Repulsive Casimir Effects
  Like Casimir's original force between conducting plates in vacuum, Casimir forces are usually attractive. But repulsive Casimir forces can be achieved in special circumstances. These might prove useful in nanotechnology. We give examples of when repulsive quantum vacuum forces can arise with conducting materials. 
K. A. Milton, E. K. Abalo, Prachi Parashar, Nima Pourtolami, Iver   Brevik, and S. A. Ellingsen
hep-th quant-ph