Retarded dispersion forces in periodic dielectric media

We show that periodic dielectric composite media such as ordered colloidal suspensions exhibit enhanced dispersion forces. In an fcc structure with a full photonic band gap these forces can produce a nearly isotropic pressure on the structure due to the vacuum field fluctuations. This pressure is shown to significantly alter the phase diagram of the Kirkwood-Alder hard-sphere transition for systems with micron-size dimensions. The calculations predict that aqueous solutions of ${\mathrm{TiO}}_2$ particles of the order of 0.35 μm in diameter should be ideal of observing these effects.