The cause of surface tension
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Surface tension is caused by the attraction between the molecules of the liquid by various intermolecular forces. In the bulk of the liquid each molecule is pulled equally in all directions by neighboring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid but they are not attracted as intensely by the molecules in the neighbouring medium (be it vacuum, air or another liquid). Therefore all of the molecules at the surface are subject to an inward force of molecular attraction which can be balanced only by the resistance of the liquid to compression. Thus the liquid squeezes itself together until it has the locally lowest surface area possible.
Another way to think about it is that a molecule in contact with a neighbor is in a lower state of energy than if it weren't in contact with a neighbor. The interior molecules all have as many neighbors as they can possibly have. But the boundary molecules have fewer neighbors than interior molecules and are therefore in a higher state of energy. For the liquid to minimize its energy state, it must minimize its number of boundary molecules and therefore minimize its surface area.[1]
Some examples of the effects of surface tension seen with ordinary water:
Beading of rain water on the surface of a waxed automobile. Because water does not adhere to wax, surface tension prevents water drops from spreading out over the wax.
Formation of drops occurs when a mass of liquid is stretched. The animation shows water adhering to the faucet gaining mass until it is stretched to a point where the surface tension can no longer bind it to the faucet. It then separates and surface tension forms the drop into a sphere. If a stream of water were running from the faucet, the stream would break up into drops during its fall. This is because of gravity stretching the stream, and surface tension then pinching it into spheres.
Surface tension has a big influence on other common phenomena, especially when certain substances, surfactants, are used to decrease it:
Bubbles have very large surface areas for very small masses. Bubbles cannot be formed from pure water because water has very high surface tension, but the use of surfactants can reduce the surface tension more than tenfold, making it very easy to increase its surface area.
Colloids are a type of solution where surface tension is also very important. Oil will not spontaneously mix with water, but the presence of a surfactant provides a decrease in surface tension that allows the formation of small droplets of oil in the bulk of water (or vice versa).