In the book of Prigogine, Modern Thermodynamics, it is given on page 147 that

Another example is the natural evolution of the shape of a bubble enclosed in a box of fixed $V$ and $T$. In the absence of gravity (or if the bubble is small enough that the gravitational energy is insignificant compared with other energies of the system), regardless of its initial shape, a bubble finally assumes the shape of a sphere of minimal size. The bubble's size decreases irreversibly until the excess pressure inside the bubble balances the contracting force of the surface. During this process, the Helmholtz energy decreases with decreasing surface area. As the area of the bubble decreases irreversibly, the surface energy is transformed into heat which escapes to the surroundings (thus $T$ is maintained constant)...

I do not understand the reason for the statement "During this process, the Helmholtz energy decreases with decreasing surface area". Why? What sort of energy does the surface store and how is the amount of energy related to the surface area?

  • $\begingroup$ Surface tension, yA, similar to pressure, pV. $\endgroup$ – alarge Mar 21 at 12:01
  • $\begingroup$ Think of it as the attractive (hence negative) energy that isn't stored in the surface, where molecules have only about half as many near neighbors and molecules in the initerior. $\endgroup$ – Bert Barrois Mar 21 at 13:04

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