Wet Foams Research Articles

Wet Foam Stability from Colloidal Suspension to Porous Ceramics: A Review

Wet Foam Stability from Colloidal Suspension to Porous Ceramics: A Review

Features of aqueous polymer-stabilized foams particularly containing bentonite particles

Using self-foaming dispenser, wet aqueous foams were prepared from isoviscous solutions of sodium lauryl sulfate (SLS) with chemically distinct polymer stabilizers. Contribution of coalescence and Ostwald ripening to the foam decay was examined from photos and video data. The bubble coarsening was estimated by dispersion analysis using the Lifshitz–Slezov–Wagner equation. Water-soluble polymers were selected from typical thickeners and suspending agents. Xanthan gum, hydroxyethyl cellulose, polyacrylamide, cross-linked sodium polyacrylate, cross-linked and hydrophobically modified (meth)acrylate copolymers were tried as foam stabilizers. By the same way, xanthan gum and submicron bentonite particles were used as foam stabilizers. Stability of the foams containing mentioned polymers as well as xanthan plus bentonite studied by the photographs and dispersion analysis. The polymers characterize by their efficacy to stabilize the wet foam. Possible reasons for poor stabilization of the foam containing cross-linked polyacrylates, both hydrophilic and hydrophobically modified, are given. The behavior of polymer-containing foams and joint stabilizing action of xanthan and bentonite are explained by variations in the structure of foam films.

Wood crib fire tests to evaluate the influence of extinguishing media and jet type on extinguishing performance at close range

abstract This paper is intended to be the first study to discuss the fire suppressing performance of the four most common extinguishing media under the same reproducible conditions. The tests were performed in bench-scale and used standardized 5A wood cribs as well as a miniature extinguishing system with a liquid flow rate of 1.4 l min − 1 . The tests results present a consistent overview of the fire suppression efficacies of water, water with a foaming agent, nozzle-aspirated foam and compressed air foam. Depending on their jet types, the cooling capabilities of the extinguishing media water and water with a foaming agent were compared to the cooling capability of a full jet of wet, general and dry foams. The results show that compressed air foam suppressed fire most effectively under the test conditions. Because of the convoluted crib structure, water and water with foaming agents used from a distance are more effective in the form of a full jet rather than a spraying jet. At close range, spraying jets multiply their effectiveness. A slight difference can be observed in the cooling performance of extinguishing foams that use foaming agents from different manufacturers. The paper establishes a link between the foaming agent's cooling capability and its wetting power by relating the results of wood crib fire tests according to DIN EN 3–7 and findings from laboratory immersion tests compliant with DIN EN 1772.

Foam as a self-assembling amorphous photonic band gap material

We show that slightly polydisperse disordered 2D foams can be used as a self-assembled template for isotropic photonic band gap (PBG) materials for transverse electric (TE) polarization. Calculations based on in-house experimental and simulated foam structures demonstrate that, at sufficient refractive index contrast, a dry foam organization with threefold nodes and long slender Plateau borders is especially advantageous to open a large PBG. A transition from dry to wet foam structure rapidly closes the PBG mainly by formation of bigger fourfold nodes, filling the PBG with defect modes. By tuning the foam area fraction, we find an optimal quantity of dielectric material, which maximizes the PBG in experimental systems. The obtained results have a potential to be extended to 3D foams to produce a next generation of self-assembled disordered PBG materials, enabling fabrication of cheap and scalable photonic devices.

Dislocation mechanisms in the plastic deformation of monodisperse wet foams within an expansion-contraction microfluidic geometry.

Densely packed wet foam was subjected to gradual expansion and contraction in a wide (1400-1800 μm) microfluidic channel to study localized plastic deformation events within the monodisperse bubble matrix. Dislocation glide, reflection, nucleation, and dipole transformations from extensional and compressive stresses were observed across a range of fluid flow rates and bubble packing densities. Disparate, cyclic reflections occur in two independent regions of the flowing foam, and the mechanisms of dislocation reflection under tension are expanded. The use of an asymmetric channel created a dichotomy in the model crystalline system between straighter, aligned bubble rows and curved, misaligned rows due to the corresponding streamlines within the channel. The resulting gradient in crystalline alignment had numerous effects on dislocation mobility and plastic deformation. 7/7 dipoles were found to rearrange to a more stable configuration aligned with the foam flow before dissociating. Dislocations comprising 5/5 dipoles (resembling the inverse-Stone-Wales defect in carbon nanostructures) were discovered to pass through one another via intermediate ring structures, which most commonly consisted of three dislocation pairs around a triangular-shaped central bubble.

Removal of trace hexavalent chromium from aqueous solutions by ion foam fractionation.

Ion foam fractionation is a green and cost-effective technology where separation of molecules exploits the difference in surface affinity. In this work, a batch ion foam fractionation system was designed and optimized for the separation of trace hexavalent chromium (Cr(VI)) from aqueous solutions. The effect of surfactant head groups (collectors) on the adsorption dynamics was analyzed. Cetyl trimethyl ammonium bromide (CTAB), a cationic surfactant showed high efficiency for the removal of Cr(VI) from aqueous solutions. An experimental investigation of the effect of different operational parameters on the separation characteristics is presented. The recovery of Cr(VI) increased with the increase in CTAB/Cr(VI) molar ratio and reached a maximum of 92.5% at optimum operating conditions. However, with CTAB concentrations close to the critical micelle concentration (CMC) wet foams were produced resulting in high liquid hold-up and poor enrichment ratio. The presence of Cr(VI) at the gas-liquid interface significantly improved the drainage characteristics of the foam decreasing the liquid hold-up. Further, a three-stage ion foam fractionation unit was developed with Cr(VI) removal efficiency of more than 99%. The concentration of Cr(VI) in the residue after the three-stage operation was less than 0.02 mg/L which is below the USEPA recommended standards for drinking water.

Stress and bubble pressure response of wet foam to continuous and oscillatory sinusoidal shear.

Wet foam, as a typical multiphase soft material, has complex spatial structure. Foam quality (i.e., gas fraction of a foam fluid), one of fundamental structure parameters of a foam system, generally has a significant influence on the mechanical response of the wet foam to the continuous and oscillatory shear. This study shows that the stress level of the wet foam, including the shear stress and the normal stress difference, rises with the foam quality. An exponential link between the yield stress of wet foam and the foam quality is demonstrated. In the oscillatory sinusoidal shear, a frequent fluctuation of the stress curve mainly occurs at the relatively higher strain rate, and the stress state in the foam is still maintained at the end of the oscillatory shear. Further, with the increase of foam quality, the loss modulus decreases when the foam does not yield, while the storage modulus as well as the loss modulus increases as the strain amplitude exceeds a certain value. Additionally, a nonlinear stress response of the wet foam is mainly attributed to the third harmonic component as the strain amplitude increases in the oscillatory shear. In the shear, the average level of bubble pressure in the foam increases with the foam quality, and it fluctuates with the strain owing to the elastic-plastic deformations of the films. Especially, in the oscillatory shear, the average bubble pressure fluctuates more frequently as the strain rate reaches a relatively higher value.

Potential of a liquid foam-bed photobioreactor for microalgae cultivation

The liquid foam-bed photobioreactor is a novel photobioreactor for microalgae cultivation. A mathematical model was developed to evaluate its potential, and to optimize the design and operation of a large-scale unit. This model describes light limited microalgal growth in a rising foam column in a foam-bed photobioreactor, which is continuously operated at constant biomass density. The microalgae-containing liquid is recirculated from the bottom of the reactor and dispersed equally on the top of the foam column, in order to ensure homogenous microalgae distribution and a wet and stable foam. The model combines calculations of liquid fraction gradient, light penetration, microalgal growth, and gas transfer in the foam-bed. The liquid fraction and light model was experimentally validated. The areal productivity of a 5 cm deep foam-bed photobioreactor operated at 30 g L−1 microalgae and 1500 μmol photons m−2 s−1 was estimated to be 67.7 g m−2 d−1. This productivity is slightly lower compared to what is achievable in flat panels, which is related to light scattering in the foam-bed. Nevertheless, the advantages of the foam-bed photobioreactor, such as high gas transfer rate and high biomass densities, were confirmed with the simulations. In addition, it was calculated that a CO2 uptake efficiency of 97% can be obtained ensuring minimal CO2 loss. These benefits result in reduced gas supply requirement and reduced energy required for downstream processing. The total energy required for the production and separation of 1 g biomass in liquid foam-beds is only 8.5% of what is required in flat panels with suspended biomass. These results highlight the potential of foam-bed photobioreactors for large scale application for microalgae production.

Discharge Characteristics of a Portable Compressed Air Foam System

Existing portable foam extinguishers generate fire-fighting foam at high pressures with the aid of an air aspirating nozzle. This system could encounter several limitations at the point of application such as poor foam quality due to the use of fire contaminated air for foam generation and insufficient momentum to reach the seat of fire. Research has shown that by incorporating compressed air into a portable foam system, the integrated foam system could generate superior quality foam with high momentum when properly installed with the right components. Several studies had been conducted on the extinguishing performance of compressed air foam systems on multiple fire types, both for small and large fires. Compressed air foam systems mitigate exposure of the operator to heat and provides faster knockdown of the fire plume as compared to air-aspirated foam because of its stronger stability and rheology. Since the expansion ratio of the foam can be regulated to combat specific fire types and sizes, compressed air foam systems can be utilized in protecting a variety of equipment of varied sizes.The aim of this study is to investigate the discharge characteristics of a portable compressed air foam at low pressure. For this study, the requirements of NFPA 10 and CAN/ULC-S508 for a new system were used to determine the feasibility of the system.The effect of air pressure on the expansion ratio of the foam was investigated with foam concentrate ranging from 2% to 4% for three different hoses with lengths of 1-m, 2-m and 3-m. Pressure used ranged from 1.72 bar to 5.52 bar. The 3% and 4% solution for the 2-m hose and 3-m hose exhibited similar trend of a rise and fall with pressure by generating fluid foam of medium expansion ratio in the range of 19 to 28. However, the expansion ratio of 3% solution and 4% solution for the 1-m hose increased monotonically with increasing pressure and generated wet foam of low expansion ratio in the range of 8 to 15. While low expansion foams are effective in extinguishing liquid pool fires, medium expansion foams are used for structural protection due to its slow drainage time and its ability to adhere to sloped, vertical, horizontal and slippery surfaces.Discharge range tests were conducted to investigate the horizontal projection of the foam from the nozzle at a height of 0.9m above the ground. The test was conducted in an open space with little interference of wind. Visual record of the maximum discharge range was taken at intervals. The foam from the 1-m hose projected from 1 m at 2.42 bar to 2.4 m at 5.52 bar while the foam from the 2-m hose projected from 1.8 m at 2.42 bar to 4.5 m at 5.17 bar. Likewise, the foam from the 3-m hose with an initial discharge of 1.85 m at 2.41 bar increased progressively to above 4.5 m at 4.83 bar. The tests demonstrated the relationship between pressure and the momentum of the foam, showing that an increase in pressure leads to an increase in the range covered. Furthermore, flow rates at different pressures were investigated using 3% foam solution with a 2 m hose. The flow rate of the foam ranged from 8 g/s to 20 g/s at 1.93 bar and 5.24 bar respectively, indicating linear progression with pressure. The flow rates correspond to application times of 244 and 102 seconds respectively for the 2-liter solution. Overall, all foams tested met the requirements of the CAN/ULC-S508 standard.

The high porosity silicon nitride foams prepared by the direct foaming method

Silicon nitride foams with porosity higher than 90% were fabricated by direct foaming method. The influence of the surfactant content, solid content and rotating speed on the foamability and stability of wet foams were discussed and the corresponding effect on the total porosity, pore size distribution and mechanical property of ceramic foams were systematically investigated. The results showed that the pore structure and porosity, the compressive strength of Si3N4 ceramic foams could be controlled by tailoring surfactant and solid content. The results indicated that rotating speed had a great impact on the pore size distribution. Si3N4 ceramic foams with porosity of 90.8–96.4% and compressive strength of 0.37–5.53 MPa were obtained.

Static and Dynamic Performance of Wet Foam and Polymer-Enhanced Foam in the Presence of Heavy Oil

Inadequate sweep efficiency is one of the main concerns in conventional heavy oil recovery processes. Alternative processes are therefore needed to increase heavy oil sweep efficiency. Foam injection has gained interest in conventional oil recovery in recent times as it can control the mobility ratio and improve the sweep efficiency over chemical or gas flooding. However, most of the studies have focused on light crude oil. This study aims to investigate the static and dynamic performances of foam and polymer-enhanced foam (PEF) in the presence of heavy oil. Static and dynamic experiments were conducted to investigate the potential of foam and PEF for heavy oil recovery. Static analysis included foam/PEF stability, decay profile, and image analysis. A linear visual sand pack was used to visualize the performance of CO2 foam and CO2 PEF in porous media (dynamic experiments). Nonionic, anionic, and cationic surfactants were used as the foaming agents. Static stability results showed that the anionic surfactant generated relatively more stable foam, even in the presence of heavy oil. Slower liquid drainage and collapse rates for PEF compared to that of foam were the key observations through foam static analyses. Besides improving heavy oil recovery, the addition of polymer accelerated foam generation and propagation in porous media saturated with heavy oil. Visual analysis demonstrated more stable frontal displacement and higher sweep efficiency of PEF compared to conventional foam flooding. Unlike foam injection, lesser channeling (foam collapse) was observed during PEF injection. The results of this study will open a new insight on the potential of foam, especially polymer-enhanced foam, for oil recovery of those reservoirs with viscous oil.

Investigation of Air Diffusion in a Wet Foam

Coefficients of air diffusion and permeability for surfactant tetradecyltrimethylammonium bromide aqueous solution and synthetic detergent Fairy are estimated by the diminishing bubble method. It is found that the diffusion coefficient is 5.9 × 10–4 cm2/s for tetradecyltrimethylammonium bromide and is (6.1–6.3) × 10–4 cm2/s for Fairy, and the volume permeаbility coefficient is 0.05 cm/s for tetradecyltrimethylammonium bromide and 0.06 cm/s for Fairy. The accuracy of experimental measurements is 10–15%. The time dependence of the squared diameter of the air bubble is close to linear.

Sonographic interstitial syndrome: Clinical aspects

Lung ultrasonography is gaining ever more consideration among physicians for detecting interstitial diseases. When the tissue is aerated, ultrasound may explore only the first peripheral layer of lung. Alveolar geometry and topology interact with ultrasound like air bubbles in a foam (wet or dry). In clinical ultrasound the normal lung appears like a specular reflector. Hyperdense pre-consolidated lung is physically similar to a wet foam and is sonographically diagnosed by artifacts generically called B-Lines (Sonographyc interstitial syndrome—SIS). Many pathologic conditions are involved. In order to give more specificity to SIS and to differentiate clinical patterns, it is crucial to analyse histologic patterns between cardiogenic and pneumogenic pathologies. Cardiogenic involvement develops through the thickening of interlobular septa, then the alveolar flooding occurs. Thickened interlobular septa can represent a specific “deterministic” acoustic trap that could generate specific artifacts. In a later stage, fluid overflows from interstitium and small free intra-alveolar bubbles are generated without structural tissue subversion. In case of pneumogenic SIS there is a structural deformation with folded and collapsed alveoli, thickened interstitium and damaged alveoli. In conclusion, specific artifacts, namely, radiofrequency signals, could be representative of specific patterns of SIS.Lung ultrasonography is gaining ever more consideration among physicians for detecting interstitial diseases. When the tissue is aerated, ultrasound may explore only the first peripheral layer of lung. Alveolar geometry and topology interact with ultrasound like air bubbles in a foam (wet or dry). In clinical ultrasound the normal lung appears like a specular reflector. Hyperdense pre-consolidated lung is physically similar to a wet foam and is sonographically diagnosed by artifacts generically called B-Lines (Sonographyc interstitial syndrome—SIS). Many pathologic conditions are involved. In order to give more specificity to SIS and to differentiate clinical patterns, it is crucial to analyse histologic patterns between cardiogenic and pneumogenic pathologies. Cardiogenic involvement develops through the thickening of interlobular septa, then the alveolar flooding occurs. Thickened interlobular septa can represent a specific “deterministic” acoustic trap that could generate specific artifacts. In a later...

Foam Injection into horizontal cells with channels of different openings

This paper discusses experimentally the advance and distribution of preformed,wet aqueous foam ow when it is injected into a horizontal rectangularcell with dierent transversal opening channels. Speeds of the simultaneousfoam fronts in each channel were obtained experimentally. Therelationship between bubble size and the opening of the channel, the apparentviscosity and the friction factor were obtained with experimental data.Experiments were performed under three dierent injection pressures. Thediversion of the foam ow into the transversal channel was also studied. Withthese experiments we show that the foam ow stream is aected by suddenchanges in the geometry.

Dynamics of Liquid Oil that Flows Inside Aqueous Wet Foam

Wet soap foam spontaneously imbibes liquid oil without defoaming when it is brought into contact. The kinetics behind this recently observed phenomenon was studied experimentally, with focus on the origin of the suction force and on the oil front dynamics. Using an aqueous foam with an air volume fraction slightly greater than the critical valueϕC, we show that the pumping pressure of oil and/or miscible liquid into the wet foam is attributed to the interfacial distortion of the bubble surfaces. Two distinct regimes along time t were observed in the oil imbibition dynamics. The proceeding oil front evolves with t1/2 dependency in the early imbibition time in accordance with the classical theory of penetration of a porous medium, whereas it departs into t1/3 at late imbibition time. The latter process is attributed to the elongation of an oil branch trapped inside the foam when pumping of the exterior oil has ceased.

Numerical study of a foam-shock trap based blast mitigation strategy

Blast mitigation using various types of foams is of interest to practitioners for the safe design of structures. Experimental and numerical studies have demonstrated the beneficial effects of different foam materials when used as protective soft coatings. However, under certain blast loading conditions and foam dimensions, the load experienced by the target/protected structure was found to be much higher in the presence of foam than in its absence. In this study, a mechanism based on geometric means known as shock trap is used along with foam as a preventive measure against shock amplification. A shock trap is a special arrangement of rigid obstacles with an air gap, designed to offer a tortuous flow path. To analyze the proposed foam-shock trap combination, a popular Lagrangian based Smoothed Particle Hydrodynamics (SPH) is employed. A novel solid wall boundary modeling technique called skew gradient wall boundary treatment is also built into the SPH solver. The material discontinuity is handled by a multimass correction strategy. The blast wave mitigation characteristics of the proposed approach are evaluated using two different foam materials, namely, wet aqueous foam and polystyrene foam. From detailed simulations, it was observed that undesirable shock enhancement effect of foams was found to be completely suppressed by combining it with the shock trap mechanism. The proposed foam shock trap combination significantly reduces the peak load and impulse experienced by the target structure.

Coarsening and mechanics in the bubble model for wet foams.

Aqueous foams are an important model system that displays coarsening dynamics. Coarsening in dispersions and foams is well understood in the dilute and dry limits, where the gas fraction tends to zero and one, respectively. However, foams are known to undergo a jamming transition from a fluidlike to a solidlike state at an intermediate gas fraction ϕ_{c}. Much less is known about coarsening dynamics in wet foams near jamming, and the link to mechanical response, if any, remains poorly understood. Here we probe coarsening and mechanical response using numerical simulations of a variant of the Durian bubble model for wet foams. As in other coarsening systems we find a steady state scaling regime with an associated particle size distribution. We relate the time rate of evolution of the coarsening process to the wetness of the foam and identify a characteristic coarsening time that diverges approaching jamming. We further probe mechanical response of the system to strain while undergoing coarsening. There are two competing timescales, namely the coarsening time and the mechanical relaxation time. We relate these to the evolution of the elastic response and the mechanical structure.

Direct Writing and Controlling of Hierarchical Functional Metal-Oxides: Bio-inspired Multiphase Processing, 3D Printing and Hierarchical Cellular Structuring

Nature-inspired processing of complex reticular/cellular architected materials by selectively incorporating multiphase materials in emulsions with targeted interactions is reported in this work. In particular, structuring of TiO2 foams at different scales is realized by combining colloidal processing and 3D printing with fundamental understanding of the TiO2 nanoparticle system. Specifically, hybrid TiO2/Ti-organic aqueous suspensions are emulsified with bio-compatible and naturally synthesized oil stabilizers where air bubbles are trapped, forming the cells of the wet foams, which are subsequently directly written as planar and three-dimensional free-standing and spanning structures. Variations in the formulation of these emulsions are implemented to tune their viscosity and render diverse cell morphologies (i.e., open- or closed-cell foam configurations) while controlling their micro-, meso- and macro-porosity. Investigation of the foams’ curing using heat versus UV light indicates differences in the emulsion’s oil-phase elimination mechanism with effects in their microstructure, which, added to the characteristic relationships between the foams’ microstructures and their photocatalytic activity, can be used to target specific properties. Our synthesis method, adjusted from the cosmetics industry, combines inorganic processing with emulsion systems of non-toxic and abundant material precursors to realize hierarchically ordered mesoporous titania foams. This method can be further exploited with other metal-oxide materials systems and even more complex emulsions to meet the needs of the currently growing technologic application fields. Moreover, we anticipate our family of foams to be readily relevant for a plethora of applications including catalysis, energy harvesting and recovery, and biomedical. Designed to be safe and water-compatible, our multi-phase emulsion systems marry high control over the materials’ microstructure and interfaces with scalable, affordable and responsible manufacturing.

Chitin Nanofibrils to Stabilize Long-Life Pickering Foams and Their Application for Lightweight Porous Materials

The demand of sustainable development is challenging researchers to convert renewable resourced biomass into functional materials via environmentally friendly and sustainable pathways. This work introduces a long-life Pickering foam stabilized by chitin nanofibers (CNFs) as colloidal rod-like particles, and a facile method for fabricating lightweight porous solid foams that recycles biomass materials derived from seafood waste. These foams were formed by combining nonionic surfactant Tween 20 (T20) and CNFs, with the CNFs being irreversibly adsorbed at the air–water interface to provide Pickering stabilization. At a concentration of 7.5 mg/mL, the foams could be stable for over 1 week without any apparent drainage. The rheological data indicated the formation of gel networks by self-aggregated CNFs at the air–water interface, which provided long-term stabilization by preventing foam coalescence and disproportionation. This long-term stability of CNF-T20 wet foam has permitted the fabrication of solid poro...

From Pickering foams to porous carbonate materials: crack-free structuring in drying ceramics

Particle-stabilized foams have attracted considerable research interest, due to their long term stability (months to years) and the possibility to use them as precursors for production of porous materials with hierarchical porosity. In our previous study [Lesov et al., J. Colloid Interface Sci. 504 (2017) 48–57] we clarified the role of the rheological properties of the foamed suspensions and the type of foam film stabilization in the production of porous silica materials with low mass density and excellent insulating properties. In the current study we extend our approach to prepare lightweight carbonate ceramics with controlled density, shrinkage and good mechanical properties. To prepare the wet foam precursors, we tested a series of eight anionic surfactants which were previously reported to provide sufficient hydrophobization of CaCO3 particles and long-term stability of the liquid foams. From those surfactants, the medium-chain fatty acids led to crack-free porous materials with superior mechanical strength, compared to the conventional surfactants. We study the reasons for the formation of cracks in drying Pickering foams and, on this basis, propose optimal conditions for obtaining dry porous carbonate materials with required porosity. Mechanistic explanations are proposed for the main observed effects.