Foams stabilised by mixtures of nanoparticles and oppositely charged surfactants: relationship between bubble shrinkage and foam coarsening

Towards a universal model for the foaming behavior of surfactants: a case study on per- and polyfluoroalkyl substances (PFAS)

Foam fractionation offers a promising solution for the separation of surface-active contaminants from water. Therefore, this work aims to comprehensively investigate foaming behavior and its correlations with the interfacial properties. As a case study, we evaluate foaming of per- and polyfluoroalkyl substances (PFAS), which are one of significant environmental issues worldwide due their pervasive presence in the environment. Since there is no universal model to describe the foaming behavior of surfactants that can be applied to PFAS, this research utilizes dimensional analysis to establish a correlation between the foaming behavior of PFAS solutions-characterized by expansion rate of foaming-and dimensionless numbers that represent both processing and interfacial characteristics. Foaming parameters, such as gas flow rate and aeration time, are varied to study their effect on PFAS foamability. In addition, we study PFAS with different headgroups and with different chain lengths in the presence of electrolytes with different concentrations. Our study elucidates distinct, condition-specific equations for individual PFAS, revealing that long-chain PFAS foaming is significantly influenced by interfacial property-related dimensionless numbers, such as the Boussinesq number. Additionally, the Froude number and Weber number affect the foamability of both long- and short-chain PFAS. Moreover, our study identifies specific trends, including a maximum foaming capacity at a certain Capillary number, aligning with the maximum in dilatational interfacial modulus. The results suggest more studies are needed on bubble interaction and foam film behavior.

Enhancing extraction efficiency of carpaine in Carica papaya L. leaves: coupling acid-base extraction with surfactant-assisted micro-flotation

Carpaine, a major alkaloid in papaya leaves, has considerable cardiovascular benefits alongside its notable effects on muscle relaxation when utilized in medicine. In this study, the coupling of acid-base extraction and flotation was developed to completely remove the use of toxic solvents. This method entails the extraction of carpaine from Carica papaya L. leaves using hot water extraction alongside ultrasound-assisted extraction followed by the condensation of the species using surfactant-assisted flotation. The acid-base extraction was applied to alter the solubility of carpaine as desired at different stages of the process. The results showed that the carpaine extraction yield using all the treatments in conjunction was significantly higher compared to the control samples in which the acid-base extraction or flotation was not applied. The TLC and GC-FID results suggested that the bubbles introduced during the flotation were highly specific toward their interactions with carpaine in its hydrophobic complex form. The quantity of carpaine extracted using our method, in comparison to the amount of carpaine obtained using a different method from a previous study that utilized ethanolic extraction, exhibited a 2.32-fold greater extraction yield. This work demonstrates the importance of flexible utilization of both surface and bulk chemistry in achieving an improved solution for a technical problem.

Evaluation of Interfacial Structure of Self-Assembled Nanoparticle Layers: Use of Standard Deviation between Calculated and Experimental Drop Profiles as a Novel Method

The self-assembly of nanoparticles (NPs) at interfaces is currently a topic of increasing interest due to numerous applications in food technology, pharmaceuticals, cosmetology, and oil recovery. It is possible to create tunable interfacial structures with desired characteristics using tailored nanoparticles that can be precisely controlled with respect to shape, size, and surface chemistry. To address these functionalities, it is essential to develop techniques to study the properties of the underlying structure. In this work, we propose an experimental approach utilizing the standard deviation of drop profiles calculated by the Laplace equation from experimental drop profiles (STD), as an alternative to the Langmuir trough or precise microscopic methods, to detect the initiation of closely packed conditions and the collapse of the adsorbed layers of CTAB-nanosilica complexes. The experiments consist of dynamic surface/interfacial tension measurements using drop profile analysis tensiometry (PAT) and large-amplitude drop surface area compression/expansion cycles. The results demonstrate significant changes in STD values at the onset of the closely packed state of nanoparticle-surfactant complexes and the monolayer collapse. The STD trend was explained in detail and shown to be a powerful tool for analyzing the adsorption and interfacial structuring of nanoparticles. Different collapse mechanisms were reported for NP monolayers at the liquid/liquid and air/liquid interfaces. We show that the interfacial tension (IFT) is solely dependent on the extent of interfacial coverage by nanoparticles, while the surfactants regulate only the hydrophobicity of the self-assembled complexes. Also, the irreversible adsorption of nanoparticles and the increasing number of adsorbed complexes after the collapse were observed by performing consecutive drop surface compression/expansion cycles. In addition to a qualitative characterization of adsorption layers, the potential of a quantitative calculation of the parameter STD such as the number of adsorbed nanoparticles at the interface and the distance between them at different states of the interfacial layer was discussed.

Oppositely charged surfactants and nanoparticles at the air-water interface: Influence of surfactant to nanoparticle ratio

HYPOTHESIS

The interactions between oppositely charged nanoparticles and surfactants can significantly influence the interfacial properties of the system. Traditionally, in the study of such systems, the nanoparticle concentration is varied while the surfactant concentration is kept constant, or vice versa. However, we believe that a defined variation of both components' concentration is necessary to accurately assess their effects on the interfacial properties of the system. We argue that the effect of nanoparticle-surfactant complexes can only be properly evaluated by keeping the surfactant to nanoparticle ratio constant.

EXPERIMENTS

Zeta potential, dynamic light scattering, high amplitude surface pressure and surface tension measurements are employed synergistically to characterize the interfacial properties of the nanoparticle-surfactant system. Interferometric experiments are performed to highlight the effect of surface concentration on the stability of thin liquid films.

FINDINGS

The interfacial properties of surfactant/nanoparticle mixtures are primarily determined by the surfactant/nanoparticle ratio. Below a certain ratio, free surfactant molecules are removed from the solution by the formation of surfactant-nanoparticle complexes. Surprisingly, even though the concentration and hydrophobicity of these complexes do not seem to have a noticeable impact on the surface tension, they do significantly affect the rheological properties of the interface. Above this ratio, free surfactant monomers and nanoparticle-surfactant complexes coexist and can co-adsorb at the interface, changing both the interfacial tension and the interfacial rheology, and thus, for example, the foamability and foam stability of the system.

Formation Mechanism of Starch-Based Double Emulsions from the Interfacial Perspective

Double emulsions are of significant practical value in protecting the core material owing to their multicomponent structure and have thus been applied in various fields, such as food, cosmetics, and drugs. However, the mechanism of double emulsion formation by native starch is not well established. Herein, we demonstrate a facile route to develop type-A, type-B, and type-C double emulsions using native starch and develop an innovative design for a carrier. Interfacial interaction, enthalpy changes of starch, and interfacial properties are key factors governing the formation of double emulsions and controlling the type of double emulsions formed. Therefore, the results of this study provide a better understanding of how and what type of starch-based double emulsions are formed.

Rheology of a 2D granular film

We study experimentally the rheology of a macroscopic particle-laden soap film, designated as a "Granular Film", in the simple shear configuration. Macroscopic particles are dispersed in a soap film, while being large enough that they bridge both fluid interfaces. We simultaneously perform macroscopic rheological measurements with a classical rheometer and investigate interactions at the particle scale with a camera underneath the film. The determination of the velocity field of the grains reveals the presence of an inhomogeneous shear within the granular film. Trying to correlate both measurements unveils the non-locality of the rheology of the granular film: similar to what has been observed in a dry granular material, we find an highly-sheared zone close to the moving wall contrasting with a large quasistatic area. This behavior can be accounted for through extended kinetic theory and correlated with a transition in the dominant component of the stress.

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

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

Study on Screening and Evaluation of Foam Drainage Agents for Gas Wells with High Temperature and High Pressure

Foam drainage gas recovery technology is a chemical method to solve the serious bottom-hole liquid loading in the middle and late stages of gas well production, and the optimization of foam drainage agents (referred to as FDAs) is the key to the technology. According to the actual reservoir conditions, a high-temperature and high-pressure (HTHP) evaluation device for FDAs was set up in this study. The six key properties of FDAs, such as HTHP resistance, dynamic liquid carrying capacity, oil resistance, and salinity resistance, were evaluated systematically. Taking initial foaming volume, half-life, comprehensive index, and liquid carrying rate as evaluation indexes, the FDA with the best performance was selected and the concentration was optimized. In addition, the experimental results were verified by surface tension measurement and electron microscopy observation. The results showed that the sulfonate compound surfactant (UT-6) had good foamability, excellent foam stability, and better oil resistance at high temperature and high pressure. In addition, UT-6 had stronger liquid carrying capacity at a lower concentration, which could meet the production requirement when the salinity was 80 000 mg/L. Therefore, compared with the other five FDAs, UT-6 was more suitable for HTHP gas wells in block X of the Bohai Bay Basin, whose optimal concentration was 0.25 wt %. Interestingly, the UT-6 solution had the lowest surface tension at the same concentration, with the generated bubbles being closely arranged and uniform in size. Moreover, in the UT-6 foam system, the drainage speed at the plateau boundary was relatively slower with the smallest bubble. It is expected that UT-6 will become a promising candidate for foam drainage gas recovery technology in HTHP gas wells.

Applications of functional nanoparticle-stabilized surfactant foam in petroleum-contaminated soil remediation

Surfactant foam (SF) can be used to remediate petroleum-contaminated soil because of its easy transfer to inhomogeneous and low-permeability formations. Nanoparticles (NPs) not only stabilize SF under extreme conditions but also impart various functions, aiding the removal of petroleum contaminants. This review discusses the stabilization mechanisms of nanoparticle-stabilized SF (NP-SF) as well as the effects of NP size, chargeability, wettability, and NP-to-surfactant ratio on foam stability. SF stabilized by inert SiO₂ NPs is most commonly used to remediate soil contaminated with crude oil and diesel. Low dose of SF stabilized by nano zero-valent iron is cost-effective for treating soil contaminated with chlorinated organics and heavy metal ions. The efficiency and recyclability of Al₂O₃/Fe₃O₄ NPs in the remediation of diesel and crude oil contamination could be enhanced by applying a magnetic field. This review provides a theoretical basis and practical guidelines for developing functional NP-SF to improve the remediation of petroleum-contaminated soils. Future research should focus on the structural design of photocatalytic NPs and the application of catalytic NP-SF in soil remediation.

Environmental impact assessment of nanofluids containing mixtures of surfactants and silica nanoparticles

Due to widespread use of nanoparticles in surfactant-based formulations, their release into the environment and wastewater is unavoidable and toxic for biota and/or wastewater treatment processes. Because of concerns over the environmental impacts of nanofluids, studies of the fate and environmental impacts, hazards, and toxicities of nanoparticles are beginning. However, interactions between nanoparticles and surfactants and the biodegradability of these mixtures have been little studied until now. In this work, the environmental impacts of nanofluids containing mixtures of surfactants and silica nanoparticles were valuated. The systems studied were hydrophilic silica nanoparticles (sizes 7 and 12 nm), a nonionic surfactant (alkyl polyglucoside), an anionic surfactant (ether carboxylic acid), and mixtures of them. The ultimate aerobic biodegradation and the interfacial and adsorption properties of surfactants, nanoparticles, and mixtures during biodegradation were also evaluated. Ultimate biodegradation was studied below and above the CMCs of the individual surfactants. The interfacial and adsorption properties of surfactant solutions containing nanoparticles were influenced by the addition of silica particles. It was determined that silica nanoparticles reduced the capability of the nonionic surfactant alkyl polyglucoside to decrease the surface tension. Thus, silica NPs promoted a considerable increase in the surfactant CMC, whereas the effect was opposite in the case of the anionic surfactant ether carboxylic acid. Increasing concentrations of surfactant and nanoparticles in the test medium caused decreases in the maximum levels of mineralization reached for both types of surfactants. The presence of silica nanoparticles in the medium reduced the biodegradability of binary mixtures containing nonionic and anionic surfactants, and this effect was more pronounced for larger nanoparticles. These results could be useful in modelling the behaviour of nanofluids in aquatic environments and in selecting appropriate nanofluids containing nanoparticles and surfactants with low environmental impact.

Polyelectrolyte/Surfactant Mixtures: A Pathway to Smart Foams

This review deals with liquid foams stabilized by polyelectrolyte/surfactant (PS) complexes in aqueous solution. It briefly reviews all the important aspects of foam physics at several scales, from interfaces to macroscopic foams, needed to understand the basics of these complex systems, focusing on those particular aspects of foams stabilized by PS mixtures. The final section includes a few examples of smart foams based on PS complexes that have been reported recently in the literature. These PS complexes open an opportunity to develop new intelligent dispersed materials with potential in many fields, such as oil industry, environmental remediation, and pharmaceutical industry, among others. However, there is much work to be done to understand the mechanism involved in the stabilization of foams with PS complexes. Understanding those underlying mechanisms is vital to successfully formulate smart systems. This review is written in the hope of stimulating further work in the physics of PS foams and, particularly, in the search for responsive foams based on polymer-surfactant mixtures.

Direct Measurement of Surfactant-Mediated Picoforces among Nanoparticles in a Quasi-Two-Dimensional Environment

The lack of methodologies which enable us to measure forces acting between nanomaterials is one of the factors limiting the full comprehension of their behavior and their more effective exploitation in new devices. Here we exploit the irreversible adsorption of surfactant-decorated nanoparticles at the air/water interface to investigate interparticle forces and the effect of the surfactant structure on them. We measured the interparticle repulsive forces as a function of the modulation of the interparticle distance by simultaneously performing compression isotherms and the grazing incidence small-angle X-ray scattering (GISAXS) structural characterization of the monolayers at water-vapor interfaces. Our results demonstrate that the short-range interparticle forces are strongly affected by the presence of the organic ligands, which are shown to be able to influence the interparticle repulsions even when added in micromolar amounts. In particular, we demonstrate the predominant steric nature of short-range forces, which are accounted for in terms of the compression-induced stretched-to-coiled conformational transition of the ligand hydrophobic tail.

Nanofluid Structural Forces Alter Solid Wetting, Enhancing Oil Recovery

Nanofluids have attracted significant research interest for their promising application in enhanced oil recovery. One striking feature leading to the outstanding efficiency of nanofluids in enhanced oil recovery is the structure of nanoparticles, which induces oscillatory structural forces in the confined space between fluid–fluid interfaces or air–liquid and liquid–solid interfaces. To promote the understanding of the oscillatory structural forces and their application in enhanced oil recovery, we reviewed the origin and theory of the oscillatory structural forces, factors affecting their magnitude, and the experimental techniques demonstrating their impacts on enhanced oil recovery. We also reviewed the methods, where the benefits of nanofluids in enhanced oil recovery provided by the oscillatory structural forces are directly manifested. The oscillatory structural forces promote the wetting and spreading of nanofluids on solid surfaces, which ultimately enhances the separation of oil from the reservoir. Some imbibition tests demonstrated as much as 50% increased oil recovery, compared to the cases where the oscillatory structural forces were absent.

Ultrastable N2/Water Foams Stabilized by Dilute Nanoparticles and a Surfactant at High Salinity and High Pressure

The rapid development of unconventional oil and gas resources presents challenges for foam flooding for reservoirs with high salinity and high heterogeneity at elevated temperatures. In this study, hydrophilic anionic sulfonate-modified nanoparticles (NPs) exhibited a synergistic effect with a cationic surfactant in stabilizing N₂/water foam in the presence of concentrated divalent ions from ambient temperature up to 70 °C. With low concentrations of both the sulfonated NPs (SNPs) and cationic surfactant, the foams remained stable for 4 days at 50 °C and atmospheric pressure, while the surfactant-stabilized foams collapsed completely in 1 day. This stability mechanism of foams by the SNPs and cationic surfactant is described in terms of phase behavior, bulk shear rheology of the aqueous phase, and the dilational modulus of the gas-brine interface. The high surface elastic dilational modulus E' observed upon addition of the SNP provided stability against coarsening according to the Gibbs criteria. The cryo-SEM images also showed the compact bubble structure of foams provided by the SNPs. Consequently, very minor changes in the foam bubble size were observed at 208 bar (3000 psi) and 50 °C for up to 48 h with only 0.1 wt % or 0.3 wt % SNPs and 0.01 wt % Arquad 12-50, indicating excellent foam stability. The ability of the surfactant and NPs to stabilize foams at low concentrations broadens the application of foams in subsurface reservoirs at high temperatures and salinities.

Oil foams stabilized by POSS/organosilica particle assemblies: application for aerobic oxidation of aromatic alcohols

A novel amphiphilic polyhedral oligomeric silsesquioxane (POSS) with surfactant-like behavior was synthesized. By combining this new POSS, used as a frother, with surface-active catalytic organosilica particles, used as a stabilizer, we designed a dual particle system able to generate foams in pure organic solvents. Tunable foamability and foam stability were achieved in a variety of organic solvents by simply adjusting the POSS concentration. As a result, the catalytic activity was drastically boosted in the aerobic oxidation of pure aromatic alcohols under 1 bar O₂ pressure. Particles were conveniently recycled with high foamability and the catalytic efficiency was maintained for at least 7 consecutive runs.

A broad perspective to particle-laden fluid interfaces systems: from chemically homogeneous particles to active colloids

Particles adsorbed to fluid interfaces are ubiquitous in industry, nature or life. The wide range of properties arising from the assembly of particles at fluid interface has stimulated an intense research activity on shed light to the most fundamental physico-chemical aspects of these systems. These include the mechanisms driving the equilibration of the interfacial layers, trapping energy, specific inter-particle interactions and the response of the particle-laden interface to mechanical perturbations and flows. The understanding of the physico-chemistry of particle-laden interfaces becomes essential for taking advantage of the particle capacity to stabilize interfaces for the preparation of different dispersed systems (emulsions, foams or colloidosomes) and the fabrication of new reconfigurable interface-dominated devices. This review presents a detailed overview of the physico-chemical aspects that determine the behavior of particles trapped at fluid interfaces. This has been combined with some examples of real and potential applications of these systems in technological and industrial fields. It is expected that this information can provide a general perspective of the topic that can be exploited for researchers and technologist non-specialized in the study of particle-laden interfaces, or for experienced researcher seeking new questions to solve.

Elastic gas/water interface for highly stable foams with modified anionic silica nanoparticles and a like-charged surfactant

HYPOTHESIS

Surface active anionic nanoparticles (NPs) with strategically designed covalent ligands may be combined with a liked-charged surfactant to form a highly elastic gas-water interface leading to highly stable gas/water foams.

EXPERIMENTS

The colloidal stability of the NPs was determined by dynamic light scattering, and the surface elastic dilational modulus E' of the interface by sinusoidal oscillation of a pendant droplet at 0.1 Hz, which was superimposed on large-amplitude compression-expansion cycles. The foam stability was measured with optical microscopy of the bubble size distribution and from the macroscopic foam height.

FINDINGS

The NPs played the key role the formation of a highly elastic air-water interface with a high E' despite a surfactant level well above the critical micelle concentration. Unlike the case for most previous studies, the NP amphiphilicity was essentially independent of the surfactant given the very low adsorption of the surfactant on the like-charged NP surfaces. With high E' values, both coalescence and coarsening were reduced leading to highly foam up to 80 °C. However, the surfactant facilitated foam generation at much lower shear rates than with NPs alone. The tuning of NP surfaces with ligands for colloidal stability in brine and simultaneously high amphiphilicity at the gas-water interface, over a wide range in surfactant concentration, is of broad interest for enabling the design of highly stable foams.

Unsupervised bubble calorimetry analysis: Surface tension from isothermal titration calorimetry

HYPOTHESIS

The injection of air into the sample cell of an isothermal titration calorimeter containing a liquid provides a rich-in-information signal, with a periodic contribution arising from the creation, growing and release of bubbles. The identification and analysis of such contributions allow the accurate determination of the surface tension of the target liquid.

EXPERIMENTS

Air is introduced at a constant rate into the sample cell of the calorimeter containing either a pure liquid or a solution. The resulting calorimetric signal is analyzed by a new algorithm, which is implemented into a computational code.

FINDINGS

The thermal power generated by our experiments is often noisy, thus hiding the periodic signal arising from the bubbles' formation and release. The new algorithm was tested with a range of different types of calorimetric raw data, some of them apparently being just noise. In all cases, the contribution of the bubbles to the signal was isolated and the corresponding period was successfully determined in an automated way. It is also shown that two reference measurements suffice to calibrate the instrument at a given temperature, regardless the injection rate, allowing the direct determination of surface tension values for the liquid contained in the sample cell.

Spectral Properties of Foams and Emulsions

The optical and spectral properties of foams and emulsions provide information about their micro-/nanostructures, chemical and time stability and molecular data of their components. Foams and emulsions are collections of different kinds of bubbles or drops with particular properties. A summary of various surfactant and emulsifier types is performed here, as well as an overview of methods for producing foams and emulsions. Absorption, reflectance, and vibrational spectroscopy (Fourier Transform Infrared spectroscopy-FTIR, Raman spectroscopy) studies are detailed in connection with the spectral characterization techniques of colloidal systems. Diffusing Wave Spectroscopy (DWS) data for foams and emulsions are likewise introduced. The utility of spectroscopic approaches has grown as processing power and analysis capabilities have improved. In addition, lasers offer advantages due to the specific properties of the emitted beams which allow focusing on very small volumes and enable accurate, fast, and high spatial resolution sample characterization. Emulsions and foams provide exceptional sensitive bases for measuring low concentrations of molecules down to the level of traces using spectroscopy techniques, thus opening new horizons in microfluidics.

Imidazolium based ionic liquid stabilized foams for conformance control: bulk and porous scale investigation

Foams are typically used as a divergent fluid for conformance control in order to divert the fluid flow from a high-permeable zone into a low-permeable zone. Nevertheless, the stability of the foam still remains a challenge due to the presence of antifoaming crude oil and the harsh environment of the reservoir, such as high-temperature, high-salinity, and high-pressure. In this study, we investigated the stability and efficacy of various surfactant generated foams with ionic liquid (IL) additives. Intrinsically, the study is targeted to represent the conditions of Arab-D reservoir formations, which are abundant in Saudi Arabian oilfields. In this, we have screened several parameters that influence foam stability like the type of foamer gases (CO₂, N₂, and air), type of ILs, type of surfactants (nonionic, anionic, cationic, and zwitterionic), concentration, salinity (formation brine, low salinity brine, and seawater brine), temperature, etc. The stability of the generated foams was analyzed in both bulk and porous scale media. The bulk foam study has demonstrated that only a very minor concentration of ILs (50-500 ppm) shows a greater improvement in both the foamability and foam stability. The stability of the foam in the presence of the studied ILs and surfactants increases by more than 50% compared to their neat surfactant solution. A similar response was also witnessed in the dynamic foaming experiments at high-temperature, high-pressure, and high-salinity. The current work also involves the determination of the foam morphology, including structure, size, shape, gas-water interface and the lamellae size for different systems with and without ILs, which helps to understand the stability mechanism of the foams with and without ILs. Confocal and optical microscopic images of the foam structure of various systems reveal that these ILs are successful in reducing the size of bubbles and increasing the lamellae size. It is very clear that the addition of ILs generates the surfactant layered-ILs, and they tend to arrange themselves in the lamellae, and at the liquid-gas interface, thereby decreasing the rate of film drainage at the lamellae and delaying the bubble rupture point. This led to the observed enhanced foam stability. Thus, we would like to conclude that the ILs investigated here improved the foam stability by their adsorption at the foam lamella which further helped in preventing liquid drainage and film thinning.

Foaming and rheological properties of aqueous solutions: an interfacial study

Although aqueous foam is composed of simple fluids, air and water, it shows a complex rheological behavior. It exhibits solid-like behavior at low shear and fluid-like behavior at high shear rate. Therefore, understanding such behavior is important for many industrial applications in foods, pharmaceuticals, and cosmetics. Additionally, air–water interface of bubble surface plays an important role in the stabilizing mechanism of foams. Therefore, the rheological properties associated with the aqueous foam highly depend on its interfacial properties. In this review, a systematic study of aqueous foam are presented primarily from rheology point of view. Firstly, foaming agents, surfactants and particles are described; then foam structure was explained, followed by change in structure under applied shear. Finally, foam rheology was linked to interfacial rheology for the interface containing particles whose surface properties were altered by surfactants.

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.

Prediction of surface tension of solution in the presence of hydrophilic silica nanoparticle and anionic surfactant by ATR-FTIR spectroscopy and chemometric methods

In the current research, an analytical method was proposed for the quantitative determination of surface tension of anionic surfactant solutions in the presence of hydrophilic silica nanoparticles using attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy and chemometric methods. The surface tension behavior of anionic surfactant solutions considerably changes by the addition of silica nanoparticles with different particle size. The spectral data of solutions were used for prediction of surface tension using two calibration methods based on support vector machine regression (SVM-R) as a non-linear algorithm and partial least squares regression (PLS-R) as a linear algorithm. For preprocessing of data, baseline correction and standard normal variate (SNV) were also applied. Root mean square error of prediction (RMSEP) in SVM-R and PLS-R methods were 4.203 and 4.507, respectively. Considering the complexity of the samples, the SVM-R model was found to be reliable. The proposed method is fast and easy for measurement of the surface tension of surfactant solutions without any sample preparation step in chemical enhanced oil recovery (C-EOR).

Particle-laden fluid/fluid interfaces: physico-chemical foundations

Particle-laden fluid/fluid interfaces are ubiquitous in academia and industry, which has fostered extensive research efforts trying to disentangle the physico-chemical bases underlying the trapping of particles to fluid/fluid interfaces as well as the properties of the obtained layers. The understanding of such aspects is essential for exploiting the ability of particles on the stabilization of fluid/fluid interface for the fabrication of novel interface-dominated devices, ranging from traditional Pickering emulsions to more advanced reconfigurable devices. This review tries to provide a general perspective of the physico-chemical aspects associated with the stabilization of interfaces by colloidal particles, mainly chemical isotropic spherical colloids. Furthermore, some aspects related to the exploitation of particle-laden fluid/fluid interfaces on the stabilization of emulsions and foams will be also highlighted. It is expected that this review can be used for researchers and technologist as an initial approach to the study of particle-laden fluid layers.

Tuning Surface Chemistry and Ionic Strength to Control Nanoparticle Adsorption and Elastic Dilational Modulus at Air-Brine Interface

The relationship between the interfacial rheology of nanoparticle (NP) laden air-brine interfaces and NP adsorption and interparticle interactions is not well understood, particularly as a function of the surface chemistry and salinity. Herein, a nonionic ether diol on the surface of silica NPs provides steric stabilization in bulk brine and at the air-brine interface, whereas a second smaller underlying hydrophobic ligand raises the hydrophobicity to promote NP adsorption. The level of NPs adsorption at steady state is sufficient to produce an interface with a relatively strong elastic dilational modulus E' = dγ/d ln A. However, the interface is ductile with a relatively slow change in E' as the interfacial area is varied over a wide range during compression and expansion. In contrast, for silica NPs stabilized with only a single hydrophobic ligand, the interfaces are often more fragile and may fracture with small changes in area. The presence of concentrated divalent cations improves E' and ductility by screening electrostatic dipolar repulsion and strengthening the attractive forces between nanoparticles. The ability to tune the interfacial rheology with NP surface chemistry is of great interest for designing more stable gas/brine foams.

Tuning Nanoparticle Surface Chemistry and Interfacial Properties for Highly Stable Nitrogen-In-Brine Foams

The design of surface chemistries on nanoparticles (NPs) to stabilize gas/brine foams with concentrated electrolytes, especially with divalent ions, has been elusive. Herein, we tune the surface of 20 nm silica NPs by grafting a hydrophilic and a hydrophobic ligand to achieve two seemingly contradictory goals of colloidal stability in brine and high NP adsorption to yield a viscoelastic gas-brine interface. Highly stable nitrogen/water (N₂/brine) foams are formed with CaCl₂ concentrations up to 2% from 25 to 90 °C. The viscoelastic gas-brine interface retards drainage of the lamellae, and the high dilational elasticity arrests coarsening (Ostwald ripening) with no observable change in foam bubble size over 48 h. The ability to design NP-laden viscoelastic interfaces for highly stable foams, even with high divalent ion concentrations, is of fundamental mechanistic interest for a broad range of foam applications and in particular foams for CO₂ sequestration and enhanced oil recovery.

Adsorption and foaming properties of edible egg yolk peptide nanoparticles: Effect of particle aggregation

The adsorption and foaming properties of an edible colloidal nanoparticle (EYPNs), self-assembled from the food-derived, amphiphilic egg yolk peptides, were investigated, with the aim of evaluating their potential as efficient particulate stabilizers for development of aqueous food foams. The influence of particle aggregation induced by the changes of environmental conditions (mainly the pH) on these properties of EYPN systems was determined. Our results showed that the EYPNs are a highly pH-responsive system, showing the pH-dependent particle aggregation behavior, which is found to strongly affect the interfacial adsorption and macroscopic foaming behaviors of systems. Compared to high pH (6.0–9.0), the EYPNs at low pH (2.0–5.0) showed higher surface activity with a lower equilibrated surface tension as well as a higher packing density of particles and particle aggregates at the interface, probably due to the reduced electrostatic adsorption barrier. Accordingly, the EYPNs at these low pH values exhibited significantly higher foamability and foam stability. The presence of large particle clusters and/or aggregates formed at low pH in the continuous phase may contribute to the foam stability of EYPNs. These results indicate that our edible peptide-based nanoparticle EYPNs can be used as a new class of Pickering-type foam stabilizer for the design of food foams with controlled material properties, which may have sustainable applications in foods, cosmetics, and personal care products.

•

Edible nanoparticle EYPNs are efficient particulate stabilizers for making food foams.

•

EYPNs have a pH-dependent particle aggregation behavior in aqueous solutions.

•

The particle aggregation strongly affects the adsorption and foaming properties.

•

The presence of particle aggregates contributes to the foam stability of EYPNs.

•

The particle aggregates show higher surface activity and interfacial packing density.

Stabilizing decontamination foam using surface-modified silica nanoparticles containing chemical reagent: foam stability, structures, and dispersion properties

The stabilization of decontamination foams containing a chemical reagent is a crucial requirement for their use in the decontamination of nuclear power plants. We have investigated the effects on decontamination foam stability of adding silica nanoparticles (NPs) modified with various functional groups, namely propyl (–CH₃), amine (–NH₂), and thiol (–SH) groups. The surface properties of these silica NPs were characterized with ATR-FTIR, solid NMR, and TGA analyses. We also established that the agglomeration in such foams of the amine-modified silica NPs is weaker than that of the other modified silica NPs due to their thorough dispersion in the liquid film. Further, the foam containing amine-modified silica NPs was found to be stable for 60 min at a pH of 2, i.e. under decontamination conditions. The bubble structure analysis showed that this decontamination foam has a bubble count that is approximately 5–8 times higher than the foams containing NPs modified with the other functional groups, which indicates that the decontamination foam with amine-modified silica NPs has the best foam structure of the three investigated foams. The well-dispersed and smaller amine-modified silica NPs enhance the foam stability by providing a barrier between the gas bubbles and delaying their coalescence. In contrast, the thiol- and propyl-modified silica NPs form aggregates with large diameters that reduce the maximum capillary pressure of coalescence and hence decrease the foam stability.

The stabilization of decontamination foams containing a chemical reagent is a crucial requirement for their use in the decontamination of nuclear power plants.

Vibration controlled foam yielding

In rheological terms, foams are time independent yield stress fluids, displaying properties of both solid and liquid materials. Here we measure the propagation of a 2D dry foam in a radially symmetric Hele-Shaw cell forcing local yielding. The yield rate is manipulated by mechanical vibration with frequencies from 0 to 150 Hz. The flow speed is then extracted from the video stream and analyzed using digital image correlation software. The data are modeled analytically by a Guzman-Arrhenius type of energy landscape where the local yielding of foam correlates with the number of oscillations, i.e. attempts to cross the energy barrier. The model is confirmed in an auxiliary experiment where the vibrated foam stays in its flowing state at the same small driving pressures, where the flow of the unvibrated foam ceases. We conclude that the yield stress behaviour of foams under an external perturbation can be summarized using a simple energy landscape model. The vibration affects the films causing the stress to occasionally and locally exceed the yield threshold. This, thus, prevents the foam from jamming as in a static configuration even when the global driving is below the yield point of the foam.

A Rapid Experimental Procedure to Assess Environmental Compatibility of Conditioning Mixtures Used in TBM-EPB Technology

Earth Pressure Balance (EPB) Tunnel Boring Machines (TBM) are currently the most widely used machines to perform tunnel excavation, particularly in urban areas. This technology involves the injection of chemicals as conditioning mixtures, which commonly raises concerns limiting the reuse of soils after excavation. This study deals with the prospect of a simplified, rapid and replicable methodology for the evaluation of the biodegradability of these conditioning mixtures. For this purpose, the biodegradation of three commercial conditioning mixtures was investigated in closed bottle tests by investigating the effect of different mixtures dosages and two different inocula (soil humus and Bacillus Clausii). While using soil humus as inoculum, a comparative study of biodegradation of the three investigated mixtures was successfully carried out; in the case of Bacillus Clausii, it was not possible to make a comparison between the different formulations in a short time. The adoption of soil humus satisfied only the criteria of rapid test, while the Bacillus Clausii, as specific inoculum, can meet the criteria of replicable results. For this reason, in the second part of this experimental study, a rapid and replicable procedure was proposed and validated. A kinetic study of organic carbon removal was also carried out.