Surface Tension: Definition, Examples, Formula

Surface tension is the force that causes the surface of fluids to compact to the minimum surface area that is possible. Have you noticed the behaviour of spilled mercury from a thermometer or when you fill a glass of water till the brim and yet there is space for a few more drops before the water spills over? All of these phenomena occur due to the presence of surface tension on the fluid surface. We will look at the concept, definition, and SI unit of surface tension.

Define Surface Tension

The phenomenon occurs when the surface of the liquid comes in contact with another element that can be a solid, liquid, or gas. Liquids are known to cover the least surface area possible making the surface of the liquid behave like an elastic sheet.

It is defined as the phenomenon on the surface of the liquid when in contact with a liquid or gas that acts like a thin elastic sheet. It is also known as interface tension when the surface is in between two liquids. It depends on the forces of attraction not just with the liquid but also the forces of attraction of the solid, liquid, or gas in contact with it.

Surface tension is popularly measured in Newton / m or dyne/cm which is elaborated as the force in Newton or dyne required to break a film of length 1m or 1cm respectively.

Mathematically surface tension is depicted as the ratio of the surface force F to the length L along the direction in which the force acts.

T (Surface Tension) = F / L

Where:
F is the surface force per unit length,

L is the length across which the force acts,

T is the surface tension of the liquid.

Dimensional Formula of Surface Tension

Surface tension is given by the formula,

T = F/L

We know that F = ma, substituting the value in the equation, we get

T =ma/L

Equating the fundamental quantities into the equation, we get

=MLT^-2L^-1

Solving further, we get

=MT^-2

Hence, the dimensional formula of surface tension is MT^-2.

Examples of Surface Tension

Water droplets from a dropper: When a dropper is used to extract water, the water does not flow in a continuous stream, but instead in a series of droplets. The circular droplet shape is defined by the surface tension of water while the gravity pulling it down tends to try and change the shape of the droplet. In the absence of gravity, the drop would tend to minimise the surface tension by reducing the area which would result in a perfect sphere.

Light insects walking on the water surface: A wide variety of lightweight insects can walk on water such as the water strider, ants, etc. The insects are structured in a way that their legs distribute their weight causing the reduction in surface tension of the liquid minimising the potential energy to create a balance of forces such that the film does not break. This is quite similar to wearing snowshoes to walk across deep snow without the sinking of your feet.

Floating of the needle on surface water: The density of the needle is greater than water, but the surface tension along the length is enough to counteract the gravitational force pulling the needle down. You should try the floating needle trick at home sometime.

Hot-water washing of clothes: The lower surface tension of hot water makes it a better wetting agent to act and get into pores and fissures rather than the action of surface tension and bridging them. Detergent cleaning of clothes to lower the surface tension of water even further enabling better action of the soap on the stains.

Rainproof tent materials: Regular tent materials are rainproof up to a certain extent where the water bridges the holes in the tent materials using surface tension. But as soon as you touch the tent material with your finger, the surface tension film will break and the rainwater will drip.

Summary

• Surface tension is defined as adhesive or cohesive forces in between the liquid and another surface/medium and behaves like a thin sheet or film.
• Surface tension finds presence in multiple areas such as insects walking on water, needles floating on water, the spherical shape of soap & water droplets, etc.