In physics, a free surface is the surface of a fluid that is subject to constant perpendicular Stress (physics)|normal stress and zero parallel shear stress,
such as the boundary between two homogenous fluids,
for example liquid water and the air in the Earth's atmosphere. Unlike liquids, gases cannot form a free surface on their own.
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.