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Free surface

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Title: Free surface  
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Subject: Stokes drift, Stokes wave, Surface rheology, Iribarren number, Flatness (liquids)
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Free surface

Free surface of the sea, viewed from below

In physics, a free surface is the surface of a fluid that is subject to both zero perpendicular normal stress and parallel shear stress,[1] such as the boundary between two homogeneous fluids,[2] for example liquid water and the air in the Earth's atmosphere. Unlike liquids, gases cannot form a free surface on their own.[3]

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]


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.[4] 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.[4] 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,[4] because the surface is increased in area by the ripples and the capillary forces are in this case large compared with the gravitational forces.[7] Capillary ripples are damped both by sub-surface viscosity and by surface rheology


Free surface of a liquid in a rotating vessel is a paraboloid

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.[4] Since the main mirror in a telescope must be parabolic, this principle is used to create liquid mirror telescopes.

Consider a cylindrical container filled with liquid rotating in the z direction in cylindrical coordinates, the equations of motion are:

\frac{\partial P}{\partial r} = \rho r \omega^2, \; \; \; \frac{\partial P}{\partial \theta} = 0, \; \; \; \frac{\partial P}{\partial z} = -\rho g

Where \rho is the density of the fluid, r is the radius of the cylinder, \omega is the angular frequency and g is the gravitational acceleration. Taking a surface of constant pressure (dP = 0 ) the total differential becomes:

dP = \rho r \omega^2 dr - \rho g dz \rightarrow \frac{ dz_{isobar}} {dr} = \frac{ r \omega^2}{g}

Integrating, the equation for the free surface becomes:

z_s = \frac{\omega^2}{2g} r^2 + h_c

Where h_c is the distance of the free surface from the bottom of the container along the axis of rotation. If one integrates the volume of the paraboloid formed by the free surface and then solves for the original height, one can find the height of the fluid along the centerline of the cylindrical container:

h_c = h_0 - \frac{\omega^2 R^2}{4g}

The equation of the free surface at any distance r from the center becomes:

z_s = h_0 - \frac{\omega^2}{4g} ( R^2 - 2 r^2)

If a free liquid is rotating about an axis, the free surface will take the shape of an oblate spheroid: the approximate shape of the Earth due to its equatorial bulge.[8]

Related terms

\ \frac{Dp}{Dt} = 0.
  • 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.[13]
  • 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.[3]

See also


  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. Retrieved on 2007-12-02.
  3. ^ a b c White, Frank (2003). Fluid mechanics. New York: McGraw-Hill. p. 4.  
  4. ^ a b c d e f  
  5. ^  
  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. 
  7. ^ Gilman, Daniel Coit; Peck, Harry Thurston; Colby, Frank Moore, ed. (1903). "Hydrostatics". The New International Encyclopædia. Dodd, Mead and Company. p. 739. 
  8. ^ "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. 
  9. ^ "Free surface". Glossary of Meteorology.  
  10. ^ Brighton, John A.; Hughes, William T. (1999). Schaum's outline of theory and problems of fluid dynamics. Boston, Mass: McGraw Hill. p. 51.  
  11. ^ "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; 
  12. ^ "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. 
  13. ^ 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.  
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