surface tension

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Related to surface tension: viscosity, capillary action, capillarity

surface tension,

tendency of liquids to reduce their exposed surface to the smallest possible area. A drop of water, for example, tends to assume the shape of a sphere. The phenomenon is attributed to cohesion, the attractive forces acting between the molecules of the liquid (see adhesion and cohesionadhesion and cohesion,
attractive forces between material bodies. A distinction is usually made between an adhesive force, which acts to hold two separate bodies together (or to stick one body to another) and a cohesive force, which acts to hold together the like or unlike
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). The molecules within the liquid are attracted equally from all sides, but those near the surface experience unequal attractions and thus are drawn toward the center of the liquid mass by this net force. The surface then appears to act like an extremely thin membrane, and the small volume of water that makes up a drop assumes the shape of a sphere, held constant when an equilibrium between the internal pressure and that due to surface tension is reached. Because of surface tension, various small insects are able to skate across the surface of a pond, objects of greater density than water can be made to float, and molten lead when dropped into a cool liquid forms suddenly into shot. See capillaritycapillarity
or capillary action,
phenomenon in which the surface of a liquid is observed to be elevated or depressed where it comes into contact with a solid. For example, the surface of water in a clean drinking glass is seen to be slightly higher at the edges, where
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Surface Tension


an important thermodynamic characteristic of the interface of phases or bodies, defined as the energy required for the reverse isothermic formation of a unit of area of a given surface. In the case of a liquid interface, the surface tension may also be rightfully considered as the force acting on a unit of length of the surface contour and tending to contract the surface to a minimum for the given volumes of the phases. Both these definitions apply to mobile surfaces, although the first is preferred since it has a clearer physical sense. The surface tension at the boundary of two condensed phases is usually called the interphase tension.

The energy required to form a new surface is expended on overcoming the forces of intermolecular cohesion in transferring molecules of a substance from the bulk of the body to the surface layer. The resultant of the intermolecular forces in the surface layer is not equal to zero, as in the bulk of the body, and is directed toward the interior of the phase with the greater cohesion. Thus, the surface tension is a measure of the lack of compensation of intermolecular forces in the surface (interphase) layer. In other words, it is a measure of the excess free energy in the surface layer relative to the free energy in the volumes of the contiguous phases. In accordance with these definitions, surface tension is expressed in units of joules/m2 or newtons (N)/m (erg/cm2 or dynes/cm).

As a result of surface tension, in the absence of external forces a liquid adopts a spherical form corresponding to the minimum value of the surface and, consequently, to the smallest value of the free surface energy. Surface tension does not depend on the size and shape of the surface when the volumes of the phases are sufficiently great relative to the dimensions of the molecules. Surface tension decreases with increasing temperature as well as from the action of surfactants. Melts of metals have the greatest surface tension among liquids; for example, the surface tension of platinum at 2000°C is equal to 1,820 dynes/cm, and the surface tension of mercury at 20°C is 484 dynes/cm. The surface tension of molten salts is considerably less, ranging from several dozens of dynes/cm to 200-300 dynes/cm. The surface tension of water at 20°C is 72.8 dynes/cm, and the surface tension of most organic solvents ranges from 20 to 60 dynes/cm. The lowest surface tensions at room temperature, those of several fluoro-carbon liquids, are under 10 dynes/cm.

With multicomponent systems, the change in the surface tension in accordance with the Gibbs thermodynamic equation for adsorotion is

–d0 = Γ11 + Γ22 + ….

Here Γ1, Γ2, … are surface excesses of the components 1, 2, … —that is, the difference between the concentrations of the components in the surface layer and the volume of the solution or gas— and dμ1, dμ2, … are the changes in the chemical potentials of the corresponding components. (The minus sign indicates that the surface tension decreases with positive adsorption.) The surface pressure is determined by the difference in the surface tensions of the pure liquid and the liquid covered by the adsorption monolayer.

At mobile liquid-gas (vapor) or liquid-liquid boundaries, the surface tension may be measured directly by many methods. The usual methods are by the mass of a drop separating from the end of a vertical tube (a stalagmometer), by the maximum pressure required to reduce a gas bubble to a liquid, and by the shape of a drop or bubble lying on a flat surface.

It is difficult to determine by experiment the surface tensions of solids, since the molecules or atoms of solids are incapable of free displacement. An exception is the plastic flow of metals at temperatures close to the melting point. Because of the anisotropy of crystals, surface tensions on the different faces of a crystal vary. The concepts of surface tension and of free surface energy are not identical for solids. Defects in the space lattice, chiefly dislocations, edges, and apexes of crystals, as well as grain boundaries extending to the surface of polycrystalline bodies, contribute to the free surface energy. The surface tension of solids is usually determined indirectly from intermolecular and interatomic interactions. Many surface phenomena are determined by the magnitude and changes of surface tension, especially in disperse systems.


In living organisms, the surface tension of the cell is one of the factors determining the shape of the entire cell and its components. It is low for cells with rigid or semirigid surfaces: such cells include those of Infusoria and many microorganisms and plant cells. For cells without a strong supramembranous structure (most animal cells, some protozoa, and the spheroplasts of bacteria), the surface tension largely determines the cell’s configuration; cells suspended within a liquid become almost spherical in shape. The shape of a cell attached to a substrate or to other cells depends primarily on such factors as contact structures and the cytoskeleton formed by the microtubules. It is assumed that local changes in surface tension are of importance in such phenomena as phagocytosis, pinocytosis, and gastrula-tion. The determination of the surface tension of the cell is a complex experimental problem; usually the surface tension of a cell does not exceed several dynes/cm (10–3 N/m).



Adam, N. K. Fizika i khimiia poverkhnostei. Moscow-Leningrad, 1947.
(Translated from English.) Surface and Colloid Science, vol. 1. Edited by E. Matijevic. New York, 1969.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

surface tension

[′sər·fəs ‚ten·chən]
(fluid mechanics)
The force acting on the surface of a liquid, tending to minimize the area of the surface; quantitatively, the force that appears to act across a line of unit length on the surface. Also known as interfacial force; interfacial tension; surface tensity.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

surface tension

1. a property of liquids caused by intermolecular forces near the surface leading to the apparent presence of a surface film and to capillarity, etc.
2. a measure of this property expressed as the force acting normal to one side of a line of unit length on the surface: measured in newtons per metre.
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
For example, the surface tension of the liquid coating material is greatly reduced, without negative effects on the surface energy of the cured coating.
Thus, only the individual values used were effective in lowering the surface tension. After these graphical and numerical analysis, a statistical prediction model was established for the operating range covered by the limits used in the CCRD.
Effect of NaAA Concentration on Surface Tension. The surface tension of polymer solutions first declined with the increase of NaAA dose (Figure 7).
The linearized condition at the free surface with the effect of surface tension can be written as ([24]) mentioned below
This discontinuous surface tension affects the morphology of the crystal [5-10].
Because of the gradient of surface tension, the flow field is strengthened in the direction of temperature gradient and the temperature change near the bottom of the interface is affected.
6 shows the rate of the growth and collapse phases of the vapor bubble in two cases: with surface tension and without surface tension.
Thermal stability of proteins in aqueous polyol solutions: Role of the surface tension of water in the stabilizing effect of polyols.
Surface tension reduction = Recorded surface tension in the media (before inoculation) - Recorded surface tension after inoculation (culture supernatant).
To determine the biosurfactantproducing ability of the studied isolate, the tests of measuring surface tension, emulsification index, drop collapse test, hemolytic activity and expanding oil test were conducted.
Thus, as far as spreading is concerned, the bottom layer liquids may have any value of surface tension, as long as the top layer has the lowest surface tension.
Water droplets are held together by surface tension, a clinging force.

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