Free surface

Not to be confused with Open surface.
Disturbed free surface of a sea, viewed from below

In physics, a free surface is the surface of a fluid that is subject to zero parallel shear stress,[1] such as the interface between two homogeneous fluids.[2] An example of two such homogeneous fluids would be a body of water (liquid) and the air in the Earth's atmosphere (gas mixture). Unlike liquids, gases cannot form a free surface on their own.[3] 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.[3] 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.[4] 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).[4]

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.[5][6]

  1. ^ "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…
  2. ^ Free surface. McGraw-Hill Dictionary of Scientific and Technical Terms. McGraw-Hill Companies, Inc., 2003. Answers.com. Retrieved on 2007-12-02.
  3. ^ a b White, Frank (2003). Fluid mechanics. New York: McGraw-Hill. p. 4. ISBN 0-07-240217-2.
  4. ^ a b Cite error: The named reference Rowland was invoked but never defined (see the help page).
  5. ^ 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.
  6. ^ 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.