Thermodynamic and Mechanical Timescales Involved in Foam Film Rupture and Liquid Foam Coalescence

Foam Stabilization Process for Nano-Al2O3 and Its Effect on Mechanical Properties of Foamed Concrete

Foamed concrete is increasingly utilized in engineering due to its light weight, excellent thermal insulation, fire resistance, etc. However, its low strength has always been the most crucial factor limiting its large-scale application. This study introduced an innovative method to enhance the strength of foamed concrete by using nano-Al2O3 (NA) as a foam stabilizer. NA was introduced into a foaming agent containing sodium dodecyl sulfate (SDS) and hydroxypropyl methylcellulose (HPMC) to prepare a highly stable foam. This approach significantly improved the foam stability and the strength of foamed concrete. Its drainage volume, settlement distance, microstructure, and stabilizing action were investigated, along with the strength, microstructure, and hydration products of foamed concrete. The presence of NA effectively reduced the drainage volume and settlement distance of the foam. NA is distributed at the gas-liquid interface and within the liquid film to play a hindering role, increasing the thickness of the liquid film, delaying the liquid discharge rate from the liquid film, and hindering bubble aggregation, thereby enhancing foam stability. Additionally, due to the stabilizing effect of NA on the foam, the precast foam forms a fine and uniform pore structure in the hardened foamed concrete. At 28 d, the compressive strength of FC0 (0% NAs in foam) is 2.18 MPa, while that of FC3 (0.18% NAs in foam) is 3.90 MPa, increased by 79%. The reason for this is that NA promotes the formation of AFt, and its secondary hydration leads to the continuous consumption of Ca(OH)2, resulting in a more complete hydration reaction. This study presents a novel method for significantly improving the performance of foamed concrete by incorporating NA.

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.

Evaporation-Induced Temperature Gradient in a Foam Column

Various parameters affect foam stability: surface and bulk rheology of the solution, gravitational drainage, mechanical vibrations, bubble gas composition, and also evaporation. Evaporation is often considered through the prism of liquid loss but also induces a cooling effect due to the enthalpy of vaporization. In this study, we combine a theoretical and experimental approach to explore the temperature field in a foam column evaporating from the top. We show that a measurable temperature profile exists in this geometry, with temperatures at the interface lower than the environmental temperature by a few degrees. We demonstrate that the temperature profile is the result of a balance between the enthalpy of vaporization and heat fluxes originating from the thermal conduction of foam and air and thermal radiation. For small foam thicknesses compared to the radius, we found that the temperature gradient is established over the foam thickness, while for large aspect ratios, the gradient spans over a length scale comparable to the tube radius.

Water content monitoring in water-in-oil emulsions using a delay line cell

The extraction process of crude oil requires addition of water, resulting in complex emulsions, in which the phases must be separated before the petrochemical processing starts. An ultrasonic cell may be used to determine in real time the water content in water-in-crude oil emulsions. The water content of emulsions can be related to parameters, such as propagation velocity, density and relative attenuation. The ultrasonic measurement cell developed here is composed of two piezoelectric transducers, two rexolite buffer rods, and a sample chamber. It is an inexpensive and robust system. The cell measures the parameters at different temperatures and flow conditions. The tests were performed using emulsions with water volume concentrations from 0% to 40%. The experimental results show that this cell is able to obtain more precise parameters, when compared to similar ultrasonic techniques. The data acquired in real time may be used to improve the emulsion separation, decreasing greenhouse gases and energy requirements.

Structure formation of PNIPAM microgels in foams and foam films

Responsive aqueous foams are very interesting from a fundamental point of view and for various applications like foam flooding or foam flotation. In this study thermoresponsive microgels (MGs) made from poly(N-isopropyl-acrylamide) (PNIPAM) with varying cross-linker content, are used as foam stabilisers. The foams obtained are thermoresponsive and can be destabilised by increasing the temperature. The structuring of MGs inside the foam films is investigated with small-angle neutron scattering and in a thin film pressure balance. The foam films are inhomogeneous and form a network-like structure, in which thin and MG depleted zones with a thickness of ca. 30 nm are interspersed in a continuous network of thick MG containing areas with a thickness of several 100 nm. The thickness of this continuous network is related to the elastic modulus of the individual MGs, which was determined by atomic force microscopy indentation experiments. Both, the elastic moduli and foam film thicknesses, indicate a correlation to the network elasticity of the MGs predicted by the affine network model.

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.

A new model to describe small-angle neutron scattering from foams

The modelling of scattering data from foams is very challenging due to the complex structure of foams and is therefore often reduced to the fitting of single peak positions or feature mimicking. This article presents a more elaborate model to describe the small-angle neutron scattering (SANS) data from foams. The model takes into account the geometry of the foam bubbles and is based on an incoherent superposition of the reflectivity curves arising from the foam films and the small-angle scattering (SAS) contribution from the plateau borders. The model is capable of describing the complete scattering curve of a foam stabilized by the standard cationic surfactant tetradecyltrimethylammonium bromide (C14TAB) with different water contents, i.e. different drainage states, and provides information on the thickness distribution of liquid films inside the foam. The mean film thickness decreases with decreasing water content because of drainage, from 28 to 22 nm, while the polydispersity increases. These results are in good agreement with the film thicknesses of individual horizontal foam films studied with a thin-film pressure balance.

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.

Hydrogel foams from liquid foam templates: Properties and optimisation

Hydrogel foams are an important sub-class of macroporous hydrogels. They are commonly obtained by integrating closely-packed gas bubbles of 10-1000 μm into a continuous hydrogel network, leading to gas volume fractions of more than 70% in the wet state and close to 100% in the dried state. The resulting wet or dried three-dimensional architectures provide hydrogel foams with a wide range of useful properties, including very low densities, excellent absorption properties, a large surface-to-volume ratio or tuneable mechanical properties. At the same time, the hydrogel may provide biodegradability, bioabsorption, antifungal or antibacterial activity, or controlled drug delivery. The combination of these properties are increasingly exploited for a wide range of applications, including the biomedical, cosmetic or food sector. The successful formulation of a hydrogel foam from an initially liquid foam template raises many challenging scientific and technical questions at the interface of hydrogel and foam research. Goal of this review is to provide an overview of the key notions which need to be mastered and of the state of the art of this rapidly evolving field at the interface between chemistry and physics.

Blowing big bubbles

Although street artists have the know-how to blow bubbles over one meter in length, the bubble width is typically determined by the size of the hoop, or wand they use. In this article we explore a regime in which, by blowing gently downwards, we generate bubbles with radii up to ten times larger than the wand. We observe the big bubbles at lowest air speeds, analogous to the dripping mode observed in droplet formation. We also explore the impact of the surfactant chosen to stabilize the bubbles. We are able to create bubbles of comparable size using either Fairy liquid, a commercially available detergent often used by street artists, or sodium dodecyl sulfate (SDS) solutions. The bubbles obtained from Fairy liquid detach from the wand and are stable for several seconds, however those from SDS tend to burst just before detachment.

The Life of a Surface Bubble

Surface bubbles are present in many industrial processes and in nature, as well as in carbonated beverages. They have motivated many theoretical, numerical and experimental works. This paper presents the current knowledge on the physics of surface bubbles lifetime and shows the diversity of mechanisms at play that depend on the properties of the bath, the interfaces and the ambient air. In particular, we explore the role of drainage and evaporation on film thinning. We highlight the existence of two different scenarios depending on whether the cap film ruptures at large or small thickness compared to the thickness at which van der Waals interaction come in to play.

Microfluidic thin film pressure balance for the study of complex thin films

Investigations of free-standing liquid films enjoy an increasing popularity due to their relevance for many fundamental and applied scientific problems. They constitute soap bubbles and foams, serve as membranes for gas transport or as model membranes in biophysics. More generally, they provide a convenient tool for the investigation of numerous fundamental questions related to interface- and confinement-driven effects in soft matter science. Several approaches and devices have been developed in the past to characterise reliably the thinning and stability of such films, which were commonly created from low-viscosity, aqueous solutions/dispersions. With an increasing interest in the investigation of films made from strongly viscoelastic and complex fluids that may also solidify, the development of a new generation of devices is required to manage reliably the constraints imposed by these formulations. We therefore propose here a microfluidic chip design which allows for the reliable creation, control and characterisation of free-standing films of complex fluids. We provide all technical details and we demonstrate the device functioning for a larger range of systems via a selection of illustrative examples, including films of polymer melts and gelling hydrogels.

Insights into CO2 Foaming Behavior of Ethoxylated Amines

Switchable ethoxylated amine surfactants are readily soluble in CO2 and high-saline brines. The objective of the current work is to maximize the foamability and stability of CO2 foam at 150 °F (65 °C) through adjustments in the surfactant concentration, pH, and brine salinity. From the results, the authors recommend potential applications of Ethomeen C12 (EC12) for CO2 foam in the oil/gas industry. Foam stability tests helped determine the optimum parameters for CO2 foam stability at 77 °F (25 °C) and 150 °F (65 °C). The surface tension of EC12 as a function of concentration was evaluated using a drop-shape analyzer. Maximum foam stability was observed for a solution comprising of 1.5 wt% EC12, 25 wt% NaCl, and pH 6.5 at 150 °F (65 °C). The interactions with the salts allowed closer packing of the surfactant molecules at the lamellae and strengthening the foam. At a pH of 2.5, the absence of salt led to poor foam stability. However, at the same pH and in the presence of sodium chloride, the foam was stable for longer periods of time due to the salt influence. The surface tension gradients had a direct relationship to foam stability. There was a strong resistance to foam degradation when multivalent ions were present with the surfactant.

Scaling Laws in the Dynamics of Collapse of Single Bubbles and 2D Foams

Avalanches of rupturing bubbles play an important role in the dynamics of collapse of macroscopic liquid foams. We hypothesized that the occurrence of cascades of rupturing bubbles in foams depends, at least in part, on the power released during the rupture of a bubble. In this paper, we present results on the dynamics of single bubble bursting obtained by analyzing the pressure wave (sound) emitted by the bubble when collapsing. We found that the released energy varies linearly with bubble size, the frequency of the emitted sound follows a power law with exponent 3/2 (compatible with the Helmholtz resonator model) and the duration of a rupturing event seems to be independent of bubble size. To correlate the dynamics of individual bubbles with the dynamics of foams, we studied the occurrence of avalanches on bubble rafts and found that the phenomenon appears to be a self-organized criticality (SOC) process. The distribution functions for the size of the avalanches are a power law with exponents between 2 and 3, depending on the surfactant concentration. The distribution of times between ruptures also follows a power law with exponents close to 1, independently of the surfactant concentration.

Physical models of infant mortality: implications for defects in biological systems

Reliability engineering concerned with failure of technical inanimate systems usually uses the vocabulary and notions of human mortality, e.g., infant mortality vs. senescence mortality. Yet, few data are available to support such a parallel description. Here, we focus on early-stage (infant) mortality for two inanimate systems, incandescent light bulbs and soap films, and show the parallel description is clearly valid. Theoretical considerations of the thermo-electrical properties of electrical conductors allow us to link bulb failure to inherent mechanical defects. We then demonstrate the converse, that is, knowing the failure rate for an ensemble of light bulbs, it is possible to deduce the distribution of defects in wire thickness in the ensemble. Using measurements of lifetimes for soap films, we show how this methodology links failure rate to geometry of the system; in the case presented, this is the length of the tube containing the films. In a similar manner, for a third example, the time-dependent death rate due to congenital aortic valve stenosis is related to the distribution of degrees of severity of this condition, as a function of time. The results not only validate clearly the parallel description noted above, but also point firmly to application of the methodology to humans, with the consequent ability to gain more insight into the role of abnormalities in infant mortality.

Enabling intensification of multiphase chemical processes with additive manufacturing

Fixed bed supports of various materials (metal, ceramic, polymer) and geometries are used to enhance the performance of many unit operations in chemical processes. Consider first metal and ceramic monolith support structures, which are typically extruded. Extruded monoliths contain regular, parallel channels enabling high throughput because of the low pressure drop accompanying high flow rate. However, extruded channels have a low surface-area-to-volume ratio resulting in low contact between the fluid phase and the support. Additive manufacturing, also referred to as three dimensional printing (3DP), can be used to overcome these disadvantages by offering precise control over key design parameters of the fixed bed including material-of-construction and total bed surface area, as well as accommodating system integration features compatible with continuous flow chemistry. These design parameters together with optimized extrinsic process conditions can be tuned to prepare customizable separation and reaction systems based on objectives for chemical process and/or the desired product. We discuss key elements of leveraging the flexibility of additive manufacturing to intensification with a focus on applications in continuous flow processes and disperse, multiphase systems enabling a range of scalable chemistry spanning discovery to manufacturing operations.

Breakup of Thin Liquid Films: From Stochastic to Deterministic

The thinning and rupture of thin liquid films is a ubiquitous process, controlling the lifetime of bubbles, antibubbles, and droplets. A better understanding of rupture is important for controlling and modeling the stability of multiphase products. Yet literature reports that film breakup can be either stochastic or deterministic. Here, we employ a modified thin film balance to vary the ratio of hydrodynamic to capillary stresses and its role on the dynamics of thin liquid films of polymer solutions with adequate viscosities. Varying the pressure drop across planar films allows us to control the ratio of the two competing timescales, i.e., a controlled hydrodynamic drainage time and a timescale related to fluctuations. The thickness fluctuations are visualized and quantified, and their characteristics are for the first time directly measured experimentally for varying strengths of the flow inside the film. We show how the criteria for rupture depend on the hydrodynamic conditions, changing from stochastic to deterministic as the hydrodynamic forces inside the film damp the fluctuations.

Stability of big surface bubbles: impact of evaporation and bubble size

Surface bubbles have attracted much interest in the past few decades. In this article, we aim to explore the lifetime and thinning dynamics of centimetric surface bubbles. We study the impact of the bubble size as well as that of the atmospheric humidity through a careful control and systematic variation of the relative humidity in the measuring chamber. We first address the question of the drainage under saturated water vapor conditions and show that a model including both capillary and gravity driven drainage provides the best prediction for this process. Additionally, unprecedented statistics on the bubble lifetimes confirm experimentally that this parameter is set by evaporation to leading order. We make use of a model based on the overall thinning dynamics of the thin film and assume a rupture thickness of the order 10-100 nm to obtain a good representation of these data. For experiments conducted far from saturation, the convective evaporation of the bath is shown to dominate the overall mass loss in the cap film due to evaporation.

Microfluidics Mediated Production of Foams for Biomedical Applications

Within the last decade, there has been increasing interest in liquid and solid foams for several industrial uses. In the biomedical field, liquid foams can be used as delivery systems for dermatological treatments, for example, whereas solid foams are frequently used as scaffolds for tissue engineering and drug screening. Most of the foam functionalities are largely correlated to their mechanical properties and their structure, especially bubble/pore size, shape, and interconnectivity. However, the majority of conventional foaming fabrication techniques lack pore size control which can induce important inhomogeneities in the foams and subsequently decrease their performance. In this perspective, new advanced technologies have been introduced, such as microfluidics, which offers a highly controlled production, allowing for design customization of both liquid foams and solid foams obtained through liquid-templating. This short review explores both the fabrication and the characterization of foams, with a focus on solid polymer foams, and sheds the light on how microfluidics can overcome some existing limitations, playing a crucial role in their production for biomedical applications, especially as scaffolds in tissue engineering.

Architected Polymer Foams via Direct Bubble Writing

Polymer foams are cellular solids composed of solid and gas phases, whose mechanical, thermal, and acoustic properties are determined by the composition, volume fraction, and connectivity of both phases. A new high-throughput additive manufacturing method, referred to as direct bubble writing, for creating polymer foams with locally programmed bubble size, volume fraction, and connectivity is reported. Direct bubble writing relies on rapid generation and patterning of liquid shell-gas core droplets produced using a core-shell nozzle. The printed polymer foams are able to retain their overall shape, since the outer shell of these bubble droplets consist of a low-viscosity monomer that is rapidly polymerized during the printing process. The transition between open- and closed-cell foams is independently controlled by the gas used, while the foam can be tailored on-the-fly by adjusting the gas pressure used to produce the bubble droplets. As exemplars, homogeneous and graded polymer foams in several motifs, including 3D lattices, shells, and out-of-plane pillars are fabricated. Conductive composite foams with controlled stiffness for use as soft pressure sensors are also produced.

Coalescence in Two-Dimensional Foams: A Purely Statistical Process Dependent on Film Area

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.

Liquid foam templating - A route to tailor-made polymer foams

Solid foams with pore sizes between a few micrometres and a few millimetres are heavily exploited in a wide range of established and emerging applications. While the optimisation of foam applications requires a fine control over their structural properties (pore size distribution, pore opening, foam density, …), the great complexity of most foaming processes still defies a sound scientific understanding and therefore explicit control and prediction of these parameters. We therefore need to improve our understanding of existing processes and also develop new fabrication routes which we understand and which we can exploit to tailor-make new porous materials. One of these new routes is liquid templating in general and liquid foam templating in particular, to which this review article is dedicated. While all solid foams are generated from an initially liquid(-like) state, the particular notion of liquid foam templating implies the specific condition that the liquid foam has time to find its "equilibrium structure" before it is solidified. In other words, the characteristic time scales of the liquid foam's stability and its solidification are well separated, allowing to build on the vast know-how on liquid foams established over the last 20 years. The dispersed phase of the liquid foam determines the final pore size and pore size distribution, while the continuous phase contains the precursors of the desired porous scaffold. We review here the three key challenges which need to be addressed by this approach: (1) the control of the structure of the liquid template, (2) the matching of the time scales between the stability of the liquid template and solidification, and (3) the preservation of the structure of the template throughout the process. Focusing on the field of polymer foams, this review gives an overview of recent research on the properties of liquid foam templates and summarises a key set of studies in the emerging field of liquid foam templating. It finishes with an outlook on future developments. Occasional references to non-polymeric foams are given if the analogy provides specific insight into a physical phenomenon.

Influence of Evaporation on Soap Film Rupture

Although soap films are prone to evaporate due to their large surface to volume ratio, the effect of evaporation on macroscopic film features has often been disregarded in the literature. In this work, we experimentally investigate the influence of environmental humidity on soap film stability. An original experiment allows to measure both the maximum length of a film pulled at constant velocity and its thinning dynamics in a controlled atmosphere for various values of the relative humidity [Formula: see text]. At first order, the environmental humidity seems to have almost no impact on most of the film thinning dynamics. However, we find that the film length at rupture increases continuously with [Formula: see text]. To rationalize our observations, we propose that film bursting occurs when the thinning due to evaporation becomes comparable to the thinning due to liquid drainage. This rupture criterion turns out to be in reasonable agreement with an estimation of the evaporation rate in our experiment.

Interplay between bulk self-assembly, interfacial and foaming properties in a catanionic surfactant mixture of varying composition

The self-aggregation, surface properties and foamability of the catanionic surfactant mixture cetyltrimethylammonium bromide (CTAB)/sodium octyl sulfonate (SOSo) have been investigated to obtain insight on the relation between bulk nanostructures, surfactant packing, and foam stability and aging. Light microscopy, SANS, cryo-TEM, DLS, surface tension, rheometry and direct photography were used to characterize mixtures with varying CTAB molar fraction, x_CTAB. In the bulk, self-assembly is richer in the excess CTAB region than in the excess SOSo one. Starting from neat CTAB micelles and on addition of anionic surfactant, there is a change from small ellipsoidal micelles (1 < x_CTAB ≤ 0.80) to large rodlike micelles (0.65 ≤ x_CTAB ≤ 0.55) and then to vesicles (0 < x_CTAB ≤ 0.50), with coexistence regions in between; SOSo-rich mixtures are thus dominated by vesicles. High size polydispersity for the micelles and vesicles is an intrinsic feature of this system. Foam stability is concomitantly impacted by x_CTAB. SOSo is a small mobile molecule and so it disrupts foam stability, irrespective of the presence of vesicles. Foams are thus only stable in the CTAB-rich regions, and SANS shows that the shape of micelles and vesicles is unchanged inside the foam. Foam drainage is thereby mostly controlled by the presence of the elongated micelles through the solution viscosity, whereas coarsening is influenced by dense surfactant packing at the gas-liquid interfaces.

Electrokinetic transport in liquid foams

Investigating electrokinetic transport in a liquid foam is at the confluence of two well developed research areas. On one hand, the study of electrokinetic flows (i.e. surface-driven flows generated close to a charged interface) is fairly well understood in regards the solid/liquid interface. On the other hand, the flow of liquid in a 3D deformable network, i.e a foam, under a volume force such as gravity has been thoroughly studied over the past decade. The overlapping zone of these two frameworks is of great interest for both communities as it gives rise to challenging new questions such as: what is the importance of the nature of the charged interface, created by mobile and soluble surfactants in the case of foam, on electrokinetic transport? How does a foam behave when submitted to a surface-driven flow? Can we compensate a volume-driven flow, i.e. gravity, by a surface-driven flow, i.e. electroosmosis? In this review, we will explore these questions on three different scales: a surfactant laden interface, a foam film and a macroscopic foam.

Stable oil-laden foams: Formation and evolution

The interaction between oil and foam has been the subject of various studies. Indeed, oil can be an efficient defoaming agent, which can be highly valuable in various industrial applications where undesired foaming may occur, as seen in jet-dyeing processes or waste water treatment plant. However, oil and foam can also constructively interact as observed in detergency, fire-fighting, food and petroleum industries, where oil can be in the foam structure or put into contact with the foam without observing a catastrophic break-up of the foam. Under specific physico-chemistry conditions, the oil phase can even be trapped inside the aqueous network of the foam, thus providing interesting complex materials made of three different fluid phases that we name oil-laden foam (OLF). In this review, we focus on such systems, with a special emphasis on dry OLF, i.e. with a total liquid volume fraction, ε smaller than 5%. We first try to clarify the physical and chemical conditions for these systems to appear, we review the different techniques of the literature to obtain them. Then we discuss their structure and identify two different OLF morphologies, named foamed emulsion, in which small oil globules are comprised within the network of the aqueous foam and biliquid foams, where the oil also comprised in the aqueous foam network is continuous at the scale of several bubbles. Last, we review the state of the art of their evolution in particular concerning topological changes, coalescence, coarsening and drainage.

Surfactant films in lyotropic lamellar (and related) phases: Fluctuations and interactions

The analogy between soap films thinning under border capillary suction and lamellar stacks of surfactant bilayers dehydrated by osmotic stress is explored, in particular in the highly dehydrated limit where the soap film becomes a Newton black film. The nature of short-range repulsive interactions between surfactant-covered interfaces and acting across water channels in both cases will be discussed.

Electro-osmosis at surfactant-laden liquid-gas interfaces: beyond standard models

Electro-osmosis (EO) is a powerful tool to manipulate liquids in micro and nanofluidic systems. While EO has been studied extensively at liquid-solid interfaces, the case of liquid-vapor interfaces, found e.g. in foam films and bubbles, remains to be explored. Here we perform molecular dynamics (MD) simulations of EO in a film of aqueous electrolyte covered with fluid layers of ionic surfactants and surrounded by gas. Following the experimental procedure, we compute the zeta potential from the EO velocity, defined as the velocity difference between the middle of the liquid film and the surrounding gas. We show that the zeta potential can be smaller or larger than existing predictions depending on the surfactant coverage. We explain the failure of previous descriptions by the fact that surfactants and bound ions move as rigid bodies and do not transmit the electric driving force to the liquid locally. Considering the reciprocal streaming current effect, we then develop an extended model, which can be used to predict the experimental zeta potential of surfactant-laden liquid-gas interfaces.

Coalescence In Draining Foams Made of Very Small Bubbles

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.

Ultradry Carbon Dioxide-in-Water Foams with Viscoelastic Aqueous Phases

For foams with ultra low water contents, the capillary pressure is very large and induces rapid drainage that destabilizes the aqueous lamellae between the gas bubbles. However, we show that high-pressure CO2-in-water foams can be stabilized with a viscoelastic aqueous phase composed of entangled wormlike micelles, even for extremely high CO2 volume fractions ϕ of 0.95 to 0.98; the viscosity of these ultradry foams increased by up to 3-4-fold, reaching more than 100 cP relative to foams formed with conventional low viscosity aqueous phases. The foam morphology consisted of fine ∼20 μm polyhedral-shaped CO2 bubbles that were stable for hours. The wormlike micelles were formed by mixing anionic sodium lauryl ether sulfate (SLES) with salt and a protonated cationic surfactant, as shown by cryogenic transmission electron microscopy (cryo-TEM) and large values of the zero-shear viscosity and the dynamic storage and loss moduli. With the highly viscous continuous aqueous phases, the foam lamella drainage rates were low, as corroborated by confocal microscopy. The preservation of viscous thick lamellae resulted in lower rates of Ostwald ripening relative to conventional foams as shown by high-pressure optical microscopy. The ability to stabilize viscous ultra high internal phase foams is expected to find utility in various practical applications, including nearly "waterless" fracturing fluids for recovery of oil and gas in shale, offering the possibility of a massive reduction in the amount of wastewater.