As a result of the surface tension inherent to liquids, curved surfaces are formed in order to minimize the area. This phenomenon arises from the energetic cost of forming a surface. As such the Gibbs free energy of the system is minimized when the surface is curved.
The Kelvin equation is based on thermodynamic principles and is used to describe changes in vapor pressure caused by liquids with curved surfaces. The cause for this change in vapor pressure is the Laplace pressure. The vapor pressure of a drop is higher than that of a planar surface because the increased Laplace pressure causes the molecules to evaporate more easily. Conversely, in liquids surrounding a bubble, the pressure with respect to the inner part of the bubble is reduced, thus making it more difficult for molecules to evaporate.  The Kelvin equation can be stated as:
where  is the vapor pressure of the curved surface,  is the vapor pressure of the flat surface,  is the surface tension,  is the molar volume of the liquid,  is the universal gas constant,  is temperature (in kelvin), and  and  are the principal radii of curvature of the surface.