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,〔(Free surface ). ''McGraw-Hill Dictionary of Scientific and Technical Terms''. McGraw-Hill Companies, Inc., 2003. Answers.com. Retrieved on 2007-12-02.〕
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.
A liquid in a gravitational field will form a free surface if unconfined from above.
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.〔 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).〔
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.
If the free surface of a liquid is disturbed, waves are produced on the surface. These waves are not elastic waves due to any elastic force; they are gravity waves caused by the force of gravity tending to bring the surface of the disturbed liquid back to its horizontal level. Momentum causes the wave to overshoot, thus oscillating and spreading the disturbance to the neighboring portions of the surface.〔 The velocity of the surface waves varies as the square root of the wavelength if the liquid is deep; therefore long waves on the sea go faster than short ones.〔 Very minute waves or ripples are not due to gravity but to capillary action, and have properties different from those of the longer ocean surface waves,〔
because the surface is increased in area by the ripples and the capillary forces are in this case large compared with the gravitational forces.
Capillary ripples are damped both by sub-surface viscosity and by surface rheology.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』