Soap film

Soap films are thin layers of liquid (usually water-based) surrounded by air.

Soap films can be used as model systems for minimal surfaces, which are widely used in mathematics.

Daily experience[citation needed] 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.

Moreover, surfactants make the film more stable toward thickness fluctuations due to the Marangoni effect.

In order to minimize its energy, a droplet of liquid in free space naturally assumes a spherical shape, which has the minimum surface area for a given volume.

Puddles and films can exist in of the presence of other forces, like gravity and the intermolecular attraction to the atoms of a substrate.

In that case, the lowest energy configuration for the body would be one where as many film atoms as possible are as close as possible to the substrate.

If surfactants are well chosen[2] and the atmospheric humidity and air movements are suitably controlled, a horizontal soap film can last from minutes to hours.

These spots are significantly thinner (< 100 nm) than the normal soap film, giving rise to their black interference colour.

Figure 1: Organisation of surfactants at both surfaces of the soap film
Figure 2: Marangoni surface forces due to inhomogeneities in surfactants concentration. The arrows represent the force direction
Figure 3: thin film interference in a soap bubble. Notice the golden yellow colour near the top where the film is thin and a few even thinner black spots
Figure 4: Picture of a film taken during its generation. The film is pulled out of a soapy solution and drains from the top.
Figure 5: Magnified view of black spots in a soap film