Methods of Measuring Surface Tension

The most useful method of measuring surface tension is by the well-known Wilhelmy plate method. If a plate-shaped metal is dipped in a liquid, the surface tension forces will be found to produce a tangential force (Figure 2.13). This is because a new contact phase is created between the plate and the liquid. 

Relation between surface tension and the pressure differences across a curved liquid surface. We must now return to a most important consequence of the existence of free surface energy, which was known to Young and Laplace, and is the foundation of the classical theory of Capillarity, and of most of the methods of measuring surface tension. If a liquid surface be curved the pressure is greater on the concave side than on the convex, by an amount which depends on the surface tension and on the curvature. This is because the displacement of a curved surface, parallel to itself, results in an increase in area as the surface moves towards the convex side, and work has to be done to increase the area. This work is supplied by the pressure difference moving the surface. 

There are many other methods of measuring surface tension as no new... 

Pendant drop method of measuring surface tension Drop of liquid hangs from pipette the z axis is vertical, the x axis is horizontal, and R is the (maximum) radius of the drop. To obtain / , many values of the radii S S— 0 to R) are measured at heights z (z = 0 to R) above the apex of the drop 4> is the tangent angle at the radius S. 

Some methods of measuring surface tension which have not been so often used as those described above will now be considered. Some of these would probably be convenient if suitably modified, and some have been used recently. 

A.W. Adamson, A.P. Cast, Physical Chemistry of Surfaces, e.g. sixth ed. (1997). (Their second chapter, entitled Capillarity, reviews several methods of measuring surface tensions, mostly in less detail than here.)... 

Drop-weight method of measuring surface tension. 

PHENOMENA OF ORIENTATION IN NEMATIC LIQUIDS. A NEW METHOD OF MEASURING SURFACE TENSION APPLICABLE TO LIQUIDS. 

Whether we assume, however, the existence of a fixed angle of contact between a given liquid and a given solid, or derive it from a static molecular model, as above, we can proceed at once to explain all the common capillary phenomena. Tlie three ideas of tension at a surface, internal pressure, and angle of contact suffice with these concepts and the key expression (1.14) for the internal pressure inside a curved surface we can solve all common equilibrium capillary problems by the methods of dassical statics. This is not a field we wish to explore in detail but show here only how the rise in a capillary tube is treated, since this forms the basis of the commonest method of measuring surface tension. 

We recall the lUPAC definition The mechanic properties of the interfacial layer between two fluids, including the equilibrium shape of the surface, may be calculated by applying the standard mathematical techniques of mechanics to the forces associated with the surface of tension. The resulting equations— which comprise the subject of capillarity— form the basis of experimental methods of measuring surface tension. lUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units, App. II, Part I,... 

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