Surface tension

Surface tension is a property of liquids that allows them to resist an external force. It is the result of cohesion (the attraction of molecules within a substance) and is measured in force per unit length. Its SI unit is newton per meter but the cgs unit of dyne per centimeter is also used.

Overview

Surface tension is responsible for the shape of liquid droplets. It allows insects (e.g. water striders), usually denser than water, to float and slide on a water surface. It also causes capillary action which allows water (and its dissolved substances) to be drawn up into the roots of plants and the tiny blood vessels in our bodies.

Causes

The cause of surface tension is the cohesive forces between liquid molecules. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submersed.

Measurement

Surface tension is typically measured in dyn/cm (dyne per centimeter), the force in dynes required to break a film of length 1 cm. Alternatively, it can be stated as the energy in ergs required to increase the surface area by one square centimeter.

Applications

Surface tension has a number of applications in the real world. It is responsible for the ability of certain insects to float on water, the capillary action that draws water up into plants, and the tendency of certain liquids to spread out or form droplets.

See also

• Capillary action
• Cohesion (chemistry)
• Water strider

References

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