Surface tension
Surface tension is a physical phenomenon that occurs in liquids due to the electromagnetic attraction between molecules, resulting in a "skin" effect at the liquid's surface. This effect enables certain liquids to resist external forces, allowing lightweight objects, like small insects or paper clips, to float on their surface. In water, the polar arrangement of its molecules creates strong cohesive forces, particularly at the surface where water molecules are more tightly bound to each other than to the air above. The strength of surface tension is quantified in units such as dynes per centimeter, with water exhibiting a surface tension of approximately 72 dynes per centimeter at room temperature. Notably, surface tension decreases as temperature rises, facilitating better cleaning properties for hot water, which can penetrate tighter spaces. The phenomenon influences various aspects of nature and technology, from the spherical shape of falling water droplets to the design of cleaning products and waterproof materials. Understanding surface tension is essential across multiple disciplines, including industrial production, food science, and materials engineering.
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Surface tension
Surface tension is caused in a liquid by the tendency of the liquid’s molecules to be electromagnetically attracted to one another to such an extent that they can resist the application of external force at the boundary between the liquid and another substance. This effect causes the liquid to appear to be encased in a sort of outer skin, as when a water droplet rests upon a nonporous surface such as a pane of glass. This resistance to external force is responsible for the ability of some liquids to support the weight of objects such as small insects (so-called water striders) and paper clips, as well as for the capillary action that can make liquids resist the pull of gravity.
Water molecules are attracted to one another because the structure of the molecule is polar—that is, one side of the molecule has a greater positive charge and the other side of the molecule has a greater negative charge. This is due to the arrangement of the two hydrogen atoms attached to the single oxygen molecule. When many water molecules are present, then, they tend to stick to one another as the negatively charged side of one molecule is attracted to the positively charged side of another water molecule. Within the mass of a body of water, each molecule is subject to attractive force from all directions, so the net effect is for the attractions to cancel each other out. At the edges where the water body comes into contact with air, however, the molecules are not subject to attractive force from the air molecules they touch, so they are more tightly attached to the water molecules next to them on the surface. This stronger horizontal attraction between water molecules at the surface of a body of water is responsible for surface tension.
Background
The strength of the surface tension is measured in relation to the amount of surface in question, with the amount of surface given either as length or area. If length is specified, then surface tension is given as dynes per centimeter, meaning how many dynes of force would be required to break a liquid surface one centimeter long. If area is specified instead, then the units used are ergs per square centimeter. (Note that surface tension can also be measured in millinewtons per meter, mN/m, corresponding to the meter-kilogram-second system of SI measurement as compared to the centimeter-gram-second system.)
To illustrate, water has a surface tension of seventy-two dynes per centimeter at room temperature (about 70 degrees Fahrenheit or 20 degrees Celsius). Interestingly, surface tension decreases as the temperature of the water increases. This happens because the rising temperature causes the molecules to move around more, counteracting the effect of intermolecular attraction and making it easier to penetrate the surface. This is why hot water does a better job of cleaning—its lower surface tension permits it to move into smaller areas than would be possible with standard surface tension.
Surface tension is also responsible for the spherical shape of drops of water. When water is observed dripping from a faucet, initially it takes the familiar shape of a teardrop. Then, when it breaks free of the faucet and begins to fall, it quickly contracts into a spherical shape, as can be seen using high-speed photography. The reason for this is that the molecules of water inside it are all attracted to each other. This causes the water to assume the most compact shape it can—the shape with the smallest surface area—which is a sphere.
Overview
The larger a quantity of water’s surface area at its boundary with air, the more energy is necessary to maintain the surface tension. Intuitively, this makes sense, because if one centimeter of water has a surface tension of 72 dynes per centimeter, two centimeters of water will have a surface tension of 144, three centimeters, 216; and so forth.
According to the second law of thermodynamics, systems naturally tend to move from high-energy states to low-energy states. This tendency manifests itself with regard to surface tension in the way in which quantities of water seek to minimize their own surface area, thereby reducing the energy required to maintain their surface tension.
Examples of this phenomenon abound in nature. For example, one may wonder why water emerging from a hose or showerhead does not remain in a continuous stream instead of breaking up into droplets. This happens because if the water were to remain in an unbroken stream, it would have a very large surface area with a correspondingly large amount of energy needed to maintain surface tension. Instead, small amounts of water separate from the stream as the force of their momentum overcomes the attractive force of the stream’s surface tension. Then, as the water flies free of the stream, it assumes a spherical shape as discussed above.
Surface tension is relevant in many different contexts: industrial production, food preparation and packaging, cleaning, and so on. The active ingredients in cleaning products are chosen in part for their property of lowering the surface tension of water with which they are mixed, increasing the ability of the mixture to penetrate small spaces and porous surfaces. Food packaging often relies on the surface tension of the product it contains, as with toothpaste being dispensable from tubes because of its surface tension. Clothing companies seeking to produce waterproof materials for use as raincoats and tents must study the surface tension of water in order to create fabric surfaces that will preserve rather than penetrate the surfaces of droplets that land on the material, to keep it from soaking in. Even the chemical composition of paint is carefully adjusted to ensure that it has a surface tension that will allow the paint to spread easily while keeping a uniform thickness. Scientists are even beginning to develop synthetic “superhydrophobic materials,” which have surfaces designed to change the angle of incidence for the moisture that lands on it, allowing the droplets to bounce and the material to remain dry.
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