A monodisperse colloid is expected to behave like atomic systems because the particles under go thermal motion. It shows phase transition from disordered phase to ordered phase. Changing its appearance from milky white to iridiscent, the iridiscence produced by the Bragg reflection of visible light by the orderely arranged particles. If the particles are large enough, they can be seen by a light microscope used in metallurgy. Several micrographs of the ordered phase (or crystalline structure) are shown. The mechanism of the transition is quite different from those seen in ordinary atomic systems, in which attractive force plays an important role. The difference is caused by repulsive force between the particles and explained as the realization in a colloidal system of the so called Alder transition, which was found by computer experiment.
