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Surface energy
Intermediate
Work needed to enlarge a surface.
W = TΔAIncreasing the surface area costs energy W = TΔA. Liquids minimise area, which is why drops are spherical.
Making a 2 cm² soap film with two surfaces needs 2TΔA.
Count two surfaces for films and bubbles.
In more detail
Surface energy is the energy per unit area of a liquid surface, equal to surface tension. Liquids minimise their surface area because a smaller surface has lower energy.
Real-world uses
- Raindrops form spheres because a sphere has the smallest surface for its volume.
- Emulsions and foams are unstable because they have large surface areas.
- Soap films and bubbles stretch to large areas only when work is done, because the surface stores energy in proportion to its area.
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Frequently asked questions
Why are drops spherical?
Because the sphere minimises surface area for a given volume, which lowers surface energy.
Is surface energy the same as surface tension?
Numerically yes, in joules per square metre or newtons per metre.
Is surface energy the same as surface tension?
They are numerically the same for a liquid surface, since surface tension is the energy per unit area.
Related topics in Fluids & Elasticity
Terminal velocity in fluids · Reynolds number · Surface tension · Excess pressure in drops and bubbles · Angle of contact · Capillary rise