Thin liquid films: Where hydrodynamics, capillarity, surface stresses and intermolecular forces meet

Comparison of Methods Used to Investigate Coalescence in Emulsions

We present a study of moderately stable dilute emulsions. These emulsions are models for water contaminated by traces of oil encountered in many water treatment situations. The purification of water and the elimination of oil rely on the emulsion stability. Despite actively being studied, the topic of emulsion stability is still far from being fully understood. In particular, it is still unclear whether experimental methods accessing different length scales lead to the same conclusions. In the study presented in this paper, we have used different methods to characterize the emulsions, such as centrifugation and simple bottle tests, as well as investigations of the collision of single macroscopic oil drops at an oil-water interface. We studied different emulsions containing added polymer or surfactant. In the case of added polymer, centrifugation and single drop experiments led to opposite trends in stability when the polymer concentration is varied. In the case of added surfactant, both centrifugation and single drop experiments show a maximum stability when the surfactant concentration is increased, whereas bottle tests show a monotonous increase in stability. We propose tentative interpretations of these unexpected observations. The apparent contradictions are due to the fact that different methods require different drop sizes or different drop concentrations. The puzzling decrease in emulsion stability at a higher surfactant concentration observed with some methods, however, remains unclear. This coalescence study illustrates the fact that different results can be obtained when different experimental methods are used. It is therefore advisable not to rely on a single method, especially in the case of emulsions of limited stability for reasons explained in the paper.

Why Bubbles Coalesce Faster than Droplets: The Effects of Interface Mobility and Surface Charge

Air bubbles in pure water appear to coalesce much faster compared to oil emulsion droplets at the same water solution conditions. The main factors explaining this difference in coalescence times could be interface mobility and/or pH-dependent surface charge at the water interface. To quantify the relative importance of these effects, we use high-speed imaging to monitor the coalescence of free-rising air bubbles with the water-air interface as well as free-falling fluorocarbon-oil emulsion droplets with a water-oil interface. We measure the coalescence times of such bubbles and droplets over a range of different water pH values (3.0, 5.6, 11.0). In the case of bubbles, a very fast coalescence (milliseconds) is observed for the entire pH range in pure water, consistent with the hydrodynamics of fully mobile interfaces. However, when the water-air interface is immobilized by the deposition of a monolayer of arachidic acid, the coalescence is significantly delayed. Furthermore, the coalescence times increase with increasing pH. In the case of fluorocarbon-oil droplets, the coalescence is always much slower (seconds) and consistent with immobile interface coalescence. The fluorocarbon droplet's coalescence time is also pH-dependent, with a complete stabilization (no coalescence) observed at pH 11. In the high electrolyte concentration, a 0.6 M NaCl water solution, bubbles, and droplets have similar coalescence times, which could be related to the bubble interface immobilization at the late stage of the coalescence process. Numerical simulations are used to evaluate the time scale of mobile and immobile interface film drainage.

Quantifying Contributions of Different Repulsion to Film Drainage Time during the Bubble-Solid Surface Attachment and Implications for the Flotation of Fine Particles

The flotation recovery of fine particles faces serious challenges due to the lack of kinetic energy required for supporting their radial displacement and attachment with bubbles. Generally, the hydrodynamic resistance and repulsive disjoining pressure successively inhibit the liquid outflow intervening between the bubble and solid surfaces. To quantitatively characterize the influence of the main repulsion on film thinning time, experiments have been designed in three different aqueous systems. Bubble surface mobility closely associated with hydrodynamic resistance was determined by the rising bubble technique, and the DLVO theory was employed to confirm the evolution of electrostatic repulsion. The film drainage process was then measured based on the high-speed microscopic interferometry. Furthermore, the influence of the main repulsion on bubble-solid surface interactions was examined by flotation recovery. Results show that the earlier buildup of hydrodynamic force ran through the whole film thinning process, and under immobile conditions, the central region gradually became dominant in film thinning due to the very limited fluid flow at the thinnest rim position. Therefore, to achieve the identical film thickness (∼100 nm), the large hydrodynamic resistance could prolong the film thinning time by about 1 order of magnitude, compared with that induced by electrostatic repulsion, which accounts for the increased flotation recovery by 10% using mobile bubbles. This study not only enhances the understanding of how typical repulsive forces work in film drainage dynamics but also opens up an avenue for enhancing flotation and avoiding wasting resources by modulating bubble surface mobility and thus micro/nanoscale fluid flow.

Spatiotemporal mapping of nanotopography and thickness transitions of ultrathin foam films

Freshly formed soap films, soap bubbles, or foam films display iridescent colors due to thin film interference that changes as squeeze flow drives drainage and a progressive decrease in film thickness. Ultrathin (thickness <100 nm) freestanding films of soft matter containing micelles, particles, polyelectrolyte-surfactant complexes, or other supramolecular structures or liquid crystalline phases display drainage via stratification. A fascinating array of thickness variations and transitions, including stepwise thinning and coexistence of thick-thin flat regions, arise in micellar foam films that undergo drainage via stratification. In this tutorial, we describe the IDIOM (interferometry digital imaging optical microscopy) protocols that combine the conventional interferometry principle with digital filtration and image analysis to obtain nanometer accuracy for thickness determination while having high spatial and temporal resolution. We provide fully executable image analysis codes and algorithms for the analysis of nanotopography and summarize some of the unique insights obtained for stratified micellar foam films.

Stratification and film ripping induced by structural forces in granular micellar thin films

HYPOTHESIS

Interactions across incredibly thin layers of fluids, known as thin films, underpin many important processes involving colloids, such as wetting-dewetting phenomena. Often in these systems, thin films are composed of complex fluids that contain dispersed components, such as spherical micelles, giving rise to oscillatory structural forces due to preferential layering under confinement. Modelling of thin film dynamics involving Derjaguin-Landau-Verwey-Overbeek (DLVO) type forces has been widely reported using the Stokes-Reynolds-Young-Laplace (SRYL) model, and we hypothesize that this theory can be extended to a concentrated micellar system by including an oscillatory structural force term in the disjoining pressure.

EXPERIMENTS

We study the drainage behaviour of thin films comprising sodium dodecyl sulfate (SDS) micelles across a range of concentrations and interaction conditions between an air bubble and a mica disk using a custom-built dual-wave interferometry apparatus.

FINDINGS

Early-stage film behaviour is dominated by hydrodynamics, which can be well reproduced by the SRYL model. However, experimental profiles drain significantly faster than predicted, transitioning into a structural force dominated phase characterised by four types of film ripping instabilities that we term 'waving', 'ridging', 'webbing', and 'hole-sheeting'. These instabilities were mapped according to SDS concentration and approach velocity, providing insight into the interplay between structural forces and hydrodynamic conditions.

Foam film stratification, viscosity, and small-angle X-ray scattering of micellar SDS solutions over an extended concentration range (1< c/CMC < 75)

Ultrathin foam films (thickness, h < 100 nm) containing micelles undergo drainage via stratification manifested as coexisting thick-thin flat regions, nanoscopic non-flat topography, and the stepwise decrease in film thickness that yields a characteristic step-size. Most studies characterize the variation in step size and stratification kinetics in micellar foam films in a limited concentration range, c/CMC < 12.5 (c < 100 mM). Likewise, most scattering studies characterize micelle dimensions, intermicellar distance, and volume fraction in bulk aqueous SDS solutions in this limited concentration range. In this contribution, we show drainage via stratification can be observed for concentrations up to c/CMC < 75 (c < 600 mM). Understanding the stratification behavior of freely draining micellar films with sodium dodecyl sulfate (SDS) concentration varying in the range 10 mM ≤ cSDS ≤ 600 mM is essential for molecular engineering, consumer product formulations, and controlling foaming in industrial processes. Here, we visualize and analyze nanoscopic thickness variations and transitions in stratifying foam films using Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols. We compare step size obtained from foam stratification to micelle dimension, micelle volume fraction, and intermicellar distance obtained from small angle X-ray scattering studies. Even though the volume fraction increases and approaches 25% at c = 600 mM, the solution viscosity only increases by a factor of four compared to the solvent, consistent with the findings from both stratification and scattering studies. These comparisons allow us to explore the effect of micelle size, morphology, and intermicellar interactions on supramolecular oscillatory structural disjoining pressure, which influences the stratification behavior of draining foam films containing micelles under confinement.

Evolution dynamics of thin liquid structures investigated using a phase-field model

Liquid structures of thin-films and torus droplets are omnipresent in daily lives. The morphological evolution of liquid structures suspending in another immiscible fluid and sitting on a solid substrate is investigated by using three-dimensional (3D) phase-field (PF) simulations. Here, we address the evolution dynamics by scrutinizing the interplay of surface energy, kinetic energy, and viscous dissipation, which is characterized by Reynolds number Re and Weber number We. We observe special droplet breakup phenomena by varying Re and We. In addition, we gain the essential physical insights into controlling the droplet formation resulting from the morphological evolution of the liquid structures by characterizing the top and side profiles under different circumstances. We find that the shape evolution of the liquid structures is intimately related to the initial shape, Re, We as well as the intrinsic wettability of the substrate. Furthermore, it is revealed that the evolution dynamics are determined by the competition between the coalescence phenomenology and the hydrodynamic instability of the liquid structures. For the coalescence phenomenology, the liquid structure merges onto itself, while the hydrodynamic instability leads to the breakup of the liquid structure. Last but not least, we investigate the influence of wall relaxation on the breakup outcome of torus droplets on substrates with different contact angles. We shed light on how the key parameters including the initial shape, Re, We, wettability, and wall relaxation influence the droplet dynamics and droplet formation. These findings are anticipated to contribute insights into droplet-based systems, potentially impacting areas like ink-jet printing, drug delivery systems, and microfluidic devices, where the interplay of surface energy, kinetic energy, and viscous dissipation plays a crucial role.

Hygroscopic Growth of Adsorbed Water Films on Smectite Clay Particles

Hygroscopic growth of adsorbed water films on clay particles underlies a number of environmental science questions, from the air quality and climate impacts of mineral dust aerosols to the hydrology and mechanics of unsaturated soils and sedimentary rocks. Here, we use molecular dynamics (MD) simulations to establish the relation between adsorbed water film thickness (h) and relative humidity (RH) or disjoining pressure (Π), which has long been uncertain due to factors including sensitivity to particle shape, surface roughness, and aqueous chemistry. We present a new MD simulation approach that enables precise quantification of Π in films up to six water monolayers thick. We find that the hygroscopicity of phyllosilicate mineral surfaces increases in the order mica < K-smectite < Na-smectite. The relationship between Π and h on clay surfaces follows a double exponential decay with e-folding lengths of 2.3 and 7.5 Å. The two decay length scales are attributed to hydration repulsion and osmotic phenomena in the electrical double layer (EDL) at the clay-water interface.

Optimal free-surface pumping by an undulating carpet

Examples of fluid flows driven by undulating boundaries are found in nature across many different length scales. Even though different driving mechanisms have evolved in distinct environments, they perform essentially the same function: directional transport of liquid. Nature-inspired strategies have been adopted in engineered devices to manipulate and direct flow. Here, we demonstrate how an undulating boundary generates large-scale pumping of a thin liquid near the liquid-air interface. Two dimensional traveling waves on the undulator, a canonical strategy to transport fluid at low Reynolds numbers, surprisingly lead to flow rates that depend non-monotonically on the wave speed. Through an asymptotic analysis of the thin-film equations that account for gravity and surface tension, we predict the observed optimal speed that maximizes pumping. Our findings reveal how proximity to free surfaces, which ensure lower energy dissipation, can be leveraged to achieve directional transport of liquids.

Coalescence Initiation in Liquid-Liquid Systems: A Stochastic Model

Coalescence is a complex phenomenon leading to the merging of deformable particles of fluid. The complexity stems largely from the simultaneous occurrence of phenomena of a different nature (hydrodynamic, electrostatic, physicochemical) acting at different scales. The stochastic effects controlling the formation of the liquid bridge between two droplets of the same liquid, immersed in another nonmiscible liquid, are studied through a series of molecular dynamics simulations. The case of heptane droplets in water, relevant to solvent extraction, a key process of the circular economy, is considered. From this series of simulations, we have confirmed that the probability function of coalescence of two identical droplets in contact follows a Poisson distribution. We moreover propose a criterion for the initiation of coalescence based on nucleation theory. A complete description of the stochastic initiation of coalescence is hence provided, opening many perspectives for the simulation of coalescence in continuous approaches used in fluid mechanics and chemical engineering. The methodology can be generalized to droplets of different size and composition, immersed in gas, or to bubbles, i.e., to other physical problems whose kinetics is influenced by the molecular scale.

Effects of Chitosan and Cellulose Derivatives on Sodium Carboxymethyl Cellulose-Based Films: A Study of Rheological Properties of Film-Forming Solutions

Bio-based packaging materials and efficient drug delivery systems have garnered attention in recent years. Among the soluble cellulose derivatives, carboxymethyl cellulose (CMC) stands out as a promising candidate due to its biocompatibility, biodegradability, and wide resources. However, CMC-based films have limited mechanical properties, which hinders their widespread application. This paper aims to address this issue by exploring the molecular interactions between CMC and various additives with different molecular structures, using the rheological method. The additives include O-carboxymethylated chitosan (O-CMCh), N-2-hydroxypropyl-3-trimethylammonium-O-carboxymethyl chitosan (HTCMCh), hydroxypropyltrimethyl ammonium chloride chitosan (HACC), cellulose nanocrystals (CNC), and cellulose nanofibers (CNF). By investigating the rheological properties of film-forming solutions, we aimed to elucidate the influencing mechanisms of the additives on CMC-based films at the molecular level. Various factors affecting rheological properties, such as molecular structure, additive concentration, and temperature, were examined. The results revealed that the interactions between CMC and the additives were dependent on the charge of the additives. Electrostatic interactions were observed for HACC and HTCMCh, while O-CMCh, CNC, and CNF primarily interacted through hydrogen bonds. Based on these rheological properties, several systems were selected to prepare the films, which exhibited excellent transparency, wettability, mechanical properties, biodegradability, and absence of cytotoxicity. The desirable characteristics of these selected films demonstrated the strong biocompatibility between CMC and chitosan and cellulose derivatives. This study offers insights into the preparation of CMC-based food packaging materials with specific properties.

Ultrathin Micellar Foam Films of Sodium Caseinate Protein Solutions

Sodium caseinates (NaCas), derived from milk proteins called caseins, are often added to food formulations as emulsifiers, foaming agents, and ingredients for producing dairy products. In this contribution, we contrast the drainage behavior of single foam films made with micellar NaCas solutions with well-established features of stratification observed for the micellar sodium dodecyl sulfate (SDS) foam films. In reflected light microscopy, the stratified SDS foam films display regions with distinct gray colors due to differences in interference intensity from coexisting thick-thin regions. Using IDIOM (interferometry digital imaging optical microscopy) protocols we pioneered for mapping nanotopography of foam films, we showed that drainage via stratification in SDS films proceeds by the expansion of flat domains that are thinner than surrounding by a concentration-dependent step-size, and nonflat features (nanoridges and mesas) form at the moving front. Furthermore, stratifying SDS foam films show stepwise thinning, such that the step-size and terminal film thickness decrease with concentration. Here we visualize the nanotopography in protein films with high spatiotemporal resolution using IDIOM protocols to address two long-standing questions. Do protein foam films formulated with NaCas undergo drainage via stratification? Are thickness transitions and variations in protein foam films determined by intermicellar interactions and supramolecular oscillatory disjoining pressure? In contrast with foam films containing micellar SDS, we find that micellar NaCas foam films display just one step, nonflat and noncircular domains that expand without forming nanoridges and a terminal thickness that increases with NaCas concentration. We infer that the differences in adsorbing and self-assembling unimers triumph over any similarities in the structure and interactions of their micelles.

Dispersion and mixing dynamics of complex oil-in-water emulsions in Taylor-Couette flows

The seminal study by G. I. Taylor (1923) has inspired generations of work in exploring and characterizing Taylor-Couette (TC) flow instabilities and laid the foundation for research of complex fluid systems requiring a controlled hydrodynamic environment. Here, TC flow with radial fluid injection is used to study the mixing dynamics of complex oil-in-water emulsions. Concentrated emulsion simulating oily bilgewater is radially injected into the annulus between rotating inner and outer cylinders, and the emulsion is allowed to disperse through the flow field. The resultant mixing dynamics are investigated, and effective intermixing coefficients are calculated through measured changes in the intensity of light reflected by the emulsion droplets in fresh and salty water. The impacts of the flow field and mixing conditions on the emulsion stability are tracked via changes in droplet size distribution (DSD), and the use of emulsified droplets as tracer particles is discussed in terms of changes in the dispersive Péclet, Capillary and Weber numbers. For oily wastewater systems, the formation of larger droplets is known to yield better separation during a water treatment process, and the final DSD observed here is found to be tunable based on salt concentration, observation time and mixing flow state in the TC cell. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (Part 2)'.

Polymer-Surfactant Complexes Impact the Stratification and Nanotopography of Micellar Foam Films

Freestanding films of soft matter drain via stratification due to confinement-induced structuring and layering of supramolecular structures such as micelles. Neutral polymers, added as rheology modifiers to cosmetics, foods, pharmaceuticals, and petrochemical formulations, often interact with monomers and micelles of surfactants, forming polymer-surfactant complexes. Despite many studies that explore interfacial and bulk rheological properties, the corresponding influence of polymer-surfactant complexes on foam drainage and lifetime is not well understood and motivates this study. Here, we report the discovery and evidence of drainage via stratification in foam films formed with polymer-surfactant (PEO-SDS) complexes. We show that the stratification trifecta of coexisting thick-thin regions, stepwise thinning, and nanoscopic topological features such as nanoridges and mesas can be observed using IDIOM (interferometry, digital imaging, and optical microscopy) protocols we developed for nanoscopic thickness mapping. We determine that for polymer concentrations below overlap concentration and surfactant concentrations beyond the excess micelle point, polymer-surfactant complexation impact the nanoscopic topography but not the step size, implying the amplitude of disjoining pressure changes, but periodicity remains unchanged.

Foam coarsening under a steady shear: interplay between bubble rearrangement and film thinning dynamics

Aqueous foams are unstable and age by drainage and coarsening. Today, these effects are well described, as also their impact on foam properties. In that respect, the foam viscoelastic properties evolve in time as a consequence of coarsening which tends to increase the mean bubble size. Here, we investigate the reverse coupling, and study if and how the continuous flow of a foam can impact its dynamics of coarsening. We introduce a new protocol where brief oscillatory measurements are inserted during a constant steady shear, allowing us to monitor the relative variation of the bubble size with time (obtained from the one of the elastic modulus G') as a function of the applied shear rate. It turns out that the coarsening rate is strongly impacted by the applied shear: this rate is continuously reduced above a critical shear rate, which itself decreases with the bubble size. This coarsening-rate reduction is interpreted as the result of out-of-equilibrium and shear-dependent film thicknesses, being higher than at rest. The critical shear rate, above which films are dynamically sustained at higher thickness than at equilibrium, emerges from the competition between the rate of rearrangements and the time required to drain the thick film created during the rearrangement. We thus report here a first experimental proof and measurements of out-of-equilibrium film thicknesses within a sheared foam, and of the impact this has on coarsening.

Relevance of the air-water interfacial and foaming properties of (modified) wheat proteins for food systems

A shift from animal protein- to plant protein-based foods is crucial in transitioning toward a more sustainable global food system. Among food products typically stabilized by animal proteins, food foams represent a major category. Wheat proteins are ubiquitous and structurally diverse, which offers opportunities for exploiting them for food foam and air-water interface stabilization. Notably, they are often classified into those that are soluble in aqueous systems (albumins and globulins) and those that are not (gliadins and glutenins). However, gliadins are at least to an extent water extractable and thus surface active. We here provide a comprehensive overview of studies investigating the air-water interfacial and foaming properties of the different wheat protein fractions. Characteristics in model systems are related to the functional role that wheat proteins play in gas cell stabilization in existing wheat-based foods (bread dough, cake batter, and beer foam). Still, to further extend the applicability of wheat proteins, and particularly the poorly soluble glutenins, to other food foams, their modification is required. Different physical, (bio)chemical, and other modification strategies that have been utilized to alter the solubility and therefore the air-water interfacial and foaming properties of the gluten protein fraction are critically reviewed. Such approaches may open up new opportunities for the application of (modified) gluten proteins in other food products, such as plant-based meringues, whippable drinks, or ice cream. In each section, important knowledge gaps are highlighted and perspectives for research efforts that could lead to the rational design of wheat protein systems with enhanced functionality and overall an increased applicability in food industry are proposed.

Variations in foam collapse and thin film stability with constant interfacial and bulk properties

The stability of foams is commonly linked to the interfacial properties of the proteins and other surfactants used. This study aimed to use these relationships to explain differences in foam stability observed among similar beer samples from different breweries. The foam stability was different for each sample (Nibem foam stability ranged from 206 to 300 s), but ranking was similar for all three foaming methods used, thus independent of the method, gas, etc. Differences in foam stability were dominated by differences in coalescence, as illustrated by the correlation with the stability of single bubbles and thin liquid films. The differences in coalescence stability could not be explained by the measured interfacial properties (e.g. surface pressure, adsorption rate, dilatational modulus and surface shear viscosity), or the bulk properties (concentration, pH, ionic strength, viscosity), since they were similar for all samples. The drainage rates and disjoining pressure isotherms measured in thin liquid films were also similar for all samples, further limiting the options to explain the differences in foam stability using known arguments. The differences in coalescence stability of the thin films was shown to depend on the liquid in between the adsorbed layers of the thin film, using a modified capillary cell to exchange this liquid (to a buffer, or one of the other samples). This illustrates the need to review our current understanding and to develop new methods both for experimental study and theoretical description, to better understand foam stability in the future.

Effects of adding methods and modification types of cellulose on the physicochemical properties of starch/PBAT blown films

This study revealed the relationship between cellulose types/adding methods and film properties, in which sodium carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose (HPMC), and microcrystalline cellulose (MCC) were added into starch/PBAT blown films in powder, aqueous solution, and emulsion forms, respectively. Cellulose interacted with starch networks via hydrogen bonds, and those added in emulsion form made more homogeneous film morphologies. MCC emulsion enhanced the film strength (40%) and modulus (149%) to the greatest extent, while comprehensively, HPMC emulsion possessed better reinforcement effects on the films, which increased mechanical properties (31% ~ 100%), moisture barrier (20%), oxygen barrier (93%), surface hydrophobicity (20%), as well as water resistance (12% ~ 76%). Findings supported the application of cellulose in high-throughput biodegradable films, and the high-content starch/PBAT blown films reinforced by HPMC emulsion had great potential in commercial packaging fields.

The Influence of the Surface Chemistry of Cellulose Nanocrystals on Ethyl Lauroyl Arginate Foam Stability

Guanidine-based surfactant ethyl lauroyl arginate (LAE) and cellulose nanocrystals (CNCs) form complexes of enhanced surface activity when compared to pure surfactants. The LAE-CNC mixtures show enhanced foaming properties. The dynamic thin-film balance technique (DTFB) was used to study the morphology, drainage and rupture of LAE-CNC thin liquid films under constant driving pressure. A total of three concentrations of surfactant and the corresponding mixtures of LAE with sulfated (sCNC) and carboxylated (cCNC) cellulose nanocrystals were studied. The sCNC and cCNC suspension with LAE formed thin films, with stability increasing with surfactant concentration and with complex rheological properties. In the presence of LAE, the aggregation of CNC was observed. While the sCNC aggregates were preferentially present in the film volume with a small fraction at the surface, the cCNC aggregates, due to their higher hydrophobicity, were preferentially located at film interfaces, forming compact layers. The presence of both types of aggregates decreased the stability of the thin liquid film compared to the one for the LAE solution with the same concentration. The addition of CNC to LAE was critical for foam formation, and foam stability was in qualitative agreement with the thin films' lifetimes. The foam volume increased with the LAE concentration. However, there was an optimum surfactant concentration to achieve stable foam. In particular, the very resistant foam was obtained with cCNC suspensions that formed the interfaces with a complex structure and rheology. On the other hand, at high LAE concentrations, the aggregates of CNC may exhibit antifoaming properties.

Probing foams from the nanometer to the millimeter scale by coupling small-angle neutron scattering, imaging, and electrical conductivity measurements

Liquid foams are multi-scale structures whose structural characterization requires the combination of very different techniques. This inherently complex task is made more difficult by the fact that foams are also intrinsically unstable systems and that their properties are highly dependent on the production protocol and sample container. To tackle these issues, a new device has been developed that enables the simultaneous time-resolved investigation of foams by small-angle neutron scattering (SANS), electrical conductivity, and bubbles imaging. This device allows the characterization of the foam and its aging from nanometer up to centimeter scale in a single experiment. A specific SANS model was developed to quantitatively adjust the scattering intensity from the dry foam. Structural features such as the liquid fraction, specific surface area of the Plateau borders and inter-bubble films, and thin film thickness were deduced from this analysis, and some of these values were compared with values extracted from the other applied techniques. This approach has been applied to a surfactant-stabilized liquid foam under free drainage and the underlying foam destabilization mechanisms were discussed with unprecedented detail. For example, the information extracted from the image analysis and SANS data allows for the first time to determine the disjoining pressure vs. thickness isotherm in a real, draining foam.

From Individual Liquid Films to Macroscopic Foam Dynamics: A Comparison between Polymers and a Nonionic Surfactant

Foams can resist destabilizaton in ways that appear similar on a macroscopic scale, but the microscopic origins of the stability and the loss thereof can be quite diverse. Here, we compare both the macroscopic drainage and ultimate collapse of aqueous foams stabilized by either a partially hydrolyzed poly(vinyl alcohol) (PVA) or a nonionic low-molecular-weight surfactant (BrijO10) with the dynamics of individual thin films at the microscale. From this comparison, we gain significant insight regarding the effect of both surface stresses and intermolecular forces on macroscopic foam stability. Distinct regimes in the lifetime of the foams were observed. Drainage at early stages is controlled by the different stress-boundary conditions at the surfaces of the bubbles between the polymer and the surfactant. The stress-carrying capacity of PVA-stabilized interfaces is a result of the mutual contribution of Marangoni stresses and surface shear viscosity. In contrast, surface shear inviscidity and much weaker Marangoni stresses were observed for the nonionic surfactant surfaces, resulting in faster drainage times, both at the level of the single film and the macroscopic foam. At longer times, the PVA foams present a regime of homogeneous coalescence where isolated coalescence events are observed. This regime, which is observed only for PVA foams, occurs when the capillary pressure reaches the maximum disjoining pressure. A final regime is then observed for both systems where a fast coalescence front propagates from the top to the bottom of the foams. The critical liquid fractions and capillary pressures at which this regime is obtained are similar for both PVA and BrijO10 foams, which most likely indicates that collapse is related to a universal mechanism that seems unrelated to the stabilizer interfacial dynamics.

Interfacial protein-protein displacement at fluid interfaces

Protein blends are used to stabilise many traditional and emerging emulsion products, resulting in complex, non-equilibrated interfacial structures. The interface composition just after emulsification is dependent on the competitive adsorption between proteins. Over time, non-adsorbed proteins are capable of displacing the initially adsorbed ones. Such rearrangements are important to consider, since the integrity of the interfacial film could be compromised after partial displacement, which may result in the physical destabilisation of emulsions. In the present review, we critically describe various experimental techniques to assess the interfacial composition, properties and mechanisms of protein displacement. The type of information that can be obtained from the different techniques is described, from which we comment on their suitability for displacement studies. Comparative studies between model interfaces and emulsions allow for evaluating the impact of minor components and the different fluid dynamics during interface formation. We extensively discuss available mechanistic physical models that describe interfacial properties and the dynamics of complex mixed systems, with a focus on protein in-plane and bulk-interface interactions. The potential of Brownian dynamic simulations to describe the parameters that govern interfacial displacement is also addressed. This review thus provides ample information for characterising the interfacial properties over time in protein blend-stabilised emulsions, based on both experimental and modelling approaches.

Investigating pore-opening in hydrogel foams at the scale of free-standing thin films

Controlling the pore connectivity of polymer foams is key for most of their applications, ranging from liquid uptake, mechanics, and acoustic/thermal insulation to tissue engineering. Despite its importance, the scientific phenomena governing the pore-opening processes remain poorly understood, requiring tedious trial-and-error procedures for property optimisation. This lack of understanding is partly explained by the high complexity of the different interrelated, multi-scale processes which take place as the foam transforms from an initially fluid foam into a solid foam. To progress in this field, we take inspiration from long-standing research on liquid foams and thin films to develop model experiments in a microfluidic "Thin Film Pressure Balance". These experiments allow us to investigate isolated thin films under well-controlled environmental conditions reproducing those arising within a foam undergoing cross-linking and drying. Using the example of alginate hydrogel films, we correlate the evolution of isolated thin films undergoing gelation and drying with the evolution of the rheological properties of the same alginate solution in bulk. We introduce the overall approach and use a first set of results to propose a starting point for the phenomenological description of the different types of pore-opening processes and the classification of the resulting pore-opening types. This article is protected by copyright. All rights reserved.

Perspective: Biochemical and Physical Constraints Associated With Preparing Thin Specimens for Single-Particle Cryo-EM

While many aspects of single-particle electron cryo-microscopy (cryo-EM) of biological macromolecules have reached a sophisticated level of development, this is not yet the case when it comes to preparing thin samples on specimen grids. As a result, there currently is considerable interest in achieving better control of both the sample thickness and the amount of area that is useful, but this is only one aspect in which improvement is needed. This Perspective addresses the further need to prevent the macromolecular particles from making contact with the air-water interface, something that can result in preferential orientation and even structural disruption of macromolecular particles. This unwanted contact can occur either as the result of free diffusion of particles during the interval between application, thinning and vitrification of the remaining buffer, or-when particles have been immobilized-by the film of buffer becoming too thin prior to vitrification. An opportunity now exists to apply theoretical and practical insights from the fields of thin-film physical chemistry and interfacial science, in an effort to bring cryo-EM sample preparation to a level of sophistication that is comparable to that of current data collection and analysis.

Cooperative physical separation of oil and suspended solids from methanol-to-olefin wastewater: A pilot study

Methanol-to-olefin (MTO) is an important non-petroleum chemical process for the preparation of light olefins. However, the MTO process consumes copious amounts of water and produces large amounts of untreated effluent. Therefore, the realization of efficient wastewater treatment and recycling is key to the green low-carbon development of MTO. Here, a cooperative process combining swirl regenerating micro-channel separation (SRMS) and combined fibrous coalescence (CFC) technologies was proposed to separate high contents of oil and suspended matter in MTO wastewater. Using a pilot device with a treatment capacity of 1 m3/h, the average oil content in MTO wastewater decreased from 750 mg/L to <30 mg/L, while the average content of suspended matter decreased from 108 mg/L to <15 mg/L. Compared with a commercial MTO wastewater treatment process (olefin production capacity of 0.6 million tons per annum), the proposed method could reduce wastewater discharges and costs by 57% and US$ 0.23 million per annum respectively. Equipment costs and operational energy consumption were also reduced by 30% and >95% respectively. The combined process may provide the basis for the green and sustainable treatment of MTO wastewater and its recycling.

Leveraging dewetting models rather than nucleation models: current crystallographic challenges in interfacial and nanomaterials research

No scientific model has shaped crystallography as much as the classical nucleation theory (CNT). The majority of all growth processes and particle formation processes are attributed to the CNT. However, alternative descriptions exist that may be better suited to explain material formation under certain conditions. One of these alternatives is the dewetting theory (DWT). To describe the possibilities of DWT in more detail, we selected three material systems for three current application areas: Gold particles on silicon as catalysts for nanowire growth, indium particles on molybdenum as precursor material in novel solar cell concepts, and silicon layers on silicon germanium as potential wells in semiconductor quantum computers. Each of these material systems showed particular advantages of DWT over CNT. For example, the properties of surface particles with high atomic mobility could be described more realistically using DWT. Yet, there were clear indications that the DWT is not yet complete and that further research is needed to complete it. In particular, modern crystallographic challenges could serve this purpose, for example the development of semiconductor quantum computers, in order to re-evaluate known models such as the CNT and DWT and adapt them to the latest state of science and technology. For the time being, this article will give an outlook on the advantages of the DWT today and its potential for future research in crystallography.

Interferometry and Simulation of the Thin Liquid Film between a Free-Rising Bubble and a Glass Substrate

Because of their practical importance and complex underlying physics, the thin liquid films formed between colliding bubbles or droplets have long been the subject of experimental investigations and theoretical modeling. Here, we examine the possibility of accurately predicting the dynamics of the thin liquid film drainage using numerical simulations when compared to an experimental investigation of millimetric bubbles free-rising in pure water and colliding with a flat glass interface. A high-speed camera is used to track the bubble bounce trajectory, and a second high-speed camera together with a pulsed laser is used for interferometric determination of the shape and evolution of the thin liquid film profile during the bounce. The numerical simulations are conducted with the open source Gerris flow solver. The simulation reliability was first confirmed by comparison with the experimental bubble bounce trajectory and bubble shape evolution during the bounce. We further demonstrate that the simulation predicted time evolution for the shape of the thin liquid film profiles is in excellent agreement with the high-speed interferometry measured profiles for the entire experimentally accessible film size range. Finally, we discuss the implications of using numerical simulation together with theoretical modeling for resolving the complex processes of high velocity bubble and droplet collisions.

Drainage via stratification and nanoscopic thickness transitions of aqueous sodium naphthenate foam films

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.

Foam film stratification studies probe intermicellar interactions

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.

Gas cell stabilization by aqueous-phase constituents during bread production from wheat and rye dough and oat batter: Dough or batter liquor as model system

Proper gas cell stability during fermentation and baking is essential to obtain high-quality bread. Gas cells in wheat dough are stabilized by the gluten network formed during kneading and, from the moment this network locally ruptures, by liquid films containing nonstarch polysaccharides (NSPs) and surface-active proteins and lipids. Dough liquor (DL), the supernatant after ultracentrifugation of dough, is a model system for these liquid films and has been extensively studied mostly in the context of wheat bread making. Nonwheat breads are often of lower quality (loaf volume and crumb structure) than wheat breads because their doughs/batters lack a viscoelastic wheat gluten network. Therefore, gas cell stabilization by liquid film constituents may be more important in nonwheat than in wheat bread making. This manuscript aims to review the knowledge on DL/batter liquor (BL) and its relevance for studying gas cell stabilization in wheat and nonwheat (rye and oat) bread making. To this end, the unit operations in wheat, rye, and oat bread making are described with emphasis on gas incorporation and gas cell (de)stabilization. A discussion of the knowledge on the recoveries and chemical structures of proteins, lipids, and NSPs in DLs/BLs is provided and key findings of studies dealing with foaming and air-water interfacial properties of DL/BL are discussed. Next, the extent to which DL/BL functionality can be related to bread properties is addressed. Finally, the extent to which DL/BL is a representative model system for the aqueous phase of dough/batter is discussed and related to knowledge gaps and further research opportunities.

Dynamic stabilisation during the drainage of thin film polymer solutions

The drainage and rupture of polymer solutions was investigated using a dynamic thin film balance. The polymeric nature of the dissolved molecules leads to significant resistance to the deformation of the thin liquid films. The influence of concentration, molecular weight, and molecular weight distribution of the dissolved polymer on the lifetime of the films was systematically examined for varying hydrodynamic conditions. Depending on the value of the capillary number and the degree of confinement, different stabilisation mechanisms were observed. For low capillary numbers, the lifetime of the films was the highest for the highly concentrated, narrowly-distributed, low molecular weight polymers. In contrast, at high capillary numbers, the flow-induced concentration differences in the film resulted in lateral osmotic stresses, which caused a dynamic stabilisation of the films and the dependency on molecular weight distribution in particular becomes important. Phenomena such as cyclic dimple formation, vortices, and dimple recoil were observed, the occurrence of which depended on the relative magnitude of the lateral osmotic and the hydrodynamic stresses. The factors which lead to enhanced lifetime of the films as a consequence of these flow instabilities can be used to either stabilise foams or, conversely, prevent foam formation.