Surface Tension

Have you ever noticed that once you fill a glass up to the brim with water, you can still add a few more drops till it spills out? Or have you ever broken a thermometer and observed how the fallen mercury behave?

This is where a property called Surface Tension comes into play. It is the tendency of liquids to acquire the least surface area possible. So what is surface tension? It is a phenomenon that occurs when the surface of a liquid is in contact with another phase (It can be a liquid as well). The surface of the liquid behaves like an elastic sheet.

Water at 1000C has a surface tension value of 0.059 N/m and mercury has a surface tension value of 0.47 N/m. If there is a small leaf or a paper clip placed on the surface of a glass of water, what causes it to float over it? The downward force of the body is balanced by the surface tension.

What is Surface Tension?

It is a property of a liquid surface displayed by its acting as if it were a stretched elastic membrane. This phenomenon can be observed in the nearly spherical shape of small drops of liquids and of soap bubbles. Because of this property, certain insects can stand on the surface of water. A razor blade also can be supported by the surface tension of water. The razor blade also can be supported by the surface tension of water. The razor blade is not floating, if pushed through the surface, it sinks through the water.

Surface tension depends mainly on the forces of attraction between the particles within the given liquid and also upon the gas, solid or liquid in contact with it. The molecules in a drop of water, for example, attract each other weakly. Water molecules well inside the drop may be through of as being attracted equally in all directions by the surrounding molecules.

Formula to find the Surface Tension?

It is expressed by the formula: Surface Tension (T) = F/L.


F = Force per unit length

L = Length in which force act

T = Surface tension of the liquid.

Some Examples of Surface Tension: Let us see some more interesting effects of the surface tension in our daily life:

  • Dew drops: Have you noticed the drops of water on the leaves in your garden in the early morning? The moisture in the atmosphere condenses in the early morning and beads on the leaves due to surface tension. The surface tension gives spherical shape to water drops.
    Dew Drops
  • Formation of drops from Liquid: When the liquid is stretched by applying force or it is poured on some surface it tends to form the droplets due to the surface tension of the fluid.
  • Mercury used in the Thermometer: The mercury used in the thermometer does not stick to the wall of the capillary tube because of the surface tension. Had it stick to the walls the measurement of temperature would not have been correct.
    Mercury used in the Thermometer
  • Separation of Oil and Water: The separation of oil and water is caused due to the difference in surface tension of the two liquids. There is also separation of the fuel and lubricating oil in the engine due to their surface tension. Had they mixed together the fuel would have lost its combustion properties while lubricating oil would have lost its viscosity and lubrication properties.

Applications of Surface Tension:

  • The oil and grease spots on clothes cannot be removed by pure water. On the other hand, when detergents (like soap) are added in water, the surface tension of water decreases. As a result of this, wetting power of soap solution increases. Also the force of adhesion between soap solution and oil or grease on the clothes increases. Thus, oil, grease and dirt particles get mixed with soap solution easily. Hence clothes are washed easily.
  • The antiseptics have very low value of surface tension. The low value of surface tension prevents the formation of drops that may otherwise block the entrance to skin or a wound. Due to low surface tension, the antiseptics spreads properly over wound. Surface tension of all lubricating oils and paints is kept low so that they spread over a large area.
  • A rough sea can be calmed by pouring oil on its surface.

How to Calculate Surface Tension?

Problem: A wooden stick 2m long is floating on the surface of water. The surface tension of water 0.07 N/m. By putting soap solution on one side of the sticks the surface tension is reduced to 0.06 N/m. The net force on the stick will be?

Solution: Force on one side of the stick

 F1 = T1 × L = 0.07 × 2 = 0.14N

Force on other side of the stick F2 = T2 × L = 0.06 × 2 = 0.12N

So, Net force on the stick = F1 – F2 = 0.14 – 0.12 = 0.02N

Therefore, Net force on the stick = 0.02 N.