Micrometer-sized colloidal particles (seen at left under the optical microscope) can be prepared from two different kinds of materials, one of which scatters light strongly, and the other of which is transparent. By controlling the balance between the two materials, a suspension of the particles in water (seen in photographs at right) can be made to look either transparent, opaque, or irridescent, and can be made to switch between these different appearances. This research might enable new kinds of coatings for windows or light filters.

D.A. Weitz and C.M. Friend