties of the various materials. The cover illustrates repulsion between a tiny gold sphere and a silica substrate (left). Replace the silica with gold (right), and the force becomes attractive.

Quantum fluctuations create intermolecular forces that pervade macroscopic bodies1, 2, 3. At molecular separations of a few nanometres or less, these interactions are the familiar van der Waals forces4. However, as recognized in the theories of Casimir, Polder and Lifshitz5, 6, 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 bodies8, 9, 10, 11. Here we show experimentally that, in accord with theoretical prediction12, 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 friction13, 14, 15.


Measured long-range repulsive Casimir–Lifshitz forces, Nature 457, 170-173 (8 January 2009) |