free surface

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