Lecture - 5 ( properties of fluids (part - 4) surface tension and capilarity)

Surface tension (σ) 

Let us consider a molecule 'A' which is under the surface of water. This molecule is completely surrounded by Cohesive forces of another molecules of water. Therefore this molecule is under the action of Balanced cohesive forces and hence it is in equilibrium. 

Let us consider another molecule 'B' which is on the surface of the water, hence it is not completely surrounded by Cohesive forces. Therefore this molecule is under the action of unbalanced Cohesive forces and hence this molecule experiences a pull or stretch. 

Due to above reason the molecules on surface experiences a pull and it appears that there is some kind of membrane over the surface of fluid. This membrane is having the ability to bear small loads and this phenomenon is known as surface tension. The main reason of surface tension is unbalanced Cohesive forces. With increase in temperature surface tension decreases due to breaking of Cohesive forces. One more thing to observe here is that in above diagram molecule B or we can say the molecules on the surface of liquid are packed due to unbalanced intermolecular forces than the molecules at the centre like molecule A. This means there is high energy density at the surface of liquid.

Surface tension can also be defined as a line force with surface tension forces acting perpendicular to the line in the plane of surface. Mathematically surface tension is defined as force per unit length and its SI unit is Newton per metre (N/m). Detergents are used while washing clothes because they reduce surface tension so that dirt particles can come out. 

σ = Fs / L 

'σ' represents surface tension

'Fs ' represents surface tension force

'L' represents length over which surface tension is acting (perimeter) 

Water can bear maximum upto 7.5 grams of weight for unit metre of length about it. Surface tension will break above 7.5 grams.
surface tension decrease with increase in temperature because the main reason for existence of surface tension is Cohesive forces. As we increase the temperature of water, the molecular forces of attraction between the molecules will start to break due to which it decreases. And with decrease in molecular forces of attraction Cohesive forces decrease which weakens the surface tension.
For more detailed explanation of surface tension check out Lecture-8 .

Capilarity 

Capillarity is defined as the Rise or fall of a liquid when a small diameter glass tube is inserted in it. 

• Capillary rise is due to adhesion. Example is water in glass tube.
• Capillary fall is due to Cohesion. Example is mercury in glass tube.

Type of fluids are -

1. Wetting fluids
2. Non wetting fluids

Wetting fluids 

In wetting fluids adhesion is more. Example is water drop on glass as shown in figure below -

Here θ represents angle of contact

In wetting fluids θ is less than 90 degrees 

As we put glass tube inside water capillary rise will there as shown in below figure - 

Non - wetting fluids 

In non wetting fluids Cohesion is more. Example is mercury on glass as shown in below figure -

In non wetting fluids θ is more than 90 degrees. As we put glass tube inside Mercury capillary fall will there as shown in below figure - 

For understanding calculations in capillarity rise and fall read Lecture - 9 .