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.
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We thank D. Iannuzzi, R. Podgornik, J. Zimmerberg, S. M. Bezrukov and M. B. Romanowsky for discussions. This project was partially supported by the Center for Nanoscale Systems at Harvard University, and by the Intramural Research Program of the NIH, Eunice Kennedy Shriver National Institute of Child Health and Human Development. J.N.M. acknowledges support from the NSF.
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Munday, J., Capasso, F. & Parsegian, V. Measured long-range repulsive Casimir–Lifshitz forces. Nature 457, 170–173 (2009). https://doi.org/10.1038/nature07610
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