Surface tension gases. The pressure inside an ideal (one surface) soap bubble can be derived from thermodynamic free energy considerations. At constant temperature and particle number, , the differential Helmholtz energy is given by where is the difference in pressure inside and outside of the bubble, and is the surface tension. In equilibrium, , and so, For a spherical bubble, the volume and surface area are given simply by and Substituting these relations into the previous expression, we find which is equivalent to the Young–Laplace equation when . For real soap bubbles, the pressure is doubled due to the presence of two
Volume solid wet film thickness is also 100 µm. Then after complete drying of the paint, the DFT will be 100 µm because no solvent will be evaporated. This is an important concept when using paint industrially to calculate the cost of painting. It can be said that it is the real volume of paint. Here is the formula by which one can calculate the volume solid of paint, In earlier days, the volume solid was measured by a disc method but now a sophisticated instrument is also available which takes only a drop of paint to check the volume solid. Volume