Emulsion, defined as a dispersion of one liquid into another, a mixture of two liquids and not a solution with one liquid devolved into the other. A foam is an emulsion in which the dispersed phase is a gas rather than a liquid. The foaming and emulsion characteristics of hydraulic fluid are important to system performance because the fluid can pick up and contain contaminants that can affect its properties and functional ability. Lubricants and oils containing excess amounts of entrained air can lead to serious disruptions of equipment in operation, increased oxidation tendency and shortened lubricating efficiency.