A colloidal crystal is an ordered array of colloids, analogous to a standard crystal whose repeating subunits are atoms or molecules.
The colloidal suspensions we are most familiar with, such as milk, blood, or latex are polydisperse, which means that the suspended particles have a broad distribution of sizes and shapes. However, suspensions of particles of identical size, shape, and interaction also occur. In these monodisperse colloids a new phenomenon, which is not found in polydisperse systems, can be observed. Under the right conditions particles spontaneously arrange themselves into spatially periodic structures.
Evidence for the existence of colloidal crystals can be found in nature, like the gem opal (which is essentially a fossilized colloidal crystal) or certain suspensions with a high concentrations of viruses.
Colloidal crystals find applications in many different areas, among the most prominent are photonics, ceramic engineering and the study of self-assembly.
These materials exhibit many interesting properties. A large number of experiments indicate that colloidal crystals in fact behave very similarly to classic crystalline structures over the appropriate length and time scales. The main results of other experiments are more perplexing however. These include the observation that defects in thin films of colloidal crystal suspensions can travel at velocities close to the speed of sound or the observation of "linear" shear band formation under high oscillatory strains (more in the paper Slip, yield, and bands in colloidal crystals under oscillatory shear).