is the intimate mixture of oil and water, generally of a
milky or cloudy appearance. Emulsions may be of two types: oil-in-water
(where water is the continuous phase) and water-in-oil (where water is the
discontinuous phase). Oil-in-water emulsions are used as cutting fluids
because of the need for the cooling effect of the water. Water-in-oil
emulsions are used where the oil, not the water, must contact a surface — as
in rust preventives, non-flammable hydraulic fluids, and compounded steam
cylinder oils (see compounded oil); such
emulsions are sometimes referred to as inverse emulsions. Emulsions are
produced by adding an emulsifier. Emulsibility is not a desirable
characteristic in certain lubricating oils, such as crankcase or turbine
oils, that must separate from water readily. Unwanted emulsification can
occur as a result of oxidation products — which are usually polar compounds
— or other contaminants in the oil. See illustration of an oil-in-water
emulsion at polar compound.
Also see air entrainment,
foaming,
microemulsion