SURFACE TENSION

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SURFACE TENSION

A paper clip can rest atop a water surface even though its density is several times that of water. Some insects can walk on the surface of water; their feet make indentations in the surface but do not penetrate it. Such phenomena are examples of surface tension: the surface of the liquid behaves like a membrane under tension. The molecules of the liquid exert attractive forces on each other; there is zero net force on a molecule inside the 14-5 SURFACE EXTENSION volume of the liquid, but a surface molecule is drawn into the volume (Fig. 14-12). Thus the liquid tends to minimize its surface area, just as does a stretched membrane. Freely are spherical (not teardrop-shaped) because a sphere has a smaller surface area for a given volume than any other shape. A beautiful example of the formation of a spherical droplet. How we can make quantitative measurements of surface tension. A piece of wire is bent into a U shape, and a second piece of wire slides on the arms of the U. When the apparatus is dipped into a soap solution and removed, creating a liquid film, the film exerts a surface-tension force on the slider that quickly pulls it up toward the top of the inverted U (if the slider’s weight w is not too great).

When we pull the slider down, increasing the area of the film, molecules move from the interior of the liquid (which IS many molecules thick, even in a thin film) into the surface layers. The surface layers do not simply stretch like a rubber sheet. Instead, more surface is created by molecules moving from the bulk liquid.

To hold the slider in equilibrium, we need a total downward force F = w + T. In equilibrium, F is also equal to the surface-tension force exerted by the soap film on the slider. Let I be the length of the wire slider. The film has both front and back surfaces, so the force F acts along a total length 2/. The surface tension r (Greek “gamma”) in the film is defined as the ratio of the surface-tension force F to the length d along which the force acts surface tension is a force per unit length. The SI unit is the newton per meter (N/m), but the unit, the per centimeter (dyn/cm), is more commonly used 1 dyn/cm = 10-3 N/m = 1 mN/m. T Ie 14-2 shows some typical values of surface tension. The lowest values of r in the liquid filed noble gases neon and helium, in which the attraction between . Ycry weak. (For the same reason, these elements do not form compounds.)  The splash from a drop of water falling into a liquid produces a column called a jet. surface tension pulls the top of the jet into a spherical droplet.

Tube and the piston face; the molecules of liquid adhere to all the surfaces. If the surfaces are very clean and the liquid very pure, then when we pull the piston face, we observe a tensile stress and a slight increase in volume; we are stretching the liquid. Adhesive forces prevent it from pulling away from the walls of the container. With water, tensile stresses as large as 300 tarn have been observed in the laboratory. This situation is highly unstable; a liquid under tension tends to break up into many small droplets. In tall trees, however, negative pressures are a regular occurrence. Negative pressure is believed to be an important mechanism for transport of water an d nutrients from the roots to the leaves in the small xylem tubes (diameter of the order of 0.1 rnm) in the growing layers of the tree.

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