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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): [14]: 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 ...
Although surface stress and surface free energy of liquid–gas or liquid–liquid interface are the same, [2] they are very different in solid–gas or solid–solid interface. Both terms represent an energy per unit area, equivalent to a force per unit length , so are sometimes referred to as " surface tension ", which contributes further to ...
Such an inequality of molecular forces induces continuous movement of molecules from the inside to the surface, which means the surface molecules has extra energy, which is called surface free energy or potential energy, [which?] and such an energy acting on reduced unit area is defined as surface tension.
Surface tension describes how difficult it is to extend the area of a surface (by stretching or distorting it). If surface tension is high, there is a large free energy required to increase the surface area, so the surface will tend to contract and hold together like a rubber sheet.
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} .
Flatness refers to the shape of a liquid's free surface. On Earth, the flatness of a liquid is a function of the curvature of the planet, and from trigonometry, can be found to deviate from true flatness by approximately 19.6 nanometers over an area of 1 square meter, a deviation which is dominated by the effects of surface tension.
When a liquid droplet interacts with a solid surface, its behaviour is governed by surface tension and energy. The liquid droplet could spread indefinitely or it could sit on the surface like a spherical cap at which point there exists a contact angle. Defining as the free energy change per unit area caused by a liquid spreading,