Oil on Water

Motor oil spreads into a micro-thin film that allows color caused by constructive interference between air-oil and water-oil interfacial reflections. Color (or wavelength) depends on the oil refractive index and film thickness. During flow from a drop the film thins while the oil separates into its hydrocarbon constituents, each with a different index and viscosity; i.e., it appears that color indicates viscosity. Incredibly, separation happens as soon as laminar flow starts, even though the falling drop is a random mixture of the hydrocarbons.