air entrainment - presence in an oil of minute bubbles in colloidal-like suspension, giving the liquid a cloudy or hazy appearance. The bubbles are difficult to expel, as distinct from foam, which consists of relatively large bubbles that tend to rise and eventually escape from the oil. In a lubricating oil, entrained air can disrupt the lubricating film and cause excessive surface wear. Because entrained air is compressible, it can cause erratic and inefficient operation of hydraulic systems. A common cause of air entrainment is excessive use of silicone in treating foaming problems. ASTM D 3427-75 ("Gas Bubble Separation Time of Petroleum Oils") measures the ability of an oil to separate entrained air. See colloid, emulsion.