Surface Tension and Capillary Action

Due to molecular attraction, liquids possess certain properties such as cohesion and adhesion. Molecules in liquids experience strong intermolecular attractive forces. When those forces act between like molecules, they are referred to as cohesive forces. When the attractive forces act between unlike molecules, they are said to be adhesive forces.

Surface tension occurs due to unbalanced cohesive forces at the interface of liquid and a gas or at the interface of two liquids.

Molecule A which is below the free surface of liquid is surrounded by various corresponding molecules and consequently under the influence of balanced cohesive forces on all sides and hence in equilibrium while molecule B experience a net attractive force toward the bulk of the liquid due to unbalanced cohesive force. This force on the molecules at the liquid surface is normal to the liquid surface. Due to the attraction of liquid molecules below the surface, a film or a membrane is formed at the surface which can resist small tensile load. For example: small insects can walk on water without getting wet

This property of the liquid surface film to exert a tension is called surface tension. It is denoted by s.
It is defined as the force along the line per unit length. Its SI unit is N/m.

Note:

• Surface tension is inversely proportional to the temperature. As the temperature rises, cohesive forces decreases. 
• It  is directly dependent on intermolecular cohesive forces.
• Surface tension for air water interface at 20°C is 0.0736 N/m and for air mercury is 0.48 N/m.
• At critical point, liquid vapor state are same thus surface tension is zero.

Pressure inside a liquid drop

 

 For equilibrium,

Pressure inside a soap bubble

A bubble has two surfaces in contact with air, one inside and the other outside, each one of which contributes the same amount of tensile force due to surface tension. Therefore,

Pressure inside a jet

Capillary Action

When adhesive forces are greater than cohesive forces, the contact angle θ lies between 0 and 90 degree. Such liquids are called wetting liquids for example : water. When adhesive forces are smaller than cohesive forces, the contact angle θ lies between 90 degree and 180 degree. Such liquids are called non - wetting liquids for example : mercury.

This rise or fall of a liquid when a small diameter tube is immersed in it is known as capillary action.

Weight of risen fluid in tube = specific weight x volume of risen fluid

where, ρ = density of liquid 

                     g = acceleration of gravity 

           D = diameter of tube 

   h= capillary rise

For equilibrium, 

Vertical component of surface tension force = weight of risen fluid

h = rise or fall in capillary                    

Note:

• For Capillary action, diameter of tube should be less than 3 cm.
• Lighter liquid experience greater capillary rise or fall.