Credit: © 2009 NPG

The Casimir effect produces forces that are tiny, but this can be a nuisance in the design of nanoscale mechanical devices where it can make components stick together destructively. However, this can be turned around if the forces could be made repulsive instead of attractive — a theoretical possibility not shown until now. Federico Capasso at Harvard University and colleagues1 have measured the sought-after repulsive Casimir forces, between a gold sphere and a silica plate immersed in fluid bromobenzene.

The group achieved this through their judicious selection of materials. In particular, bromobenzene was chosen for its dielectric permittivity, which is in between that of the other two materials — a necessary criterion to obtain repulsive Casimir forces. The measurements were carried out in an atomic force microscope fluid cell, where the gold sphere is attached to the cantilever. With a clever calibration technique that isolates hydrodynamic forces in the fluid, clean measurements of the repulsive Casimir forces between the gold sphere and the silica substrate are possible. These are turned into attractive forces when the substrate is replaced with gold.

This experiment is a long-awaited verification that repulsive Casimir forces exist and could point the way to practical design of ultra-low friction devices and sensors.