Domain expansion dynamics in stratifying foam films: experiments

Abstract

The stability, rheology and applications of foams, emulsions and colloidal sols depend on the hydrodynamics and thermodynamics of thin liquid films that separate bubbles, drops and particles respectively. Thin liquid films containing micelles, colloidal particles, liquid crystals or polyelectrolyte-surfactant mixtures exhibit step-wise thinning or stratification, often attributed to the layer-by-layer removal of the aforementioned supramolecular structures. Stratification proceeds through emergence and growth of thinner circular domains within a thicker film, and the domain expansion dynamics are the focus of this study. Domain and associated thickness variation in foam films made from sodium dodecyl sulfate (SDS) micellar solutions are examined using a Scheludko-type cell with a novel technique we call Interferometry Digital Imaging Optical Microscopy (IDIOM). Below 100 nm, stratification and drainage cause a thickness-dependent variation in reflected light intensity, visualized as progressively darker shades of gray. We show that the domain expansion dynamics exhibit two distinct growth regimes with characteristic scaling laws. Initially, the radius of the isolated domains grows with square root time, and the expansion rate can be characterized by an apparent diffusion constant. In contrast, after a section of the expanding domain coalesces with the Plateau border, the contact line between domain and the surrounding thicker region moves a constant velocity. We show that a similar transition from a constant diffusivity to a constant velocity regime is also realized when a topological instability occurs at the contact line between the growing thinner isolated domain and the surrounding thicker film. Though several studies have focused on the expansion dynamics of isolated domains that exhibit a diffusion-like scaling, the change in expansion kinetics observed after domains contact with the Plateau border has not been reported and analyzed before.

Quantitative imaging of the complexity in liquid bubbles’ evolution reveals the dynamics of film retraction

“…In this way, it is possible to image the very first moments of the bubble rupture close to the tip of the needle. Finally, virtually all the systems currently used for the study of flat bubbles could be easily integrated into a holographic microscope 7,8,18 (see Figure S4d).…”

“…Modernized versions of this setup are currently used by several research groups 15,16 . Conversely, the full-field techniques measure the thickness across the entire surface of the film throughout the experiment 17,18 . Even though these systems can determine the film thickness with a resolution of a few nanometers, they lack the (lateral) spatial or temporal resolution necessary to follow the complex dynamics of an evolving thin liquid film.…”

Foam film stratification studies probe intermicellar interactions

“…M olecules in simple liquids and supramolecular structures in complex fluids can stratify or undergo confinement-induced layering induced by symmetry breaking at a solid-liquid or a fluidfluid interface (1)(2)(3)(4)(5)(6)(7)(8). In freestanding or foam films, the confinementinduced layering of supramolecular structures including micelles (9)(10)(11)(12)(13)(14)(15)(16)(17), lipid layers (18,19), polyelectrolyte-surfactant complexes (20,21), nanoparticles (9,22), and liquid crystalline assemblies (23) can result in drainage via stratification. Due to thin film interference, foam films visualized under white light illumination display iridescent colors for thick films (h > 100 nm) (24)(25)(26)(27)(28), but ultrathin films (h < 100 nm) exhibit shades of gray that get progressively darker as the film gets thinner (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21).…”

“…In freestanding or foam films, the confinementinduced layering of supramolecular structures including micelles (9)(10)(11)(12)(13)(14)(15)(16)(17), lipid layers (18,19), polyelectrolyte-surfactant complexes (20,21), nanoparticles (9,22), and liquid crystalline assemblies (23) can result in drainage via stratification. Due to thin film interference, foam films visualized under white light illumination display iridescent colors for thick films (h > 100 nm) (24)(25)(26)(27)(28), but ultrathin films (h < 100 nm) exhibit shades of gray that get progressively darker as the film gets thinner (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21). In reflected light microscopy, micellar foam films exhibit coexisting thick−thin regions with distinct shades of gray.…”

Hyperspectral imaging for dynamic thin film interferometry

“…This approach is quite robust, however it is quite slow and suffers from human subjectivity. Another common approach utilizes fringe counting from a known absolute reference thickness in the film [25][26][27] . This approach is quite fast but is not robust and requires assumptions on the spatial structure of the film.…”