Experiments to demonstrate surface tension

Surface tension of a liquid

Surface tension is the property of the free surface of a liquid at rest to behave like a stretched membrane in order to acquire minimum surface area.

Imagine a line AB in the free surface of a liquid at rest (Fig.). The force of surface tension is measured as the force acting per unit length on either side of this imaginary line AB. The force is perpendicular to the line and tangential to the liquid surface. If F is the force acting on the length l of the line AB, then surface tension is given by T=F/l.

Surface tension is defined as the force per unit length acting perpendicular on an imaginary line drawn on the liquid surface, tending to pull the surface apart along the line. Its unit is N m-1 and dimensional

formula is MT^-2.

Experiments to demonstrate surface tension

(i)                   When a painting brush is dipped into water, its hair gets separated from each other. When the brush is taken out of water, it is observed that its hair will cling together. This is because the free surface of water films tries to contract due to surface tension.

(ii)                 When a sewing needle is gently placed on water surface, it floats. The water surface below the needle gets depressed slightly. The force of surface tension acts tangentially. The vertical component of the force of surface tension balances the weight of the needle.