In physics, a free surface is the surface of a fluid that is subject to constant perpendicular 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.
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
If a liquid is contained in a cylindrical vessel and is rotating around a vertical axis coinciding with the axis of the cylinder, the free surface will assume a parabolic surface of revolution known as a paraboloid. The free surface at each point is at a right angle to the force acting at it, which is the resultant of the force of gravity and the centrifugal force from the motion of each point in a circle. Since the main mirror in a telescope must be parabolic, this principle is used to create liquid mirror telescopes.
Related terms 
- In fluid dynamics, a free-surface vortex, also known as a potential vortex or whirlpool, forms in an irrotational flow, for example when a bathtub is drained.
- In naval architecture and marine safety, the free surface effect occurs when liquids or granular materials under a free surface in partially filled tanks or holds shift when the vessel heels.
- In hydraulic engineering a free-surface jet is one where the entrainment of the fluid outside the jet is minimal, as opposed to submerged jet where the entrainment effect is significant. A liquid jet in air approximates a free surface jet.
- In fluid mechanics a free surface flow, also called open channel flow, is the gravity driven flow of a fluid under a free surface, typically water flowing under air in the atmosphere.
See also 
- Free surface effect
- Surface tension
- Laser-heated pedestal growth
- Splash (fluid mechanics)
- Slosh dynamics
- Riabouchinsky solid
- "Glossary: Free Surface". Interactive Guide. Vishay Measurements Group. Retrieved 2007-12-02. "Surface of a body with no normal stress perpendicular or shear stresses parallel to it…"
- Free surface. McGraw-Hill Dictionary of Scientific and Technical Terms. McGraw-Hill Companies, Inc., 2003. Answers.com. Retrieved on 2007-12-02.
- White, Frank (2003). Fluid mechanics. New York: McGraw-Hill. p. 4. ISBN 0-07-240217-2.
- Rowland, Henry Augustus; Joseph Sweetman Ames (1900). "Free Surface of Liquids". Elements of Physics. American Book Co. pp. 70–71.
- Millikan, Robert Andrews; Gale, Henry Gordon (1906). "161. Shape assumed by a free liquid". A First Course in Physics. Ginn & company. p. 114. "Since, then, every molecule of a liquid is pulling on every other molecule, any body of liquid which is free to take its natural shape that is which is acted on only by its own cohesive forces, must draw itself together until it has the smallest possible surface compatible with its volume; for, since every molecule in the surface is drawn toward the interior by the attraction of the molecules within, it is clear that molecules must continually move toward the center of the mass until the whole has reached the most compact form possible. Now the geometrical figure which has the smallest area for a given volume is a sphere. We conclude, therefore, that if we could relieve a body of liquid from the action of gravity and other outside forces, it would at once take the form of a perfect sphere."
- Dull, Charles Elwood (1922). "92. Shape Assumed by a Free Liquid". Essentials of Modern Physics. New York: H. Holt. "Since the molecules of liquids slide over one another readily, the force of gravity causes the surface of liquids to become level. If the force of gravity can be nullified, a small portion of free liquid will then assume a spherical form."
- Gilman, Daniel Coit; Peck, Harry Thurston; Colby, Frank Moore, ed. (1903). "Hydrostatics". The New International Encyclopædia. Dodd, Mead and Company. p. 739.
- "Hydrostatics". Appletons' Cyclopædia of Applied Mechanics. New York: D. Appleton and company. 1880. p. 123. "If a perfectly homogeneous mass of liquid be acted upon by a force which varies directly as the distance from the centre of the mass, the free surface will be of spherical form; if the mass rotates about an axis, the form assumed will be that of an oblate spheroid, which is the shape of the earth."
- "Free surface". Glossary of Meteorology. American Meteorological Society. Retrieved 2007-11-27.
- Brighton, John A.; Hughes, William T. (1999). Schaum's outline of theory and problems of fluid dynamics. Boston, Mass: McGraw Hill. p. 51. ISBN 0-07-031118-8. "A simple example of irrotational flow is a whirlpool, which is known as a potential vortex in fluid mechanics."
- "Ricerca Italiana - PRIN - Global stability of three-dimensional flows". Retrieved 2007-12-02. "The free-surface vortex (whirlpool) that occurs during the draining of a basin has received different interpretations along its history;"
- "The Free Surface Effect - Stability". Retrieved 2007-12-02. "In a partly filled tank or fish hold, the contents will shift with the movement of the boat. This "free surface" effect increases the danger of capsizing."
- Suryanarayana, N. V. (2000). "3.2.2 Forced Convection - External Flows". In Kreith, Frank. The CRC Handbook of Thermal Engineering (Mechanical Engineering). Berlin: Springer-Verlag and Heidelberg. pp. 3–44. ISBN 3-540-66349-5. "In free-surface jets — a liquid jet in an atmosphere of air is a good approximation to a free-surface jet — the entrainment effect is usually negligible…"