Hydrogels, such as those that comprise soft contact lenses, are inaccessible to the standard arrangement; so the captive bubble method is also used in such cases.
[3] A contact angle is formed on a smooth, periodically heterogeneous solid surface.
The global minimum has the lowest free energy within the system and is defined as the stable equilibrium state.
Through studies, a higher contact angle may correspond to a more hydrophobic surface in membrane analysis.
Source: [6] In comparison to the use of the captive bubble method in the measurement of contact angles in other cases, the contact angle in the study of the lung surfactant monolayer is kept at a constant 180 degrees, due to the property of the hydrated agar gel on the ceiling of the bubble.
In addition to measuring the surface tension, bubble formation can also be utilized in the measurement of the adsorption of lung surfactant, which defines how quickly substances build up on the air-liquid interface of pulmonary surfactants to form a film.