Optical micrograph (differential interference contrast) of an oil-in-water emulsion. Monodisperse (equal-sized) droplets of oil, about 2 micrometers in diameter, form an ordered arrangement when suspended in water. Surfactant covers the interfaces of the droplets, preventing them from coalescing. In this image, the droplets are confined between two glass plates so that they are only able to form a monolayer. But in bulk the droplets can form a three-dimensional crystal with a lattice spacing on the order of the wavelength of visible light. Such colloidal crystals can serve as templates for materials (photonic crystals) that diffract light in unique ways.

See V.N. Manoharan, A. Imhof, and D.J. Pine, “Photonic Crystals from Emulsion Templates”, Advanced Materials 13 (6): 447–450 (2001)

Image credit: V.N. Manoharan