noun
free surface
Technical term in chemistry and physics for a surface of a fluid that is not in contact with a solid boundary, such as the top surface of a liquid in an open container.
液体の自由表面では、表面張力が働く。
Surface tension acts on the free surface of a liquid.
Compound of 自由 (free) and 表面 (surface), a calque of the English term 'free surface'.