Repulsive Casimir-Lifshitz force observed

In the late 1940s, Hendrik Casimir proposed that two perfectly conducting parallel plates should feel a feeble attractive force between them, due to the zero-point energy of the surrounding electromagnetic field and its dependence on the plates’ positions. (See the article by Steve Lamoreaux, PHYSICS TODAY, February 2007, page 40 .) About a decade later, Evgeny Lifshitz and colleagues generalized Casimir’s work to real conductors and dielectrics and found that the force persisted. In most cases the proposed force was still attractive, but for some configurations--a high-permittivity material and a low-permittivity material separated by a medium of intermediate permittivity--it could be repulsive. In fact, the repulsive Casimir-Lifshitz force is responsible for liquid helium’s tendency to climb the walls of its container: The container repels the ambient vapor, and the liquid rises to fill the gap. Now, a group of researchers led by Harvard University’s Federico Capasso have observed a repulsive Casimir-Lifshitz interaction between two solid objects, a silica surface and a 40-µm-diameter gold-coated sphere, immersed in bromobenzene. To monitor the force, they attached the sphere to an atomic force microscope cantilever and measured the cantilever’s deflection using a light beam and a split-quadrant photodetector, as shown in the figure. A repulsive force of a few tens of piconewtons was measurable when the objects were brought within 40 nm of each other, and it increased as their separation decreased. The researchers suggest that the force they observed could levitate a solid within a liquid, which may lead to very low-friction sensors of force and torque. (J. N. Munday, F. Capasso, V. A. Parsegian, Nature 457, 170, 2009 .) — Johanna L. Miller