Surface Tension

Surface Tension. If we try to mix oil and water on a plate, we find that they will separate directly they are left alone. The liquids have therefore done work in order to parate, and have lowered their energy; the tendency was to decrease the surface between them as much as possible. Hence decrease of energy and decrease of surface take place together, looking at it from the other-point of view, the greater number of particles existing in the surface the greater is the energy of the liquid; so that we may draw the conclusion that a particle of liquid has more energy when in the surface than when far from it. This energy is known as surface energy, and is proportional to the area of the fluid surface. The energy of the two surfaces of a soap-bubble bubble is equal to the work done in blowing the bubble, but it is usually stated in the form of the energy per unit area. Surface energy is not always the same for any fluid, but depends on the substance which meets it at that surface. With the same two non-mixable fluids, however, the surface energy is a fixed quantity. The surface of a fluid is exerting a tension in all directions, and it is the action of this force which causes the contraction of the surface and the lowering of the energy of the fluid. It is usual to consider this force as acting across a line of film and to express it as the force per unit length. It can then be shown that this force per unit length, or surface tension, is equal to the energy per unit area, and tbe surface tenslon also depends upon the nature of the two fluids bounded by the surface. When three fluids can exist in contact with each other, the surface tensions for each pair are three forccs in equilibrium, and their values determine the angles between the three surfaces. The same three fluids always meet at the saame angles. If the surface tension between two is greater than the sum of the surface tensions between the other two, there cannot be equilibrium. This is the case when air, water, and oil are the three fluids; the surface tension of water in contact with air is so much greater than the sum of the tensions between air and oil, and between oil and water, that the oil is pulled out. It tries, in fact, to cause the last two surface tensions to act in one line, so that their resultant shall be as great as possible, and thus tends to make the surface between water and air and that between oil and air lie in one plane, This occurs when the angle between oil and water is reduced to zero - i.e. when the oil is drawn out into in infinitely thin film. Surface tension also acts when a fluid is in contact with a solid. A liquid in a fine capillary tube is in contact with the solid tube and with the air, and as long as the surface tensions between pairs of the three substances are in equilibrium, the surface of the liquid will make a definite angle with the tube. [CAPILLARITY.]