A dispersion of one immiscible liquid into another through the use of a chemical that reduces the interfacial tension between the two liquids to achieve stability. Two emulsion types are used as muds: (1) oil-in-water (or direct) emulsion, known as an "emulsion mud" and (2) water-in-oil (or invert) emulsion, known as an "invert emulsion mud. " The former is classified as a water-base mud and the latter as an oil-base mud.