When large number of small droplets coalesces to from a single drop, it warms up. Explain why.

Consider a soap bubble of radius R and surface tension T. There are two free surfaces of soap bubble. Due to surface tension the molecules on the surface film experience the net force in inward direction normal to the surface. Therefore there is more pressure inside than outside. Let p_i be pressure inside the liquid drop and p_o the pressure outside the drop. Therefore excess of pressure inside the liquid drop is,

p =p₁–P_o

Due to excess of pressure inside the liquid drop the free surface of the drop will experience the net force in outward direction due to which the drop expands. Let the free surface be displaced by dR under isothermal conditions.

Therefore excess of pressure does the work in displacing the surface and that work will be stored in the form of potential energy.

The work done by excess of pressure in displacing the surface is,

dW= Force x displacement 
     = (excess of pressure x Surface area) x displacement of surfce

    
Increase in the potential energy
dU = surface tension x increase in area of the free surface



From (1) and (2)

   

When we dip the capillary tube in liquid, at the contact two types of forces are acting on liquid molecule as shown:

(i) Adhesive force by molecules of tube perpendicular to the surface of tube.

(ii) Cohesive force by liquid molecules at angle 45°

  
The resultant force or liquid molecule is outside the tube and meniscus is concave, and 

   

then meniscus is convex.

Applications of Surface tension in daily life are:

(i) The surface tension of antiseptic ointments is low which make them to spread over the wound.

(ii) Adding detergent in water decreases the surface tension of water and enhances the cleaning action of water and detergent both.