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Measuring instruments designed to measure the cohesive energy present at an interface; the instruments usually can measure the surface tension between a liquid and air and/or the interfacial tension between the boundaries of two liquids. Other tensiometers are intended to measure the surface tension between a liquid and a solid (also known as contact angle meters). There are several methods to measure the surface tension between a liquid and air and/or the interface tension between two liquids; the most common techniques are: (1) force tensiometers, instruments that measure by rising a ring submerged in the liquid from the liquid to the air or between the common surface of two liquids and measure the force needed to break through the surface (that is related to the surface tension). A variant of the method using a plate (that is placed but not submerged in the liquid) is also available. Force tensiometers are mainly used to determine the surface tensions or substances such as oils, detergents, and other chemicals. (2) Instruments that are based in the relationship between liquid heights in a capillary tube to the surface tension and (3) tensiometers intended to measure the surface tension between a liquid and a solid are based in the measurement of the angle between a liquid droplet and a solid surface usually employing optical techniques; they are mainly used to determine the wettability of surfaces when testing pills, surfactants, ink adhesion, and materials before coating. Some instruments employing other techniques (e.g., bubble pressure) are also used for these measurements.
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