As a system is formed from a solid surface and a drop of liquid, energy minima and maxima are produced by the free energy of the system. When the solid surface is rough or homogeneous, the system, which is made up of a solid, a liquid, and a fluid, could have multiple minima produced from the free energy at different minima points. One of these minima is called the global minimum. The global minimum has the lowest free energy within the system and is defined to be the stable equilibrium state. Furthermore, the other minima illustrate the metastable equilibrium states of the system. In between these minima are the energy barriers which hinder the motion of energy between the various metastable states in the system. The transition of energy between metastable states is also affected by the availability of external energy to the system, which is associated with the volume of the liquid drop on top of the solid surface. As such, the volume of the liquid may have an impact on the locations of the minima points, which could influence the contact angles created by the solid and the liquid. The contact angles are directly related to whether the solid surface is ideal, in other words, whether it is a smooth, heterogeneous surface.
In the captive bubble method, the bubble indicating the liquid drop forms an angle with the solid surface called the contact angle. The stability of the liquid phase on the solid is measured with an intrinsic contact angle. Theoretically, the contact angle equals the value of the local intrinsic contact angle, assuming line tension has a negligible effect.