In physics, a free surface is the surface of a fluid that is subject to zero parallel shear stress,[1] such as the boundary between two homogeneous fluids,[2] for example liquid water and the air in the Earth's atmosphere. Unlike liquids, gases cannot form a free surface on their own.[3] Fluidized/liquified solids, including slurries, granular materials, and powders may form a free surface.

A liquid in a gravitational field will form a free surface if unconfined from above.[3] Under mechanical equilibrium this free surface must be perpendicular to the forces acting on the liquid; if not there would be a force along the surface, and the liquid would flow in that direction.[4] Thus, on the surface of the Earth, all free surfaces of liquids are horizontal unless disturbed (except near solids dipping into them, where surface tension distorts the surface in a region called the meniscus).[4]

In a free liquid that is not affected by outside forces such as a gravitational field, internal attractive forces only play a role (e.g. Van der Waals forces, hydrogen bonds). Its free surface will assume the shape with the least surface area for its volume: a perfect sphere. Such behaviour can be expressed in terms of surface tension. It can be demonstrated experimentally by observing a large globule of oil placed below the surface of a mixture of water and alcohol having the same density so the oil has neutral buoyancy