  Hypothesis: New dynamic phenomena can be observed in evaporating free liquid
films from colloidal solutions with bimodal particle size distribution. Such
distributions are formed in a natural way in mixed (slightly turbid) solutions
of cationic and anionic surfactants, where nanosized micelles coexist with
micronsized precipitated particles. Experiment: Without evaporation of water,
the films thin down to thickness < 100 nm. Upon water evaporation from the
film, one observes spontaneous film thickening (above 300 nm) and appearance of
a dynamic vortex with a spot of thinner film in the center. The vortex wall has
a stepwise profile with step-height equal to the effective micelle diameter
(ca. 8 nm) and up to 20-30 stratification steps. Results: For thicknesses
greater than 100 nm, stratification in foam films from micellar solutions has
never been observed so far. It evidences for the formation of a thick colloidal
crystal of micelles in the evaporating film. The role of the bigger,
micronsized particles is to form a filtration cake in the Plateau border, which
supports the thick film. The developed quantitative mechanical model shows that
the stepwise vortex profile is stabilized by the balance of hydrodynamic and
surface tension forces. Vortex is observed not only in films from catanionic
surfactant solutions, but also in films from silica and latex particle
suspensions, which contain smaller surfactant micelles.
