Surface tension
Definition
The invisible 'skin' on the surface of a liquid that lets water striders walk on ponds and water bead up on a waxed car.
Symbol: · Unit: newtons per meter (N/m)
Technical definition
The tendency of a liquid surface to minimize its area, arising from stronger cohesive forces between surface molecules than between surface and air molecules. Measured as force per unit length (N/m) or energy per unit area (J/m²).
Example
A steel paperclip — denser than water — can float if placed gently on water, because surface tension supports it. Soaps and detergents work by lowering water's surface tension so it can spread into fabrics.
Surface tension is not a separate membrane sitting on the water; it is a property of the water molecules themselves pulling sideways on each other. It is also not buoyancy — a floating paperclip is held by surface tension, while a floating boat is held by buoyancy.