Irregular mobile films

The boundary between the black and silver region is horizontal except near the vertical border. This sudden change in colour from black to silver at a horizontal level indicates that the thickness of the film has altered rapidly. This region is indicated in Fig. 2.8 by the horizontal level at which the thin black film meets the draining film. This is an important characteristic that distinguishes the simple mobile film from the irregular mobile film. The rapid variation in the thickness of the film at the black-silver border occurs when the thickness of the film becomes comparable to the range of the van der Waals interaction between the water molecules in the film. 

The irregular mobile film initially drains in a similar manner to the simple mobile film. The differences appear after the formation of the black film. The boundary between the black region and the coloured region becomes... 

A vertical irregular mobile film displaying critical fall... 

---