Editing Soap film (section)

== Stability ==
[[File:SchemeSoapFilm.png|thumb|Figure 1: Organisation of surfactants at both surfaces of the soap film]]
[[File:MarangoniInSoapFilms.png|thumb|Figure 2: Marangoni surface forces due to inhomogeneities in surfactants concentration. The arrows represent the force direction]]

Daily experience{{Citation needed|date=October 2020|reason=Who's daily experience?}} shows that soap bubble formation is not feasible with water or with any pure liquid. Actually, the presence of soap, which is composed at a molecular scale of [[surfactants]], is necessary to stabilize the film. Most of the time, surfactants are [[amphiphile|amphiphilic]], which means they are molecules with both a [[hydrophobic]] and a [[hydrophilic]] part. Thus, they are arranged preferentially at the air/water interface (see figure 1).

Surfactants stabilize films because they create a repulsion between both surfaces of the film, preventing it from thinning and consequentially bursting. This can be shown quantitatively through calculations relating to [[disjoining pressure]]. The main repulsion mechanisms are [[steric]] (the surfactants can not interlace) and electrostatic (if surfactants are charged).

Moreover, surfactants make the film more stable toward thickness fluctuations due to the [[Marangoni effect]]. This gives some elasticity to the interface: if surface concentrations are not homogeneously dispersed at the surface, Marangoni forces will tend to re-homogenize the surface concentration (see figure 2).

Even in the presence of stabilizing surfactants, a soap film does not last forever. Water evaporates with time depending on the humidity of the atmosphere. Moreover, as soon as a film is not perfectly horizontal, the liquid flows toward the bottom due to gravity and the liquid accumulates at the bottom. At the top, the film thins and bursts.