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what is surface tension in physics

what is surface tension in physics

Surface tension is the property of a liquid rest, due to to which the top surface of a liquid contained in a vessel behaves like a stretched membrane occupying minimum surface area.

spherical shape acquired by liquid drop due to surface tension
spherical shape acquired by liquid drop due to surface tension

Explanation for Surface Tension 

Consider a cube of liquid. The molecules of the liquid experience a pull by the neighboring molecules equally. Hence, the molecule is acted upon by balanced attractive cohesive forces. But the molecules present near the edges (top surface) are pulled inward because of the unbalanced cohesive forces. But these molecules cannot move inward due to the opposition offered by inner molecules leading to a tension at the surface. Thus, the molecules at the surface realign themselves giving a spherical shape to the drop of a liquid. For example, a drop of water is always spherical.

Simularly, at the top surface of a liquid in a vessel the molecules are pulled inward due to the unbalanced cohesive forces acting downward. Due to this, the surface of the liquid behaves like a stretched membrane. This phenomenon is called 'surface tension'.   

However, near the walls of the container, there exists adhesive force between the liquid molecules and the molecules of the material of the container. This adhesive force being different from the cohesive force, the surface near the container walls is either pulled upward or downward as shown in the figure.  

This is seen clearly in a narrow tube such a burette or pipette, in which a curved surface is formed at the liquid surface. Such a curved surface formed is known as a meniscus

There are two types of menisci. They are 

  1. convex meniscus 
  2. concave Bieniscus

A convex meniscus is formed due to more cohesive force between the molecules of liquid than the adhesive force between the molecules of liquid and the vessel (or container) taken.

Meniscus for surface tension example 

In a test tube containing mercury, the top surface of mercury takes convex shape. This is due to the presence of strong cohesive forces between the molecules of mercury at the exposed surface, as compared to less adhesive force of molecules of mercury with glass. Hence, mercury forms a convex meniscus.

If water is taken in a test tube, the molecules of water have a strong adhesive force with glass surface and a weak cohesive force. Hence, water forms a concave meniscus on account of high adhesive forces.

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