%0 Journal Article %J NATURE %D 2009 %T Measured long-range repulsive Casimir-Lifshitz forces %A Munday, J. N. %A Federico Capasso %A Parsegian, V. Adrian %X 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). %B NATURE %I NATURE PUBLISHING GROUP %C MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND %V 457 %P 170-173 %8 JAN 8 %G eng %N 7226 %9 Article %R 10.1038/nature07610