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Sodium naphthenates (NaNs), found in crude oils and oil sands process-affected water (OSPW), can act as surfactants and stabilize undesirable foams and emulsions. Despite the critical impact of soap-like NaNs on the formation, properties, and stability of petroleum and OSPW foams, there is a significant lack of studies that characterize foam film drainage, motivating this study. Here, we contrast the drainage of aqueous foam films formulated with NaN with foams containing sodium dodecyl sulfate (SDS), a well-studied surfactant system, in the relatively low concentration regime ( c /CMC < 12.5). The foam films exhibit drainage via stratification, displaying step-wise thinning and coexisting thick–thin regions manifested as distinct shades of gray in reflected light microscopy due to thickness-dependent interference intensity. Using IDIOM (interferometry digital imaging optical microscopy) protocols that we developed, we analyze pixel-wise intensity to obtain thickness maps with high spatiotemporal resolution (thickness <1 nm, lateral ∼500 nm, time ∼10 ms). The analysis of interference intensity variations over time reveals that the aqueous foam films of both SDS and NaN possess an evolving, dynamic, and rich nanoscopic topography. The nanoscopic thickness transitions for stratifying SDS foam films are attributed to the role played by damped supramolecular oscillatory structural disjoining pressure contributed by the confinement-induced layering of spherical micelles. In comparison with SDS, we find smaller concentration-dependent step size and terminal film thickness values for NaN, implying weaker intermicellar interactions and oscillatory structural disjoining pressure with shorter decay length and periodicity. 

Bile salts, especially in their aggregated or micellar form, play a critical role in health and medicine by solubilizing cholesterol, fat-soluble vitamins, and drugs. However, in contrast to the head–tail (HT) surfactants like sodium dodecyl sulfate (SDS), amphiphilic bile salts have an unusual steroid structure and exhibit a smaller aggregation number ( N agg < 20 molecules per micelle vs. N agg > 50 for SDS). Foam films formed by micellar solutions of typical surfactants like SDS exhibit stratification manifested as stepwise thinning and coexistence of flat thick–thin regions that differ by a step-size proportional to the intermicellar distance. We consider drainage via stratification studies as an effective and insightful probe of the strength and magnitude of intermicellar interactions and resulting supramolecular oscillatory structural (SOS) surface force contribution to disjoining pressure. However, there are neither prior reports of stratification in foam films formed with bile salt solutions nor measurements of SOS surface forces. Here we report the discovery and characterization of stratification in foam films formed by aqueous solutions of four bile salts – sodium cholate (NaC), sodium taurocholate (NaTC), sodium deoxycholate (NaDC), and sodium glycodeoxycholate (NaGDC) – that have a similar steroid nucleus, but difference in conjugation sites and the number of hydroxyl groups (3 for NaC and NaTC, 2 for NaDC and NaGC). Using IDIOM (interferometry digital imaging optical microscopy) protocols we developed recently to characterize and analyze thickness variations and transitions, we find that foam films made with bile salts exhibit fewer stepwise transitions and smaller step-size than SDS solutions. Also, we measured a lower drop in surface tension and lower magnitude of thickness-dependent disjoining pressure compared to SDS solutions. We find that the bile salts with a matched number of hydroxyl groups exhibit similar properties in tensiometry and foam film studies. We show that the stratification studies can characterize the influence of chemical structure on the magnitude and range of intermicellar interactions as well their influence on drainage and stability of foam films. 

Understanding and characterizing the influence of polymers and surfactants on rheology, application, and processing is critical for designing complex fluid formulations for enhanced oil recovery, pharmaceuticals, cosmetics, foods, inks, agricultural sprays, and coatings. It is well-established that the addition of anionic surfactant like sodium dodecyl sulfate (SDS) to an aqueous solution of an oppositely-charged or uncharged polymer like poly(ethylene oxide) (PEO) can result in the formation of the polymer–surfactant association complexes (P 0 S − ACs) and a non-monotonic concentration-dependent variation in zero shear viscosity. However, the extensional rheology response of polymer–surfactant mixtures remains relatively poorly understood, partially due to characterization challenges that arise for low viscosity, low elasticity fluids, even though the response to strong extensional flows impacts drop formation and many processing operations. In this article, we use the recently developed dripping-onto-substrate (DoS) rheometry protocols to characterize the pinching dynamics and extensional rheology response of aqueous P 0 S − solutions formulated with PEO (P 0 ) and SDS (S − ), respectively. We find the PEO–SDS mixtures display a significantly weaker concentration-dependent variation in the extensional relaxation time, filament lifespan, and extensional viscosity values than anticipated by the measured shear viscosity. 

Ultrathin foam films containing supramolecular structures like micelles in bulk and adsorbed surfactant at the liquid–air interface undergo drainage via stratification. At a fixed surfactant concentration, the stepwise decrease in the average film thickness of a stratifying micellar film yields a characteristic step size that also describes the quantized thickness difference between coexisting thick–thin flat regions. Even though many published studies claim that step size equals intermicellar distance obtained using scattering from bulk solutions, we found no reports of a direct comparison between the two length scales. It is well established that step size is inversely proportional to the cubic root of surfactant concentration but cannot be estimated by adding micelle size to Debye length, as the latter is inversely proportional to the square root of surfactant concentration. In this contribution, we contrast the step size obtained from analysis of nanoscopic thickness variations and transitions in stratifying foam films using Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols, that we developed, with the intermicellar distance obtained using small-angle X-ray scattering. We find that stratification driven by the confinement-induced layering of micelles within the liquid–air interfaces of a foam film provides a sensitive probe of non-DLVO (Derjaguin–Landau–Verwey–Overbeek) supramolecular oscillatory structural forces and micellar interactions.

 

Nail lacquer formulations are multi-ingredient complex fluids with additives that affect color, smell, texture, evaporation rate, viscosity, stability, leveling behavior, consumer's sensory experience, and dried coating's decorative and wear performance. Optimizing and characterizing the formulation rheology is critical for achieving longer shelf-life, better control over the nail painting process and adhesion, continuous manufacturing of large product volumes, and increasing overall consumer satisfaction. Dispensing, bottle filling, brush application, and dripping, as well as perceived tackiness of nail polishes, all involve capillarity-driven pinching flows associated with strong extensional deformation fields. However, a significant lack of characterization of pinching dynamics and extensional rheology response of multicomponent formulations, especially particle suspensions in viscoelastic solutions, motivates this study. Here, we characterize the shear rheology response of twelve commercial nail lacquer formulations using torsional rheometry and characterize pinching dynamics and extensional rheology response using dripping-onto-substrate (DoS) rheometry protocols we developed. We visualize and analyze brush loading, nail coating, dripping from brush, sagging, and lacquer application on a nail to outline the challenges posed by free-surface flows and non-Newtonian rheology. We find that the radius evolution over time obtained using DoS rheometry displays power law exponents distinct from those exhibited in shear thinning. Both shear and extensional viscosity decrease with deformation rate. However, the extensional viscosity appears to be rate-independent at the highest rates and displays nearly an order of magnitude larger values than the high shear rate viscosity. We envision that the findings and protocols described here will help and motivate industrial scientists to design better multicomponent formulations through a better characterization and understanding of the influence of ingredients like particles and polymers on rheology, processing, and applications. 

We report the discovery of a hitherto unreported mechanism of drainage and rupture of micellar foam films that presents unexplored opportunities for understanding and controlling the stability, lifetime and properties of ubiquitous foams. It is well-known that ultrathin micellar foam films exhibit stratification, manifested as stepwise thinning and coexistence of thin–thick flat regions that differ in thickness by a nanoscopic step size equal to the intermicellar distance. Stratification typically involves the spontaneous formation and growth of thinner, darker, circular domains or thicker, brighter mesas. Mechanistically, domain expansion appears similar to hole growth in polymer films undergoing dewetting by nucleation and growth mechanism that can be described by considering metastable states resulting from a thickness-dependent oscillatory free energy. Dewetting polymer films occasionally phase separate into thick and thin regions forming an interconnected, network-like morphology by undergoing spinodal dewetting. However, the formation of thick–thin spinodal patterns has never been reported for freestanding films. In this contribution, we show that the thickness-dependent oscillatory contribution to free energy that arises due to confinement-induced layering of micelles can drive the formation of such thick-thin regions by undergoing a process we term as spinodal stratification. We visualize and characterize the nanoscopic thickness variations and transitions by using IDIOM (interferometry digital imaging optical microscopy) protocols to obtain exquisite thickness maps of freestanding films. We find that evaporation and enhanced drainage in vertical films play a critical role in driving the process, and spinodal stratification can occur in both single foam films and in bulk foam.