Surface Tension – Vital property of the Fluid

In science, we come across various topics that deal with the surface properties of substances. Similarly, in fluids, surface chemistry is the branch that explains the phenomenon occurring on the surfaces of substances. In that, surface tension is the vital property of the fluids. Surface tension is described as the tendency of fluid surfaces to diminish into the reduced surface area possible by the fluid. In this article, we will focus on the surface tension of a fluid substance. 

What is Surface Tension?

In general, we can say that surface tension is the phenomenon that occurs when the surface of a liquid is in contact with another phase. It can be defined as the tension on the surface of a liquid due to the attraction of the particles present in the surface layer of the liquid, which helps to minimise surface area. In simple words, one can say that it is the attractive force that is present mostly in liquids, and this force pulls molecules into the surface together; as a result, it reduces the surface area of the fluid. 

Causes of Surface Tension

We know that in liquids, the molecules are pulled into each and every side. The intermolecular forces, such as Van der Waals force, pull the liquid particles together. Throughout the surface, the particles are attracted to the rest of the liquid. The presence of cohesive forces between liquid molecules also plays a major role in creating surface tension of the liquid. Similarly, adhesive force is also a vital participant in this phenomenon. 

Formula and SI Unit

The formula of surface tension is given by the ratio of the surface force F to the length L along which the force acts.

T=F/L

Where,

T is the surface tension of the liquid

F is the force per unit length

L is the length in which force act

The dimensional formula of surface tension is: [ML0T-2].

• The SI unit of surface tension is a Newton per meter or N/m.
• The CGS unit of surface tension is dyne per centimetre.
• It can also be measured as surface energy using J/m2 or ergs per square centimetre.

How to Measure?

In laboratories, various instruments and methods are utilised in the measurement of surface tension. This surface phenomenon can be measured using various techniques that are listed below:

• Spinning drop method
• Pendant drop method
• Stalagmometric method
• Capillary rise method
• Bubble pressure method

The capillary rise method is the most commonly used technique. In this process,  capillary action and meniscus are the effects of adhesion. This occurs between two different molecules or substances.

The Surface Tension of a Few Liquids

Liquids have a standard value of measurement, which is determined by conducting experiments, and their values are estimated. Listed below are a few liquids with their surface tension value:

(i) Hydrogen: 2.4 N/m

(ii) Helium: 0.16  N/m

(iii) Water: 0.072  N/m

(iv) Ethanol: 22  N/m

(v) Sodium chloride: 114  N/m

Examples

In our day to day life activities, we come across various examples of surface tension that are found in our surroundings. A few examples are listed below:

• Floating of a needle on the surface of the water: The density of the needle is greater than water.
• Rainproof tent materials used in adventure camps: The surface tension of water will bridge the pores in the tent material.
• Surface tension disinfectants: Disinfectants are solutions of moderate or low surface tension that we use daily.
• Cleansing action: Washing of clothes with soaps and detergents lowers the surface tension of the water.
• Formation of a bubble: Round bubbles form when the surface tension of water establishes the wall tension for the formation of water bubbles.

The other properties of fluid such as density, viscosity, temperature, specific gravity, pressure also contribute a major role in estimating the fluid. Fluid is a substance that can flow. Hence these all properties help in knowing the fluidity of the liquid.