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Foams made of gas bubbles dispersed in a liquid have limited stability and disappear rapidly unless surface active species are used. Foams can be a nuisance or very much sought after, however the control over the foaming and stability is still hampered because of the limited understanding of foam properties. On Earth, liquid rapidly drains out of the foam because of gravity, the liquid films formed between bubbles thin and break. In microgravity conditions, gravity drainage is suppressed and stability is expected to be greatly enhanced. We describe investigations of foams that are very unstable on Earth, including foams made with liquids containing antifoaming agents. Experiments performed in the International Space Station (ISS) show that foam generation can still be limited, however once created these foams are very stable. [ABSTRACT FROM AUTHOR]

Abstract: We review here how millifluidic techniques can be used to construct versatile Labs-on-a-Chip, in which on-chip bubble generation and flow chemistry are efficiently combined to generate porous solids with highly monodisperse and ordered pore structures. With this approach, a liquid foam with well-controlled structural and chemical properties is first generated, which is then solidified in situ. We discuss here the interplay of various important processing parameters. In particular, we demonstrate that solidification time and foam life time need to be matched wisely in order to control precisely the properties of the final porous material. We also discuss a simple and inexpensive route to the manufacture of chemically and pressure-resistant, millifluidic Labs-on-a-Chip via the machining and hot-pressing of Cyclic Olefin Copolymers. We illustrate our review using the example of the generation of two types of hydrogel foams (synthetic acrylamide-based and biopolymer chitosan-based) and of particle foams, which may be used as green bodies for the manufacture of porous ceramics. The newly designed materials can be used to investigate fundamental questions of the structure–property relationship of porous solid. [Copyright &y& Elsevier]

Abstract: The thermal behaviour, the physical ageing and the amorphous phase dynamics of polysulfone (PSU) and sulfonated polysulfone (SPSU) are characterized by thermogravimetric analysis and temperature modulated differential scanning calorimetry. The sulfonic group introduction (the sulfonation degree is 70%) in the polymer implies a drastic decrease of the thermal decomposition temperature (220 and 517°C for SPSU and PSU respectively) and a modification of calorimetric parameters (for SPSU, the heat capacity in the glassy state is lower and the glass transition temperature T g is higher than for PSU). In terms of molecular dynamics, the amorphous phase heterogeneities are greater and the cooperative rearranging region size at the glass transition temperature is smaller for SPSU than for PSU. Moreover, after a physical ageing process, the enthalpy recovery kinetic has slowed down by sulfonation. These results can be explained from the existence of sulfonic domains confining the amorphous phase domains. [ABSTRACT FROM AUTHOR]

Abstract: The review addresses the effect of geometrical confinement on the structure formation of colloidal dispersions like particle suspensions, (non)micellar surfactant solutions, polyelectrolyte solutions and mixed dispersions. The dispersions are entrapped either between two fluid interfaces (foam film) in a Thin Film Pressure Balance (TFPB) or between two solid interfaces in a Colloidal Probe Atomic Force Microscope (Colloidal Probe AFM) or a Surface Force Apparatus (SFA). The oscillating concentration profile in front of the surface leads to an oscillating force during film thinning. It is shown that the characteristic lengths like the distance between particles, the distance between micelles, or the mesh size of the polymer network remain the same during the confining process. The influence of different parameters like ionic strength, molecular structure, and the properties of the outer surfaces on the structure formation are reported. The confinement of mixed dispersions might lead to phase separation and capillary condensation, which in turn causes a pronounced attraction between the two opposing film surfaces. [Copyright &y& Elsevier]

Abstract: Mixtures of the two non-ionic surfactants hexaoxyethylene dodecyl ether (C12E6) and n-dodecyl-β-d-maltoside (β-C12G2) were studied with regard to surface properties, bulk properties, foam films, and foams. The reason for studying a mixture of an ethylene oxide (C i E j ) and a sugar (C n G m ) based surfactant is that despite being non-ionic, these two surfactants behave quite differently. Firstly, the physico-chemical properties of aqueous solutions of C n G m surfactants are less temperature-sensitive than those of C i E j solutions. Secondly, the surface charge density q 0 of foam films stabilized by C n G m surfactants is pH insensitive down to the so-called isoelectric point, while that of foam films stabilized by C i E j surfactants changes linearly with the pH. The third difference is related to interaction forces between solid surfaces. Under equilibrium conditions very high forces are needed to expel β-C12G2 from between thiolated gold surfaces, while for C12E6 low loads are sufficient. Fourthly, the adsorption of C12E6 and β-C12G2 on hydrophilic silica and titania, respectively, is inverted. While the surface excess of C12E6 is large on silica and negligible on titania, β-C12G2 adsorbs very little on silica but has a large surface excess on titania. What is the reason for this different behaviour? Under similar conditions and for comparable head group sizes, it was found that the hydration of C i E j surfactants is one order of magnitude higher but on average much weaker than that of C n G m surfactants. Moreover, C n G m surfactants possess a rigid maltoside unit, while C i E j surfactants have a very flexible hydrophilic part. Indeed, most of the different properties mentioned above can be explained by the different hydration and the head group flexibilities. The intriguing question of how mixtures of C i E j and C n G m surfactants would behave arises organically. Thus various properties of C12E6 +β-C12G2 mixtures in aqueous solution have been studied with a focus on the 1:1 mixture. The results are compared with those of the single surfactants and are discussed accordingly. [Copyright &y& Elsevier]

Abstract: We study the sponge phase of the mixed non-ionic/ionic surfactant system C14DMAO–TTAB–hexanol–brine. Our aim is to determine if this phase exists in this mixed system and if it preserves or changes its structure when the relative amount of the charged surfactant is increased in the mixture. SAXS, FFEM, and conductivity results show that for the same bilayer volume fraction the sponge phase preserves its global structure. We propose a method to determine the geometrical obstruction factor from electrical conductivity measurements in ionic sponge phases. Analysis of lamellar phases in the same system shows that the bilayer thickness increases when the ionic surfactant concentration is increased. [Copyright &y& Elsevier]

Abstract: Composite materials are usually reinforced by synthetic fibers as carbon, glass etc…. Because of their good mechanical properties and low density, natural fibers are being considered more preferentially as reinforcement. The application of natural fibers as reinforcements in composite materials requires a strong adhesion between the fiber and the matrix. The poor resistance towards water absorption is one of the drawbacks of natural fibers which makes it more important to understand the dynamic properties of penetration of water molecules through these heterogeneous materials. Water vapour sorption kinetics in natural flax fibers have been performed at 25°C by using an electronic microbalance (IGA, Hiden). By using the Fickian model for a complete cylinder water diffusion coefficients have been determined and calculated at short times (first half-sorption) and long times (second half-sorption) of kinetic curve and for different water activities. The values obtained for D 1 and D 2 are rather similar on the all range of water activity. Generally, water diffusivity increases and then decreases with water activity. The increase of diffusivity at low water activities may be explained on the basis of the dual mode sorption (Langmuir and Henry sorption’s combination) whereas the decrease for the higher activities can be attributed to the immobilization of sorbed water molecules due to the water clustering. [Copyright &y& Elsevier]

Abstract: Poly(dimethylsiloxane) (PDMS)-HTiNbO5 nanocomposite membranes with various HTiNbO5 nanofiller content were prepared by melt intercalation. WAXS diffraction measurements and TEM observations have suggested that the HTiNbO5 mineral was exfoliated in the PDMS matrix. The influence of the filler in the membrane was evaluated by water diffusion, gas permeation (CO2, N2, O2, ethane and ethylene), toluene pervaporation and by CO2 sorption measurements. A filler content of only 2wt.% in PDMS-HTiNbO5 nanocomposite membranes slows down the water diffusion significantly, and a filler content of 5wt.% reduces also the permeability of the films for toluene. The addition of a filler content up to 10wt.% do not significantly influences the gas permeability (P) except for CO2. The PDMS matrix appears to be highly permeable and, therefore, a decreasing effect on P is only marked for a very high HTiNbO5 content. This effect is more pronounced for CO2, the P value of which decreases by 80% when the amount of nanofiller is 40wt.%. The sorption measurements show that the interaction between CO2 and PDMS is weak (isotherms agree with Henry’s law). The filler decreases the solubility of CO2 in the films (S =7.94×10−3 and S =5.44×10−3 cm3 STPcm−3 filmcmHg−1 for PDMS and PDMS-HTiNbO5 40wt.%, respectively). [Copyright &y& Elsevier]

Abstract: We report results on foamability, stability and coarsening of foams made either of surfactant (SDS) or of milk protein (casein) solutions. Studies have been performed at the scales of the gas–liquid interface, thin liquid film and bubble size, in order to find the correlations between these different scales, and to elucidate the microscopic origins of the macroscopic features. For both systems, foamability concentration thresholds have been measured, and a bubble size dependence has been found. A clear correlation between the stability of an isolated thin film and the foam stability is always evidenced. However, the mechanism of stability of the casein thin films is different from the surfactant one, and related to the confinement and percolation of casein aggregates. We also report results on coarsening at constant liquid fraction, showing that the protein foams coarsen more slowly than the surfactant ones, and that it is due to differences in thin film thickness. [Copyright &y& Elsevier]

Metallic gratings can be used as infrared filters, but their performance is often limited by bandwidth restrictions due to metallic losses. In this work, we propose a metallic groove-slit-groove (GSG) structure that overcomes these limitations by exhibiting a large bandwidth, angularly independent, extraordinary optical transmission. Our design achieves high transmission efficiency in the longwave infrared range, driven by Fano-type resonances created through the interaction between the grooves and the central slit. This mechanism results in a tunable 2 µm transmission window with high rejection rate. We extend the concept to a two-dimensional GSG array, exhibiting a polarization-insensitive 80% transmission window for incident angles up to 50°, offering significant potential for infrared filtering applications.

We have studied the coalescence of oil in water emulsions under the influence of gravity. The emulsions were made with alkane oils and surfactants with varying physical chemistry. We chose cationic alkyl trimethylammonium bromides of different chain lengths and nonionic surfactants of ethylene oxide and sugar head groups, including polymeric surfactants. We observed phase separation in two steps. Creaming of the oil drops is followed by their rapid coalescence, increasing the average drop size and resulting in complete surfactant surface coverage of the interfaces. Full phase separation occurs after much longer times T c when the emulsion drops coalesce dramatically. We have used a model by Dinh et al. to relate T c to the coalescence frequency and hence to the activation energy for the rupture of the films between two neighboring drops. Our results support the view that the coalescence of stable emulsions (stable at least for a few hours) is a thermally activated process and is controlled by the surface compression elastic modulus. This modulus was determined using surface tension measurements and calculations using the Gibbs adsorption equation. The observed differences between ionic and nonionic systems are attributed to a two-step film rupture process in the case of ionic surfactants, which is not found in nonionic systems.

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.

Effective cross sections of nano-objects are fundamental properties that determine their ability to interact with light. However, measuring them for individual resonators directly and quantitatively remains challenging, particularly because of the very low signals involved. Here, we experimentally measure the thermal emission cross section of metal-insulator-metal nanoresonators using a stealthy hyperuniform distribution based on a hierarchical Poisson-disk algorithm. In such distributions, there are no long-range interactions between antennas, and we show that the light emitted by such metasurfaces behaves as the sum of cross sections of independent nanoantennas, enabling direct retrieval of the single resonator contribution. The emission cross section at resonance is found to be on the order of λ_{0}^{2}/3, a value that is nearly 3 times larger than the theoretical maximal absorption cross section of a single particle, but remains smaller than the maximal extinction cross section. This measurement technique can be generalized to any single resonator cross section, and we also apply it to a lossy dielectric layer.

Nanophotonic devices manipulate light at sub-wavelength scales, enabling tasks such as light concentration, routing, and filtering. Designing these devices to achieve precise light-matter interactions using structural parameters and materials is a challenging task. Traditionally, solving this problem has relied on computationally expensive, iterative methods. In recent years, deep learning techniques have emerged as promising tools for tackling the inverse design of nanophotonic devices. While several review articles have provided an overview of the progress in this rapidly evolving field, there is a need for a comprehensive tutorial that specifically targets newcomers without prior experience in deep learning. Our goal is to address this gap and provide practical guidance for applying deep learning to individual scientific problems. We introduce the fundamental concepts of deep learning and critically discuss the potential benefits it offers for various inverse design problems in nanophotonics. We present a suggested workflow and detailed, practical design guidelines to help newcomers navigate the challenges they may encounter. By following our guide, newcomers can avoid frustrating roadblocks commonly experienced when venturing into deep learning for the first time. In a second part, we explore different iterative and direct deep learning-based techniques for inverse design, and evaluate their respective advantages and limitations. To enhance understanding and facilitate implementation, we supplement the manuscript with detailed Python notebook examples, illustrating each step of the discussed processes. While our tutorial primarily focuses on researchers in (nano-)photonics, it is also relevant for those working with deep learning in other research domains. We aim at providing a solid starting point to empower researchers to leverage the potential of deep learning in their scientific pursuits., Competing Interests: Conflict of interest: Authors state no conflicts of interest., (© 2023 the author(s), published by De Gruyter, Berlin/Boston.)

Coarsening of two-phase systems is crucial for the stability of dense particle packings such as alloys, foams, emulsions, or supersaturated solutions. Mean field theories predict an asymptotic scaling state with a broad particle size distribution. Aqueous foams are good model systems for investigations of coarsening-induced structures, because the continuous liquid as well as the dispersed gas phases are uniform and isotropic. We present coarsening experiments on wet foams, with liquid fractions up to their unjamming point and beyond, that are performed under microgravity to avoid gravitational drainage. As time elapses, a self-similar regime is reached where the normalized bubble size distribution is invariant. Unexpectedly, the distribution features an excess of small roaming bubbles, mobile within the network of jammed larger bubbles. These roaming bubbles are reminiscent of rattlers in granular materials (grains not subjected to contact forces). We identify a critical liquid fraction [Formula: see text], above which the bubble assembly unjams and the two bubble populations merge into a single narrow distribution of bubbly liquids. Unexpectedly, [Formula: see text] is larger than the random close packing fraction of the foam [Formula: see text]. This is because, between [Formula: see text] and [Formula: see text], the large bubbles remain connected due to a weak adhesion between bubbles. We present models that identify the physical mechanisms explaining our observations. We propose a new comprehensive view of the coarsening phenomenon in wet foams. Our results should be applicable to other phase-separating systems and they may also help to control the elaboration of solid foams with hierarchical structures.

We report foam coarsening studies which were performed in the International Space Station (ISS) to suppress drainage due to gravity. Foams and bubbly liquids with controlled liquid fractions ϕ between 15 and 50% were investigated to study the transition between bubble growth laws previously reported near the dry limit ϕ → 0 and the dilute limit ϕ → 1 (Ostwald ripening). We determined the coarsening rates for the driest foams and the bubbly liquids, they are in close agreement with theoretical predictions. We observe a sharp cross-over between the respective laws at a critical value ϕ *. At liquid fractions beyond this transition, neighboring bubbles are no longer all in contact, like at a jamming transition. Remarkably ϕ * is significantly larger than the random close packing volume fraction of the bubbles ϕ rcp which was determined independently. We attribute the differences between ϕ * and ϕ rcp to a weakly adhesive bubble interaction that we have studied in complementary ground-based experiments.

In this perspective paper, we highlight the numerous open problems in the topic of stability of emulsions and foams, focusing on the simplest case of dispersions stabilized by surfactants. There are three main destabilization processes, gravity induced evolution, Ostwald ripening, and drops or bubble coalescence, which are analyzed separately. The discussion is restricted to the case of Newtonian fluids, deprived of microstructure, except for the presence of micelles. Thanks to continuing efforts and recent breakthroughs, we show that the understanding of emulsion and foam stability is progressing. Many problems are still open, however, and much work remains to be done along the lines outlined in the paper.

Sub-wavelength metallic grooves behave as Fabry-Perot nanocavities able to resonantly enhance the absorption of light as well as the intensity of the electromagnetic field. Here, with a one-mode analytical model, we investigate the effect of a correlated disorder on 1D groove arrays i.e., randomly shaped and positioned grooves on a metallic layer. We show that a jitter-based disorder leads to a redistribution of energy compared to the periodic case. In an extreme case, a periodic diffracting array can be converted into a highly scattering array (98% at λ = 2.8 µm with a 1 µm full width at half maximum). Eventually, we show that the optical response of combinations of variously shaped grooves can be well described by the individual sub-set behaviors.

This short review describes the work on aqueous foam film stability with the important past contributions of Dotchi Exerowa and Dimo Platikanov, together with advances from other research groups. The review is focused on film rupture, for which few controlled experiments can be found in the literature and as a consequence, our understanding is still limited. The work on rupture of films in foams is described, together with the correlations with the rupture of isolated films. The review addresses mainly the case of aqueous films and foams, but analog studies of emulsions and emulsion films are also briefly discussed., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The mechanical properties of lipid monolayers and their responses to shear and compression stresses play an important role in processes such as breathing and eye blinking. We studied the mechanical properties of Langmuir monolayers of a model mixture, composed of an unsaturated lipid, 1-palmitoyl-2-oleoyl- sn -glycero-phosphoethanolamine (POPE), and a saturated lipid, 1,2-dipalmitoyl- sn -glycero-phosphocholine (DPPC). We performed isothermal compressions and sinusoidal shear deformations of these mixed monolayers. Also, the different phases were observed with Brewster angle microscopy. We found that the mechanical behavior is affected by the miscibility of both lipids. In the two-phase region, the compression elastic modulus increases with the amount of the LC phase but does not follow the predictions of a simple effective medium model. The discrepancies arise from the fact that, upon compression, the domains grow at a rate faster than the compression rate but not fast enough to reach thermodynamic equilibrium. Before reaching the LC phase, domain percolation is observed and compression and shear moduli become equal to those of the pure LC phase. Most of the monolayers behave as viscoelastic fluids at the frequencies investigated. A minimum in the compression modulus and shear viscosity was observed for mixtures close to equimolar composition, with the minimum being accompanied by a change in domain shapes.

We studied the dynamics of a cationic surfactant monolayer, Gemini 12-2-12, at the air?water interface for surfactant aqueous solutions at concentrations below the critical micelle concentration. We present surface rheology experiments performed in a Langmuir trough by the oscillatory barrier technique. From these, we found negative surface viscosities at certain frequencies. We demonstrate that this unphysical result is a consequence of an unconsidered surfactant dynamics within the interfacial region. By surface pressure relaxation experiments, after a sudden modification of the interfacial area and by dynamic surface tension and surface potential measurements, several relaxation phenomena and relaxation times were identified. We found that surfactant adsorption and desorption processes are asymmetric: the characteristic times and the number of processes involved in the mechanisms of adsorption and desorption are different. This asymmetry invalidates the usual data analysis procedure that leads to the negative viscosities. Similar mechanisms could be at the origin of the negative viscosities reported in other systems, a possibility that remains to be explored.

The coarsening of quasi-2D wet foams is well described theoretically by the model of Schimming and Durian, that takes into account the diffusion through the Plateau borders and the vertices in a rigorous manner. In this article, we describe an experimental study of coarsening in which the foam film permeability is measured in such quasi-2D wet foams. We first performed a full characterization of the structure of the studied foams. Then we measured the coarsening rates. It appears that, in these foams, the film thicknesses are still too small for the Plateau borders and the vertices to contribute, but the surface Plateau borders lead to a smaller coarsening rate compared to dry foams. This rate increases with capillary pressure and follows well the prediction of the model. We demonstrate the importance of working in controlled pressure conditions during permeability measurements. Indeed, permeability depends on film thickness itself depending on capillary pressure.

While coalescence is ultimately the most drastic destabilization process in foams, its underlying processes are still unclear. To better understand them, we track individual coalescence events in two-dimensional foams at controlled capillary pressure. We obtain statistical information revealing the influence of the different parameters which have been previously proposed to explain coalescence. Our main conclusion is that coalescence probability is simply proportional to the area of the thin film separating two bubbles, suggesting that coalescence is mostly stochastic.

We have studied emulsions made with two- and three-phase oil-water-surfactant systems in which one of the phases is a microemulsion, the other phases being water or/and oil excess phases. Such systems have been extensively studied in the 1970-1980s for applications in enhanced oil recovery. It was found at that time that the emulsions became very unstable in the three-phase systems, but so far few explanations have been proposed. In the most complete one, Kabalnov and colleagues related the emulsion stability to the probability of hole nucleation in the liquid film separating two nearby emulsion drops and associated this probability to the curvature elastic energy of the surfactant layer covering drop surfaces. We propose a different explanation, linked to another type of interfacial elastic energy, associated with compression of the surfactant layers. As found long ago, the three-phase systems are found near optimum formulation (hydrophile lipophile difference, HLD = 0), where the interfacial tension exhibits a deep minimum. The determination of interfacial elastic properties in low interfacial tension systems is not straightforward. In our present work, we used a spinning drop tensiometer with an oscillating rotation velocity. We show that the interfacial compression elastic modulus and viscosity also exhibit a minimum at optimum formulation. We propose that this minimum is related to the acceleration of the surfactant exchanges between the interface, oil and water, near the optimum formulation. Furthermore, we find that the surfactant partitions close to equally between oil and water at the optimum, as in earlier studies. The interfacial tension gradients that slow the thinning of liquid films between drops are reduced by surfactant exchanges between drops and the interface, which are fast whatever the type of drop, oil or water; film thinning is therefore very rapid, and emulsions are almost as unstable as in the absence of surfactant.

When surfactants adsorb at liquid interfaces, they not only decrease the surface tension, they confer rheological properties to the interfaces. There are two types of rheological parameters associated to interfacial layers: compression and shear. The elastic response is described by a storage modulus and the dissipation by a loss modulus or equivalently a surface viscosity. Various types of instruments are available for the measurements of these coefficients, the most common being oscillating pendent drops instruments and rheometers equipped with bicones. These instruments are applicable to systems with large enough interfacial tensions, typically above a few mN/m. We use a new type of instrument based on spinning drop oscillations, allowing to extend the interfacial rheology studies to low and ultralow interfacial tension systems. We present examples of measurements with systems of high and low tension, discuss the possible artifacts and demonstrate the capability of this new technique. We emphasize that the data shown for low interfacial tensions are the first reported in the literature. The instrument is potentially interesting for instance in enhanced oil recovery or demulsification studies., (Copyright © 2018 Elsevier Inc. All rights reserved.)

A central challenge for the safe design of nanomaterials (NMs) is the inherent variability of NM properties, both as produced and as they interact with and evolve in, their surroundings. This has led to uncertainty in the literature regarding whether the biological and toxicological effects reported for NMs are related to specific NM properties themselves, or rather to the presence of impurities or physical effects such as agglomeration of particles. Thus, there is a strong need for systematic evaluation of the synthesis and processing parameters that lead to potential variability of different NM batches and the reproducible production of commonly utilized NMs. The work described here represents over three years of effort across 14 European laboratories to assess the reproducibility of nanoparticle properties produced by the same and modified synthesis routes for four of the OECD priority NMs (silica dioxide, zinc oxide, cerium dioxide and titanium dioxide) as well as amine-modified polystyrene NMs, which are frequently employed as positive controls for nanotoxicity studies. For 46 different batches of the selected NMs, all physicochemical descriptors as prioritized by the OECD have been fully characterized. The study represents the most complete assessment of NMs batch-to-batch variability performed to date and provides numerous important insights into the potential sources of variability of NMs and how these might be reduced.

Foamed emulsions are ubiquitous in our daily life but the ageing of such systems is still poorly understood. In this study we investigate foam drainage and measure the evolution of the gas, liquid and oil volume fractions inside the foam. We evidence three regimes of ageing. During an initial period of fast drainage, both bubbles and drops are very mobile. As the foam stabilises drainage proceeds leading to a gradual decrease of the liquid fraction and slowing down of drainage. Clusters of oil drops are less sheared, their dynamic viscosity increases and drainage slows down even further, until the drops become blocked. At this point the oil fraction starts to increase in the continuous phase. The foam ageing leads to an increase of the capillary pressure until the oil acts as an antifoaming agent and the foam collapses.

We review the coarsening process of foams made with various surfactants and gases, focusing on physico-chemical aspects. Several parameters strongly affect coarsening: foam liquid fraction and foam film permeability, this permeability depending on the surfactant used. Both parameters may evolve with time: the liquid fraction, due to gravity drainage, and the film permeability, due to the decrease of capillary pressure during bubble growth, and to the subsequent increase in film thickness. Bubble coalescence may enhance the bubble's growth rate, in which case the bubble polydispersity increases. The differences found between the experiments reported in the literature and between experiments and theories are discussed., (Copyright © 2015 Elsevier B.V. All rights reserved.)

We review the existing literature on asphaltenes at various types of interfaces: oil-water, air-water, gas-oil and solid-liquid, with more emphasis on the oil-water interfaces. We address the role of asphaltene aggregation, recently clarified for asphaltenes in bulk by the Yen-Mullins model. We discuss the questions of adsorption reversibility and interfacial rheology, especially in connection with emulsion stability., (Copyright © 2015 Elsevier B.V. All rights reserved.)

We studied the stability of foams containing small bubbles (radius ≲ 50  μm). The foams are made from aqueous surfactant solutions containing various amounts of glycerol. The foams start breaking at their top, when the liquid volume fraction has decreased sufficiently during liquid drainage. Unlike in foams with larger bubbles, the liquid fraction at which the foam destabilizes is surprisingly high. In order to interpret this observation we propose that film rupture occurs during reorganization events (T1) induced by bubble coarsening, which is particularly rapid in the case of small bubbles. New films are therefore formed rapidly and if their thickness is too small, they cannot be sufficiently covered by surfactant and they break. Using literature data for the duration of T1 events and the thickness of the new films, we show that this mechanism is consistent with the behavior of the foams studied.

The stability of foams made with sponge phases (L3 phases) and lamellar phases (L(α) phases), both containing surfactant bilayers, has been investigated. The extreme stability of foams made with lamellar phases seems essentially due to the high viscosity of the foaming solution, which slows down gravity drainage. Moreover, the foams start draining only when the buoyancy stress overcomes the yield stress of the L(α) phase. The bubble growth associated with gas transfer is unusual: it follows a power law with an exponent smaller than those corresponding to Ostwald ripening (wet foams) and to coarsening (dry foams). The foams made with sponge phases are in turn very unstable, even less stable than pure surfactant foams made with glycerol solutions having the same viscosity. The fact that the surfactant bilayers in the sponge phase have a negative Gaussian curvature could facilitate bubble coalescence.

We show that, while the gelation of colloidal silica proceeds much faster in the presence of added KCl than NaCl, the final gels are very similar in structure and properties. We have studied the gelation process by visual inspection and by small angle X-ray scattering for a range of salt and silica particle concentrations. The characteristic times of the early aggregation process and the formation of a stress-bearing structure with both salts are shown to collapse onto master curves with single multiplicative constants, linked to the stability ratio of the colloidal suspensions. The influence of the salt type and concentration is confirmed to be mainly kinetic, as the static structure factors and viscoelastic moduli of the gels are shown to be equivalent at normalized times. While there is strong variation in the kinetics, the structure and properties of the gel at long-times are shown to be mainly controlled by the concentration of particles, and hardly influenced by the type or the concentration of salt. This suggests that the differences between gels generated by different salts are only transient in time., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Ultrastable foams are made very simply by adding salt (NaCl or KCl) to sodium dodecyl sulfate. The addition of high concentrations of salt leads to the precipitation of the surfactant on the bubble surfaces and as crystals in the interstices between the bubbles. As a consequence, the ageing of the foams is stopped to make them stable indefinitely, or until they are heated above the melting temperature of the crystals. The use of KCl is shown to be much more effective than that of NaCl because potassium dodecyl sulfate has a higher melting temperature and faster rates of crystallization. The crystalline structures have been investigated inside the foam using small angle neutron scattering. The larger lattice spacing of the crystals formed with NaCl in comparison with KCl has been evidenced. These simple temperature stimulable foams could have many potential applications., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The paper discusses adsorption and desorption energy barriers for macroscopic interfaces of surfactant solutions. Literature data suggest that adsorption and desorption are not always fully diffusion controlled. Apart from electrostatic barriers that lead to strong deviations, other types of barriers are less easy to identify, because smaller deviations from diffusion controlled mechanisms are evidenced. Complete models involving both diffusion and sorption barriers are very complex and involve many adjustable parameters, making the data analysis frequently unreliable. Empirical equations of state are used in most cases, although they are inaccurate, especially close to the cmc. The variation of sorption energies with surface concentration is not accurately described in the models. Finally, convection can mask the effect of sorption energy barriers. Experiments are presented to illustrate the main difficulties encountered., (Copyright © 2014 Elsevier B.V. All rights reserved.)

We describe forced drainage experiments of foams made with model surfactant solutions with different surface rheology. We analyze the origin of two distinct drainage transitions reported in the literature, between regimes where the bubble surfaces are mobile or rigid. We propose that both transitions are related to the surface shear viscosity and to its shear thinning behavior. Shear thinning could also account for the huge discrepancies between measurements reported in the literature. The role of surface tension gradients, i.e. Marangoni effect, could not possibly explain the behavior observed with the different solutions., (Copyright © 2014 Elsevier Inc. All rights reserved.)

We address the rheology of assemblies of surfactant-decorated silica nanoparticles irreversibly adsorbed at the gas/liquid interface. Positively charged surfactant molecules (such as CTAB) bind to silica nanoparticle surfaces, and the resulting particle-surfactant complexes adsorb at gas/liquid interfaces. The surfactant molecules control the wettability of such decorated nanoparticles and their adsorption. The interparticle forces can be tuned by changing the surfactant concentration Cs. Increasing Cs, in addition to a decrease of the particles wettability, leads to an increase of the area fraction of particles at the interface. Oscillatory shear measurements (strain- and frequency-sweep) have been performed. Here, we explore the effect of the surfactant concentration Cs. At high enough Cs, the interface is highly packed, and an overall solidlike response is observed, with 2D glass properties.

Antifoams are widely used to control or to avoid foam production. In order to work, antifoam particles need to break foam films efficiently, which many antifoams do very well. However, once they have broken a film, to continue to be effective they need to be transported to the next film. We show, for the first time, that buoyancy has an important part in the transport of the antifoam particles. In microgravity, where buoyancy and gravitational drainage are strongly slowed down, diffusion leads to poor antifoam performance. The foam is stable for the duration of the experiment, whereas on Earth the foam starts to disappear immediately. Indeed, microgravity renders highly efficient antifoam practically useless., Competing Interests: The authors declare no conflict of interest.

We aim to produce emulsions that can act as contrast agents and drug carriers for cancer imaging and therapy. To increase tumor detection and decrease drug side effects, it is desirable to take advantage of the enhanced permeability and retention effect that allows nanoparticles to accumulate in tumor tissues. To do so, the emulsion droplets need to be small enough and stable over time in addition to enhancing image contrast and carrying a drug payload. In the present study, we have investigated the properties and potentiality as theranostic agents of perfluorocarbon emulsions stabilized by a biocompatible fluorinated surfactant called FTAC. To obtain better control of our system, the synthesis of those surfactants was studied and their physico-chemical properties were explored in different configurations such as micelles, in the perfluorocarbon droplet shell and at water/air and water/perfluorocarbon interfaces. The originality of this work lies in the determination of numerous characteristics of emulsions and fluorinated surfactants including surface tension, interfacial tension, critical micelle concentration, adiabatic compressibility, density, size distribution (aging studies), and ultrasonic echogenicity. These characterization studies were conducted using different types of FTAC and several perfluorocarbons (perfluoropentane, perfluorohexane, and perfluorooctyl bromide). We have also shown that a hydrophobic drug could be encapsulated in the FTAC-stabilized perfluorocarbon droplets thanks to triacetin addition. Finally, the perfluorocarbon emulsions were detectable in vitro by a clinical 3 T MRI scanner, equipped with a double frequency 19 F/ 1 H transmit-receive coil.

Ceramides have unique biophysical properties. Their high melting temperature and their ability to form lateral domains have converted ceramides into the paradigm of rigid lipids. Here, using shear surface rheology of egg-ceramide Langmuir monolayers, a solid to fluid transition was evidenced as a vanishing shear rigidity at lower temperatures than the lipid melting temperature. Such a mechanical transition, which depends on the lipid lateral pressure, was found in a broad range temperature (40-50 °C). The solid to fluid transition was correlated to a LC to LC+LE phase transition, as confirmed by BAM experiments. Interestingly, together with the softening transition, a supercooling process compatible with a glassy behavior was found upon freezing. A new phase scenario is then depicted that broadens the mechanical behavior of natural ceramides. The phase diversity of ceramides might have important implications in their physiological roles.

Quasi-two-dimensional (quasi-2D) foams consist of monolayers of bubbles squeezed between two narrowly spaced plates. These simplified foams have served successfully in the past to shed light on numerous issues in foam physics. Here we consider the electrical conductivity of such model foams. We compare experiments to a model which we propose, and which successfully relates the structural and the conductive properties of the foam over the full range of the investigated liquid content. We show in particular that in the case of quasi-2D foams the liquid in the nodes needs to be taken into account even at low liquid content. We think that these results may provide different approaches for the characterization of foam properties and for the in situ characterization of the liquid content of foams in confining geometries, such as microfluidics.

We have studied foams stabilised by surfactant-decorated nanoparticles adsorbed at the bubble surfaces. We show that the controlled compression of a single bubble allows one to understand the coarsening behavior of these foams. When bubbles are compressed, the particles become tightly packed in the surface layer. They lose their mobility, and the interface becomes solid-like when the jammed state is reached. Further compression leads to interfacial buckling characterised by crumpled surfaces. We find that the surface concentration of particles at which the jamming and the buckling transitions occur are independent of the surfactant concentration. This is a surprising feature. It suggests that the surfactants are mandatory to help the particles adsorb at the interface and that they change the equilibrium surface concentration of the decorated particles. But they do not affect the surface properties once the particles are adsorbed. We measured the compression elastic modulus of the surface in the jammed state and found it to be compatible with the Gibbs condition for which the spontaneous dissolution of bubbles is arrested. Due to this effect, the coarsening process of a foam composed of many close-packed bubbles occurs in two steps. In the first step, coarsening is slow and coalescence of the bigger bubbles is observed. In the second step, a number of very small bubbles remains, which exhibit crumpled surfaces and are stable over long times. This suggests that foam coarsening is arrested once the smallest bubbles become fully covered after the initial shrinking step.

The knowledge of surface shear rheology is important to understand and model flow in systems where interfaces are present: multiphase flow, wetting, foaming and others. The topic has been investigated for more than 100 years, but the knowledge accumulated is still partial. The experimental devices used for the measurement of the viscoelastic parameters are delicate to operate and the response of the monolayers is complex, usually non-linear and time dependent. Furthermore, it is difficult to decouple from the response of the bulk liquid. Important discrepancies between microscopic and macroscopic methods were reported and remain to be clarified. The knowledge of shear properties does not suffice in general to achieve proper descriptions of the flow behavior and measurements of compression properties are needed as well. This paper presents examples taken from the literature and discusses the current level of understanding., (Copyright © 2013 Elsevier B.V. All rights reserved.)

What are the lifetime and maximum length of a soap film pulled at a velocity V out of a bath of soapy solution? This is the question we explore in this article by performing systematic film rupture experiments. We show that the lifetime and maximal length of the films are fairly reproducible and controlled only by hydrodynamics. For surfactants with high surface elastic modulus, we argue that the rupture is triggered by the expansion of a thinning zone at the top of the film. The length ltz of this zone expands with time at a velocity equal to V/2, which can be obtained by a balance between gravity and viscous forces. The film lifetime is then found to decrease with the pulling velocity V, which implies that the surface tension gradient along the film increases with V. This surface tension gradient is found to be surprisingly small. Finally, the lifetime of films stabilised by solutions with small surface elastic modulus turns out to be much shorter than the ones for films with rigid interfaces.

In the presence of soluble surfactants, the motion of liquid surfaces involves Marangoni effects. As a consequence, the surfaces exhibit elastic responses, even frequently behaving as rigid surfaces, especially at low surfactant concentration. The Marangoni effects can be conveniently quantified introducing surface viscoelastic compression parameters that characterize the mechanical response of the surface near equilibrium. Many experimental techniques allow measuring the viscoelastic parameters. However, many difficulties are encountered during the interpretation of the surface response in the various types of hydrodynamic velocity fields involved in the different techniques. The role of adsorption and desorption energy barriers appears crucial, despite the fact that little is known yet about their values. In this short review, we will present examples illustrating the different problems., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Foams and foaming pose important questions and problems for both fundamental research and practical applications. Despite the fact that foams have been extensively studied, many aspects of foam physics and chemistry still remain unclear. Experiments on foams performed under microgravity can be extended far beyond their counterpart where gravity is fully present (i.e. most experiments on Earth). They allow, in particular, observation of the wet foams obtained during the foaming process; on Earth, foams at this stage evolve too quickly due to gravity drainage and cannot be studied. This paper reviews the existing studies of foams under microgravity, which include studies in parabolic flights, in sounding rockets and in the International Space Station.

Obtaining stable liquid foams is an important issue in view of their numerous applications. In some of these, the liquid foam in itself is of interest, in others, the liquid foam acts as a precursor for the generation of solid foam. In this short review, we will make a survey of the existing results in the area. This will include foams stabilised by surfactants, proteins and particles. The origin of the stability is related to the slowing down of coarsening, drainage or coalescence, and eventually to their arrest. The three effects are frequently coupled and in many cases, they act simultaneously and enhance one another. Drainage can be arrested if the liquid of the foam either gels or solidifies. Coalescence is slowed down by gelified foam films, and it can be arrested if the films become very thick and/or rigid. These mechanisms are thus qualitatively easy to identify, but they are less easy to model in order to obtain quantitative predictions. The slowing down of coarsening requests either very thick or small films, and its arrest was observed in cases where the surface compression modulus was large. The detail of the mechanisms at play remains unclear., (© 2013 Elsevier B.V. All rights reserved.)

The present work addresses the fundamental question of membrane elasticity of ceramide layers with a special focus on the plastic regime. The compression and shear viscoelasticity of egg-ceramide Langmuir monolayers were investigated using oscillatory surface rheology in the linear regime and beyond. High compression and shear moduli were measured at room temperature-a clear signature for a solid behavior. At deformations larger than one per mill, egg-ceramide monolayers display plastic features characterized by a decrease of the storage modulus followed by a viscous regime typical of fluid lipids. This behavior is accompanied by a marked decrease of the loss modulus with increasing stress above a yield point. The results permit to univocally classify ceramide monolayers as 2D solids able to undergo plastic deformations, at the difference of typical fluid lipid monolayers. These unusual features are likely to have consequences in the mechanical behavior of ceramide-rich emplacements in biological membranes.

We show an increase of the sedimentation velocity as small particles are confined in circular capillaries. In general, confinement slows down sedimentation. But, we show that at low Reynolds numbers and in 1D confinement this is not the case. Particle sedimentation velocity is not homogeneous, which can lead to the formation of structures. These structures are enhanced and stabilized in the presence of walls and in the absence of other dissipative mechanisms. As a consequence, it is possible to achieve sedimentation velocities that even exceed the Stokes velocity. The segregation at critical capillary diameters has been directly observed using a large scale model. These simple experiments offer a new insight into the old problem of sedimentation under confinement.

We have investigated the adsorption and organization at the air/water interface of catanionic molecules released from a dispersion of solid-like catanionic vesicles composed of myristic acid and cetyl trimethylammonium chloride at the 2:1 ratio. These vesicles were shown recently to be promising foam stabilizers. Using Brewster angle microscopy, we observed the formation of a catanionic monolayer at the air/water interface composed of liquid-condensed domains in a liquid-expanded matrix. Further adsorption of catanionic molecules forced them to pack, thereby forming a very dense monolayer that prevented further vesicle rupture by avoiding contact of the vesicles with air. Moreover, confocal fluorescence microscopy revealed the presence of layers of intact vesicles that were progressively creaming toward this catanionic monolayer; the surface tension of the vesicle dispersion remained constant upon creaming. The catanionic monolayer behaved as a soft glassy material, an amorphous solid with time- and temperature-dependent properties. Using interfacial oscillatory rheology, we found that the monolayer relaxed mechanical stresses in seconds and melted at a temperature very close to the melting transition temperature of the vesicle bilayers. These results have potential application in the design of smart foams that have temperature-tunable stability.