The surface tension of a liquid governs the rise of liquids in capillary tubes and the formation of menisci (curved surfaces) at the top of liquids (Hiemenz 1986, Hunter 1986). When a glass capillary tube is dipped into a beaker of water, the liquid climbs up the tube and forms a curved surface (Figure 5.10). The origin of this phenomenon is the imbalance of intermolecular forces at the various surfaces and interfaces in the system (Evans and Wennerstrom 1994). When water climbs up the capillary tube, some of the air-glass contact area is replaced by water-glass contact area, while the air-water contact area remains fairly constant. This occurs because the imbalance of molecular interactions between glass and air is much greater than that between glass and water. Consequently, the system attempts to maximize the glass-water contacts and minimize the glass-air contacts by having the liquid climb up the inner surface of the capillary tube. This process is opposed by the downward gravitational pull of the liquid. When the liquid has climbed to a certain height, the surface energy it gains by optimizing the number of favorable water-glass interactions is exactly balanced by the potential energy that must be expended to raise the mass of water up the tube (Hiemenz 1986). A mathematical analysis of this equilibrium leads to the derivation of the following equation:
2 cos 0
where g is the gravitational constant, h is the height that the meniscus rises above the level of the water, r is the radius of the capillary tube, Ap is the difference in density between the water and the air, and 8 is the contact angle (Figure 5.10). This equation indicates that the surface tension can be estimated from a measurement of the height that a liquid rises up a capillary tube and the contact angle: the greater the surface tension, the higher the liquid rises up the tube. This is one of the oldest and simplest methods of determining the surface tension of pure liquids, but it has a number of problems which limit its application to emulsifier solutions (Couper 1993).
Capillary forces are responsible for the entrapment of water in biopolymer networks and oil in fat crystal networks. When the gaps between the network are small, the capillary force is strong enough to hold relatively large volumes of liquid, but when the gaps exceed a certain size, the capillary forces are no longer strong enough and syneresis or "oiling off"
occurs. A knowledge of the origin of capillary forces is therefore important for an understanding of the relationship between the microstructure of foods and many of their quality attributes.
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