Surface Tension

Cause of Surface Tension

A molecule is pulled equally in all directions because of cohesive forces from neighbouring molecules of liquid, which results in a zero net force. Since molecules close to the surface are not surrounded by similar molecules on all sides, they are pushed inwards. This creates internal pressure which further results in liquid surfaces being compressed to the smallest possible area. 

Also, due to the cohesive structure of water molecules, there is a tension which is parallel to the surface that will oppose an external force.

Cohesive and Adhesive Forces

Cohesive Force

The force which exists between the molecules of certain liquids is considered as a cohesive force. Raindrops are held together by the cohesive force before they fall to earth. Surface tension is a phenomenon which is known to us but not many of us know that surface tension is also due to the cohesive force. Surface tension allows bodies which are denser than liquids to float on them without any support and prevent them from sinking.

Adhesive Force

Adhesive force is the force of attraction between two different bodies like a solid container and a liquid. Adhesive force allows water to stick to glass. 

If the phenomenon of adhesion is larger than that of cohesion, then the liquids will wet the surface of the solid with which it is in contact and we may also notice that the liquid curves towards the edge of the container. Liquids such as mercury have more cohesive force as compared to adhesive force and can therefore be defined as non-wetting liquids. Non – wetting liquids curve inward when they are near the rim of the container.

Viscosity

Viscosity is defined as the measure of resistance of flow of any fluid.

A fluid with large viscosity resists movement or motion because the intermolecular forces of fluids are strong which provides a lot of internal friction that resists the motion of the layers past one another.

On the other hand, fluid which has low viscosity can flow easily because its molecular composition produces very small friction when it moves. Gases also have viscosity, but it is difficult to detect under normal circumstances.

Viscosity of liquid decreases rapidly when temperature of liquid increases. Whereas, the viscosity of gases increases when temperature of the gas increases. Therefore, on heating, liquids flow more easily whereas the flow of gases becomes slow as compared to liquid. When the amount of matter changes then no changes occur in viscosity, thus viscosity is an intensive property.

Viscosity is the ratio of shear stress to the velocity gradient in fluid. When a sphere is dropped into a fluid, then the viscosity is determined by using the formula which is given below.

η=2g×a²×∆ρ/9v

Here, 

= viscosity

g = gravitational acceleration

a = radius of sphere

∆ρ = density difference

v = velocity

Principle of viscosity

When a liquid layer is moved on a surface or other layer of the same liquid, the fluid particles tend to resist the movement and this force of resistance which is created by a liquid is known as viscosity.

Conclusion

The forces of attraction between the particles in the layer of surface and the particles in the bulk of the liquid, tending to minimize their surface area, cause surface tension.

Surface tension is given as

T=F/L  

And also

T=m×a/L 

Unit of surface tension is joules per square meter ( J/m²).

Dimensional formula of surface tension is