^BOil on water. ^b A swirl of coloured oil (pink) on water (blue), showing the immiscibility of the two liquids. The vortex of oil forms a more continuous film on the water surface at centre, but is breaking up at the outer edges. Oil and water are mutually insoluble, so will not mix without the addition of an agent (such as a detergent) that breaks the surface tension of both. Despite any amount of agitation, the oil and water will always separate out at rest.