1. INTRODUCTION

One of the most fascinating properties of monodisperse colloids is that they can form ordered phases. These phases can be thermodynamically stable if the volume fraction of particles is high enough, or they can form under the influence of an external field such as gravity or an electric field. The particles are small enough to exhibit Brownian motion, which causes them to move around randomly until the collective has found a state of minimal free energy. In a colloidal crystal the (almost always spherical) particles stack regularly to form a long-range, three-dimensional array. Particles shaped like rods or plates form additional liquid crystalline phases with nematic or smectic order. The spontaneous formation of such phases is nowadays often called self-assembly or self-organization. Colloidal crystallization has been studied intensively ever since the time that methods became available to synthesize sufficiently monodisperse colloids, i.e. those with particles having sizes that are uniform to better than about 8%. ^1-3 Apart from being interesting for their statistical mechanical behaviour colloidal crystals have special and useful optical properties.

The best-known colloidal crystal with special optical properties is almost certainly the opal. It consists of a three-dimensional array of submicrometer silica spheres, which must have formed naturally in a sediment and then cemented together. ^4-6 Visible light diffracts from the lattice planes, just like Bragg diffraction of X-rays from molecular crystals. The result is a beautiful iridescence, earning opals the qualification gemstones. Opals can also be made artificially by...