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The surface energy of a liquid may be measured by stretching a liquid membrane (which increases the surface area and hence the surface energy). In that case, in order to increase the surface area of a mass of liquid by an amount, δA, a quantity of work, γ δA, is needed (where γ is the surface energy density of the liquid).
Gibbs emphasized that for solids, the surface free energy may be completely different from surface stress (what he called surface tension): [13]: 315 the surface free energy is the work required to form the surface, while surface stress is the work required to stretch the surface. In the case of a two-fluid interface, there is no distinction ...
(σ: surface tension, ΔP max: maximum pressure drop, R cap: radius of capillary) Later, after the maximum pressure, the pressure of the bubble decreases and the radius of the bubble increases until the bubble is detached from the end of a capillary and a new cycle begins. This is not relevant to determine the surface tension. [3]
Cloth, treated to be hydrophobic, shows a high contact angle. The theoretical description of contact angle arises from the consideration of a thermodynamic equilibrium between the three phases: the liquid phase (L), the solid phase (S), and the gas or vapor phase (G) (which could be a mixture of ambient atmosphere and an equilibrium concentration of the liquid vapor).
This may be written in the following form, known as the Ostwald–Freundlich equation: =, where is the actual vapour pressure, is the saturated vapour pressure when the surface is flat, is the liquid/vapor surface tension, is the molar volume of the liquid, is the universal gas constant, is the radius of the droplet, and is temperature.
Here () denotes the surface tension (or (excess) surface free energy) of a liquid drop with radius , whereas denotes its value in the planar limit. In both definitions (1) and (2) the Tolman length is defined as a coefficient in an expansion in 1 / R {\displaystyle 1/R} and therefore does not depend on R {\displaystyle R} .
This method is especially used to compare and measure the critical surface tension of low-energy solids (mainly plastics) very quickly and easily. Figure 4 in ZIsman's published article from 1964 [1] shows the critical surface tension as a measure of wettability of Polyethylene. Zisman published this analysis in 1964 and used a variety of ...
The surface energy is measured in units of joules per square meter, which is equivalent in the case of liquids to surface tension, measured in newtons per meter.The overall surface tension/energy of a liquid can be acquired through various methods using a tensiometer or using the pendant drop method and maximum bubble pressure method.