Nanoridge Formation and Dynamics of Stratification in Micellar Freestanding Films

Abstract

Controlling and predicting the stability and lifetime of freestanding films, including foam and emulsion films, is crucial for many industrial and biological applications. Freestanding films (thickness <100 nm), stabilized by surfactants above the critical micelle concentration, exhibit stratification or stepwise thinning. Stratification proceeds by formation of thinner domains that grow at the expense of surrounding films. In this Article, we address several longstanding challenges related to the experimental characterization and theoretical description of thickness variations, forces, fluxes and flows underlying stratification. We show that nanoridges form and grow at the moving front around expanding domains, and we visualize their shape evolution using Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols with an unprecedented spatiotemporal resolution (thickness <10 nm, time <1 ms). We develop a theoretical model for drainage via stratification under the influence of supramolecular oscillatory surface forces arising from the confinement-induced layering of micelles, and we show that the nanoridge growth and domain expansion dynamics can be modeled quantitatively.

Foam film stratification studies probe intermicellar interactions

“…The ID-IOM setup, shown schematically in Fig. 1A, is utilized for visualization of a single foam film undergoing drainage and simultaneously for quantitative analysis of thickness variations and transitions (15)(16)(17)(48)(49)(50). A high-speed camera attached to a microscope captures the pixel-wise reflected light intensity that is determined by interference between light reflected from the two liquid-air interfaces of a single foam film, as sketched in Fig.…”

“…2 (and in movies included as supplementary information). In addition to the snapshots that show the contrast in grayscale intensity, here we include representative thickness maps obtained by converting intensity into pixel-wise thickness using IDIOM protocols, as detailed elsewhere (16,48,49). Even though the measurements of step size are routinely carried out by using monochromatic light sources and photodiodes as detectors that measure the average reflected light intensity (27), the visualization and analysis of nanoscopic topographical features shown in Fig.…”

“…Even though the measurements of step size are routinely carried out by using monochromatic light sources and photodiodes as detectors that measure the average reflected light intensity (27), the visualization and analysis of nanoscopic topographical features shown in Fig. 2 only began with the advent of IDIOM protocols (15)(16)(17)(48)(49)(50).…”

“…For micellar fluids containing charged micelles, the step size, Δh, obtained using thickness-time plots from stratification experiments, and periodicity, λ, of Π OS (h) directly measured using thin-film balance (47,48) show that both periodicity and step size exceed micelle size, a, implying λ > a and Δh > a. In 1971, Bruil and Lyklema (51) were the first to report that the concentrationdependent decrease in step size measured for sodium dodecyl sulfate (SDS) solutions followed a power law of the form Δh ∝ c soap −1=3 and wrote that step size values "seem to be related to intermolecular distance in the (unmicellized) bulk solution."…”

Thin film drainage dynamics of wheat and rye dough liquors and oat batter liquor

“…Indeed, each number (> 1) of micellar layers creates an additional metastable state in the Π isotherm (Nikolov and Wasan, 1989; (Fig 1C). The magnitude of Πsup at these metastable TF thicknesses generally increases as the number of micellar layers decreases and can reach values up to 200 Pa (Zhang and Sharma, 2018) or even 1,000 Pa (Basheva et al 2007). However, the magnitude of Πsup at such metastable thicknesses is system-specific and amongst others depends on the molecular structure and concentration of the involved surfactants (Basheva et al 2007;Zhang et al 2016).…”