Normally what determines whether something floats or not is its density. For instance, hot air is less dense than cooler air, which is why hot-air balloons float. Crude oil has a density of about 58 pounds per square foot, and so floats on seawater, which has a density of 64 pounds per square foot.
By making water float on oil, scientists now find that conventional wisdom can be wrong.
The key behind the counterintuitive results involves how tightly the molecules in a liquid stick to one another as opposed to something else. The strength of this force between like molecules determines the liquid's surface tension — how likely it resists an external force. Insects such as water striders can walk on water by taking advantage of this surface tension.
The researchers added tiny amounts of water to drops of various types of oil. These water droplets could float on the oil, depending on how large they were and the type of oil that was used. Commercial vegetable oil has enough surface tension to support the droplets; in contrast, pure mineral oils such as hexane, octane and decane do not. This effect also worked on commercial vegetable oil as long as the drops were smaller than about one-hundredth of a cubic inch — slightly smaller than a pea.