Surface tension

Surface tension

Surface tension is the property of a liquid that allows it to obtain the smallest surface area possible.

Physics

Keywords

surface tension, capillary action, capillary, cohesion, adhesion, wetting, mercury, water drop, pond skater, meniscus, fluid, liquid, particle, water surface, water molecule, insect, density, physics, sphere

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Surface tension occurs because molecules of a liquid attract one another. The attractive force between similar molecules is called cohesive force. Cohesive forces inside the liquid cancel out so interior molecules have a low energy state. However, boundary molecules have a high energy state as cohesive forces do not cancel out on the surface of the liquid. Consequently, in accordance with the principle of minimum energy, the boundary molecules are pulled constantly towards the inside of the liquid. As a result, it tries to obtain the smallest surface area possible. This is why a free-falling water droplet takes the shape of a sphere.

The phenomenon of surface tension becomes more complex when molecules of a solid surface interact with the liquid. The attractive force between the molecules of a solid surface and a liquid is called adhesive force.

Wetting is a characteristic ability of a particular liquid/solid pair which depends on the magnitude of the adhesive force and the cohesive force between the molecules of a liquid. If this force is strong enough, the liquid spreads on the solid surface. In other words, it is capable of wetting it. However, if the adhesive force is weak, the liquid in contact with the solid surface can reduce its own surface area, and no wetting occurs. Mercury, for instance, is a non-wetting liquid.

The wettability of liquids determines how they behave in thin tubes, that is, capillaries. The law of communicating vessels does not apply to liquids in capillary tubes, meaning that the liquid is not on the same level in these tubes; instead, capillary action can be observed. A capillary containing water exhibits capillary rise, while a capillary containing mercury exhibits capillary fall.

Capillary rise occurs when the liquid wets the wall of the tube, that is, when the adhesive forces between the tube and the liquid are stronger than the cohesive forces between the water molecules. If it is the other way round, capillary fall happens.

Surface tension and capillary action play an important role in nature. For example, pond skaters could not run on the surface of the water without the presence of surface tension. The legs of these aquatic insects are covered with tiny hairs which provide a small surface area, thus making them non-wettable. Due to surface tension, the surface of the water acts as a membrane that can support the weight of pond skaters, and tiny depressions form on the water under their legs. They can launch themselves from these depressions reaching speeds of up to 90 cm/s (35.4 in/s).

Capillary action also occurs in the transport system of plants. This phenomenon is needed for the plants’ xylem vessels to be able to transport water from the roots to the upper parts of the plant.

  • - Neighboring particles attract these molecules only from the inside of the liquid. Surface tension is produced by the tendency of these molecules to move towards the center of the droplet.
  • - Neighboring particles attract these molecules from all directions.

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