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Surface tension
Intermediate
Surface acts like a stretched membrane.
T = F/LMolecules at a liquid surface are pulled inward, so the surface behaves like a stretched membrane with tension T = F/L. It allows insects to walk on water.
Water: 0.072 N/m.
Soap lowers surface tension.
In more detail
Surface tension is the force per unit length at a liquid surface that makes the surface behave like a stretched elastic film. It arises from the attraction between molecules at the surface.
Real-world uses
- Insects walk on water because of surface tension.
- Soap bubbles form because surface tension acts on thin films.
- Water droplets form spheres on a waxed surface, and insects can walk on water because surface tension supports them.
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Frequently asked questions
Why does a needle float?
Surface tension supports the needle's weight, as long as it does not break the surface.
What are the units?
Newtons per metre.
Why does surface tension make droplets round?
The molecules at the surface are pulled inward, which minimises the surface area for a given volume.
Related topics in Fluids & Elasticity
Stokes' law · Terminal velocity in fluids · Reynolds number · Surface energy · Excess pressure in drops and bubbles · Angle of contact