Capillarity

CAPILLARITY (hair-like property), in the proper sense of the word, is a phenomenon by virtue of which liquid in two communicating vessels does not settle at the same level if one of the two vessels has the shape of a very narrow (hair-like) tube. A liquid that wets the walls of the tube rises in it above the hydrostatic level (see figure); in the case of a liquid that does not wet the walls, a lowering of the level is observed. In a broader sense, capillarity is the totality of phenomena caused by a special compacted state of the surface layer of a liquid. A particle located inside a liquid experiences attraction from all surrounding particles. But since these attractions act on average equally in all directions, they mutually compensate for each other, and the particle remains free. Conversely, a particle located in the upper "capillary layer" experiences a one-sided attraction to the mass of liquid adjacent from below, which creates a special state of compaction. Despite the insignificance of the force experienced by each particle individually, the magnitude of the pressure per 1 cm² reaches, for example, 15,000 kg for water (surface pressure). The forces acting on individual molecules, due to their one-sidedness, give them all a similar arrangement—with their longitudinal axis perpendicular to the surface of the liquid. Thus, the surface of the liquid is covered with an armor of tightly adhering and uniformly arranged molecules, which brings the properties of the liquid surface closer to the surface of a crystal. Since the movement of new particles to the surface is possible only by overcoming attractive forces, any increase in surface area is associated with the expenditure of energy (surface energy), and conversely, the contraction of the surface is accompanied by the release of energy. Therefore, every liquid surface has a tendency toward the greatest possible contraction. The magnitude of the surface energy e is different for different liquids and depends on the properties of the medium with which the liquid is in contact. Hereinafter, we assume that the second medium is air. Thus, for water, the value of e is 75 ergs per 1 cm², for mercury—500 ergs per 1 cm², for ethyl alcohol—22 ergs per 1 cm², for sulfuric ether—18 ergs per 1 cm². Thus, a drop of liquid is as if wrapped in an elastic film that is in a state of tension. If the surface of the liquid is bounded by walls to which the film adheres, then the walls experience an attractive force from the film, called surface capillary tension k. If we express the magnitude of k in dynes per 1 cm of the length of the film edge, we obtain numbers for capillarity identical to the above numbers for e. To characterize the capillary properties of a liquid, in addition to the values e and k, tables often contain the value of the "capillary constant a²." This value characterizes capillary phenomena in the narrow sense of the word. If a thin tube open at both ends, made of a substance wetted by a given liquid, is immersed in a vessel with liquid, the surface film inside the channel will spread along the walls of the tube, taking the form of a concave meniscus. In its striving to contract, the meniscus pulls the column of liquid in the tube upward until the hydrostatic pressure balances the capillary forces. The rise is greater the narrower the channel. Let h denote the height of the rise, k—surface tension, r—radius of the capillary, and s—specific weight of the liquid; then it is easy to

obtain the relationship h = 2k/rs. The ratio 2k/s has a constant value for a given liquid, denoted by a² (capillary constant). Introducing it, we obtain a simple relationship: h = a²/r or h·r = a². If, as is usually done, r and h are measured in mm, then the value a² takes the values: for water a² = 15 mm², for ethyl alcohol = 5.89 mm², for ether = 7.1 mm². The rise of liquids in capillaries has great and well-known significance. The penetration of liquids deep into porous bodies is based on it. The relationship between the phenomena of capillarity and adsorption is very important. If molecules of a substance with weak capillary properties (ether) are mixed, even in a negligible amount, with molecules of a base liquid possessing high surface tension (water), then under the influence of attractive forces, the water particles will be drawn inward, and a layer of the less strongly attracted ether molecules will remain on the surface. A drop of ether applied to a water surface spreads rapidly, forming a thin protective layer on the water (adsorption). If the molecules mixed into the water are electrolytically dissociated, the film may exhibit selective adsorption to a specific ion, and the consequence will be the emergence of surface charges (electrocapillary phenomena). With the development of the doctrine of the finely divided state of matter (disperse systems), the importance of capillarity for understanding the nature of the colloidal medium has increased extraordinarily, giving rise to a new discipline—capillary chemistry. A physician has to encounter phenomena of capillarity, for example, when draining wounds. Fluid located in a wound is absorbed through the capillary spaces between the threads of gauze. Strong compression of the gauze destroys these spaces and entails the cessation of absorption.