Surface Tension – The Invisible Skin of Water

Published on: April 22, 2025in

Have you ever noticed how water forms droplets or how insects can walk on water? This magic happens because of surface tension – the "invisible skin" that forms on liquids. In this blog, we’ll explore what surface…

Have you ever noticed how water forms droplets or how insects can walk on water? This magic happens because of surface tension – the “invisible skin” that forms on liquids. Additionally, we’ll explore what surface tension is, why it happens, and where we see it in real life.

What is Surface Tension?

Additionally, surface tension is the force that makes liquid surfaces act like a stretched elastic sheet. It happens because:

Surface Tension

Molecules attract each other due to cohesive forces. As a result, surface molecules experience unbalanced forces, which pull them inward. Because of this, liquids naturally try to minimize their surface area—this is why droplets form round shapes.

Molecules attract each other (cohesive forces).
Surface molecules have unbalanced forces, pulling inward.
Liquids try to minimize surface area means that drops are round.

Formula:
Surface Tension (γ) = Force per unit length (N/m)

Example:
Water has γ = 0.072 N/m at 20°C.

A great discovery solves a great problem, but there is a grain of discovery in the solution of any problem. Your problem may be modest, but if it challenges your curiosity and brings into play your inventive faculties, and if you solve it by your own means, you may experience the tension and enjoy the triumph of discovery.

— George Pólya

Real-Life Examples

Water Striders: These bugs are super light. Water has a “skin” on the surface because the water molecules like to stick together. Water striders don’t weigh enough to break that “skin,” so they can walk on top.
Soap Bubbles: The soapy water also likes to stick together (surface tension). Additionally, it wants to pull itself into the smallest shape possible, and that shape is a sphere (a round ball). That’s why bubbles are always round.
Capillary Action: Imagine tiny tubes. Water can climb up these tubes, even against gravity, because it likes to stick to the tube walls (and to itself). Plant roots use this to suck water up from the soil.

Waterdrop due to surface Tension — **Fig 1. Water Drop Due to Surface Tension**

FAQ’s

Q1. Why do liquids form spherical drops?

A1. Spheres have the smallest surface area, so liquids naturally take this shape to minimize energy.

Q2. How does soap reduce surface tension?

A2. Soap molecules disrupt water’s cohesive forces. Hence, making it easier to spread.

Q3. What is the unit of surface tension?

A3. Newton per meter (N/m).

Reference

Eriksson, M., Claesson, P. M., Järn, M., Wallqvist, V., Tuominen, M., Kappl, M., Teisala, H., Vollmer, D., Schoelkopf, J., Gane, P. A. C., Mäkelä, J. M., & Swerin, A. (2023). Effects of liquid surface tension on gas capillaries and capillary forces at superamphiphobic surfaces. Scientific Reports, 13, 6794. https://doi.org/10.1038/s41598-023-33875-9

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