Rupture of thin liquid films from Newtonian and viscoelastic liquids - Bursting behaviour of Newton-black films

Holes in very thin liquid films, stabilized by surfactants, sometimes show retardation in their expansion rates. This is a new, and quite unexpected observation, that we made by the use of ultra-rapid flash photographic techniques. It contradicts the accepted idea, that, owing to conservation of momentum, the rim velocity of a hole that expands in a thin liquid film must be constant. Indeed, thicker films of sodium dodecylsulfate (SDS) solutions, including the 'common-black' equilibrium him of ca. 10 nm thickness have the normal, constant bursting rates. However, the thinner 'Newton-black' films which can be formed from these same solutions burst in an anomalous, retarded fashion. Moreover, the rim of the expanding holes in these films is not toroidal; it has a scalloped appearance, small droplets issuing from the loci where the rim trails behind. Free vertical films drawn from a viscoelastic film also show retardation of the bursting rate. However, whereas, in SDS films, the transition to this type of bursting is abrupt between the common-black film and the Newton-black film, it is gradual from the thicker films to the thinnest Newton-black film for these solutions. We assume, therefore, that, although it is formed from a Newtonian liquid, a Newton-black film from an SDS solution has a storage modulus like a thicker viscoelastic film. The anomalous bursting behaviour of such films reflects a molecular structure which is more like a liquid-crystalline bilayer. The deviations from the classical (Culick) model for the rim velocity of an expanding hole in a liquid film can be described in terms of him elements being deformed at the transition from the film to the rim. Elastic energy is then (temporarily) stored in a molecular network structure, which is characterized by a correlation length 5. When the film thickness becomes smaller than 5, the elastic modulus becomes dependent on the film thickness. On this basis we have found a qualitative explanation for the observations on decelerating film bursts with viscoelastic solutions and Newton-black films.