What is Surface Tension in Liquids?

Class 11 Physics Chapter 10 Mechanical Properties of Fluids

In day to day life surface tension has various example like floating of water striders on water, fluid in test tube raised at the boundaries, floating of oil on water and many more. Phenomena of Surface tension is well explained by science. In this article, let’s speak about surface tension in detail.

Surface tension

Fluids consist of large number of molecules. Molecule inside the fluid are surrounded by same type of molecules from all side. Thus, it experience force of attraction from all sides. The attractive force between same type of molecules is called cohesive force. Molecules of liquid present on surface have molecule of same liquid on one side, whereas, molecule of different substance on other side. Thus, strong force of attraction acts from inside of the fluid than outside. At the surface, due to stronger bonds from sides, there is formation of thin elastic layer of molecules on the surface.Elastic layer do not deform, thus, force of very small magnitude are opposed by the layer. This is because small insects with very small weight can float on water. Thus surface tension is defined as,

“The property of fluid to resist external force applied on it with the help of cohesive force within the molecules.”

Tendency of fluids to reduce surface area

Due to cohesive forces, surface of fluid tend to shrink its surface area. This is reason fluid always take shape in which it is poured. For the same volume of fluid, the smallest area can be obtained only by taking the shape sphere. This is the reason why raindrop appears to be spherical and painting brush which forms fine tip when removed from fluid. Thus, surface tension can be defined,

“Property of fluid to minimize its free surface.”

Surface Energy

Now let’s talk about the molecules on the surface. Here molecule experience strong force of attraction from inside. Hence the resultant force tends to pull the molecule inside the liquid. As a result molecules tend to get pulled inside the fluid. To fill the vacant place of pulled molecules, molecules from inside should be moved to the surface,however, force of attraction oppose this action. Thus, work is done on the molecule to move it on the surface. This work done is stored as potential energy at the surface. Hence, molecules at the surface has some extra energy as compared to molecules present inside the fluid. Such an extra energy is called surface energy.

Surface tension of liquid is surface energy per unit surface area. S= U/A. Also, surface tension is force per unit length exerted by fluid on the sliding wire i.e. S = F/ 2l. Unit of surface tension is Jm-2.

Angle of Contact

The angle between, tangent to liquid surface at a point of contact and the solid surface is called Angle of contact.

It is denoted by θ. Mathematically it can be expressed as cos θ = (Ssa– Ssl)/ Sla. If Ssa – Ssl less than 0 or Ssa less than Ssl, then cos θ will be negative. However cos θ is negative when θ greater than 90°. Hence, angle of contact for solid liquid combination is obtuse angle. In this case intermolecular forces are stronger than molecular forces between solid and liquid. If Ssa – Ssl greater than 0 i.e. Ssa greater than Ssl then cos θ will be positive. You know that cos θ will be positive only if angle of contact θ less than 90°. It means that angle of contact for given solid liquid combination is acute angle. In such cases the intermolecular forces are weaker than the molecular forces between liquid and solid.

Wetting Agents

If angle of contact θ is an obtuse angle then in such case we say that the liquid could not wet the solid. In our households you might have observed it difficult to wet some clothes. It is because the angle of contact of water of for such clothes is obtuse angle. In such case we add detergents and dying substances to water so as to reduce the angle of contact and help to penetrate water in the cloth. Hence to wet the clothes. Such substances which when mixed in fluid reduces their angle of contact and increases the its wettability are called Wetting Agents.

Water Proofing Agents

These substances works opposite of wetting agents. Water proofing agents are added to create a large angle of contact. E.g. waterproofing paints have water proofing agents added in it which prevents penetration of water in walls.

Detergents and surface tension

Angle of contact between water and oil is obtuse angle. In this case water could not wet oil. Hence, if we try to wash clothes with water then it does not remove the oil dirt from the clothes. Detergent are chemicals which have hairpin shaped molecule, the head of the molecule forms bond with water molecule and tail forms bond with oil. Thus the surface tension of oil layer gets reduced as there is force of attraction from the upper side. After some time it becomes even weaker such that the molecules of detergent forms a globe around the molecules of dirt. This globe can be easily removed by water.
Keywords: Surface energy, Wettability, Intermolecular forces, Mechanical Properties of Fluids

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