Surface Tension | Definition, Formula, Examples, Capillary Effect

What is surface tension?

Surface tension (or ST) is a property of fluid which is responsible for the formation of enclosed surfaces. For example, Mercury droplets are near spherical (and can also be rolled very easily) and water drops hanging from a tree branch are also near spherical.

It is the property which is responsible for the near spherical shape of droplets.

How surface tension works?

 

Surface tension is created by the unbalanced forces (both cohesive and adhesive) on the surface of fluid.

Liquid molecules at the surface are only subjected to cohesive forces from behind (unlike liquid molecules at center) which results into generation of a spherical surface.

Surface tension formula

The pulling force (which creates round shape) per unit length is called surface tension (ST) or coefficient of surface tension.

ST (or coefficient of ST) = force/Length

The SI units of surface tension is N/m

Dimensional formula of surface tension is MT^ (-2)

Examples of surface tension

Below are some examples which will help you in understanding the topic.

• Round shape of liquid droplets
• Adding detergent in water to decrease its ST so that it can penetrate clothes easily
• Heating food before eating decreases its ST and hence we get more taste out of it (and same is the reason for washing clothes with hot water)
• Walking of some insects on water
• Floating of small needle on water
• Tent material
• Clinical test of jaundice
• Disinfectants for bacteria
• Making shape of bubbles round etc.
• Capillary effect

Capillary effect

 

Capillary effect is the rise or fall of the liquid in the capillary, it is the result of surface tension. Whether the liquid will rise or fall in capillary is dependent on the magnitude of cohesive and adhesive forces acting.

Capillary effect is also partially responsible for the rise of water to the top of tall trees.

Also read:

Different types of fluid flows

Image source:

Surface tension: By User:Booyabazooka – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5313203

Capillary rise and fall: By Eduard Konečný – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1684735