Editing Soap film (section)

==Black spots==

[[File:Soap film 3.jpg|thumb|left|Figure 5: Magnified view of black spots in a soap film]]
During the late stages of draining, sharp-edged black spots start to form. These spots are significantly thinner (< 100&nbsp;nm) than the normal soap film, giving rise to their black interference colour. Whether black spots can form depends on the concentration of the soap, and moreover there are two types of black films:<ref name="Pugh2016">{{cite book|last1=Pugh|first1=Robert J.|title=Bubble and Foam Chemistry|chapter=Soap bubbles and thin films|year=2016|pages=84–111|publisher=Cambridge|doi=10.1017/CBO9781316106938.004|isbn=9781316106938}}</ref>
* Common black films, around 50&nbsp;nm in thickness, and
* Newton black films, around 4&nbsp;nm in thickness, require a higher electrolyte concentration. In these films the outer soap surfaces have effectively snapped together and pinched out most of the inner liquid.

As drainage continues, the black spots eventually take over the entire soap film, and despite its extreme thinness, the final black film can be quite stable and can survive for many minutes.