Foam Lifetime Research Articles

Ion-specific effects on foam films and foam from diluted solution of sodium dodecyl sulfate

The ion-specific effects on extremely diluted solutions of surfactants are poorly studied. The effect of Li+ , Na+ and K+ ions on the properties of foam films and foam stabilized by 0.05 mmol/L sodium dodecyl sulfate (SDS) was studied. Each one of the salts LiCl, NaCl and KCl in the concentration range of 1.1 mmol/L to 50 mmol/L was added to 0.05 mmol/L solution of SDS, for studying the properties of its foam films and foams. It was established that the surface potential of the foam films does not depend on the type of salt added, which means that no ion-specific effects are registered. On the contrary, such effects are evident in the case of foam produced by sparging of gas through porous bottom of column containing the same solutions. Thus, the rates of foaming, foam decay and the foam lifetime vs. the concentration of added salt (LiCl, NaCl and KCl) were studied by means of Dynamic Foam Analyzer (DFA-100, Kruss Ltd, Germany). The foam production number for each one of these cases was calculated as well. It was established that both NaCl and KCl boost the foam, the first one of which being stronger than the second one. On the contrary, LiCl has no effect on the foamability of the system. The present study is an example for manifestation of a salt-specific effect under dynamic conditions, which vanishes under static conditions.

Stability of an aqueous foam lamella at simulated fire conditions containing surfactants relevant to firefighting foams

To better understand interactions between firefighting foams and a fire environment, we isolated interactions to a single surfactant containing bubble lamellae. A horizontal thin liquid film experiment was designed to study film rupture in the presence of heated air and heptane vapors. Thin films were formed from aqueous solutions of alkylpolyglycosides (APGs, G215 and G225), trisiloxane (502W and L77), and fluorinated (Cap) surfactants and their mixtures. The Cap surfactant film was the most stable to heat but in the presence of heptane, the surfactant film had a comparable (and the Cap:G215 mixture had a shorter) rupture time to the fluorine-free surfactant films. APG films showed the least destabilization with heptane followed by the trisiloxane films and the Cap film which was the most destabilized by heptane. L77, Cap, and their respective mixtures formed possible emulsions with the fuel vapors, turning cloudy with vapor introduction. The correlation between film rupture behavior and fuel induced foam lifetime may be better defined by minimizing error in lifetime and producing longer film lifetimes. This thin film experiment represents a new way of observing surfactant/fuel interactions that may elucidate important surfactant properties for firefighting foams.

Tailoring Amphiphilic Copolymers for Improved Aqueous Foam Stability.

Amphiphilic copolymers of various-molecular-weight (MW) poly(ethylene glycol) (PEG) were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. The first PEG series, poly(ethylene glycol)monomethacrylate (PEGMA, average Mn 200 and 400 MW), contained an -OH terminal group, and the second series, poly(ethylene glycol) monomethyl ether monomethacrylate (PEGMMA, average Mn 200, 400, and 1000 MW), possessed an -OCH3 terminal group. A total of five PEG-functionalized copolymers contained the same hydrophobic monomer, butyl acrylate (BA), and were successfully reproduced via a one-pot synthesis. The resulting PEG-functionalized copolymers provide a systematic trend of properties including surface tension, critical micelle concentration (CMC), cloud point (CP), and foam lifetime based on the average MW of the PEG monomer and final polymer properties. In general, the PEGMA series produced more stable foams with PEGMA200 demonstrating the least change in foam height with time over a 10 min period. The important exception is that at elevated temperatures, the PEGMMA1000 copolymer had longer foam lifetimes. The self-assembling copolymers were characterized by gel permeation chromatography (GPC), 1H nuclear magnetic resonance (NMR), attenuated total reflection Fourier transform infrared (FTIR-ATR), CMC, surface tension, dynamic light scattering (DLS), as a foam using a dynamic foam analyzer (DFA), and foam lifetime at ambient and elevated temperatures. The copolymers described highlight the importance of the PEG monomer MW and terminal end group for surface interaction and final polymer properties for foam stabilization.

Cross over to collective rearrangements near the dry-wet transition in two-dimensional foams

Liquid foams respond plastically to external perturbations over some critical magnitude. This rearrangement process is directly related to the mechanical properties of the foams, playing a significant role in determining foam lifetime, deformability, elasticity, and fluidity. In this paper, we experimentally investigate the rearrangement dynamics of foams near a dry-wet transition. When a foam transforms from a dry state to a wet state, it is found that considering collective events, separated T1 events propagate in dry foams, while T1 events occur simultaneously in wet foams. This cross over to collective rearrangements is closely related to the change in local bubble arrangements and mobility. Furthermore, it is also found that a probability of collective rearrangement events occurring follows a Poisson distribution, suggesting that there is little correlation between discrete collective rearrangement events. These results constitute progress in understanding the dynamical properties of soft jammed systems, relevant for biological and material sciences as well as food science.

Effect of temperature on stability and film thinning behavior of aqueous film forming foam

In the extinguishing process, the stability of the foam extinguishing agent would be inevitably affected by the flame with high temperature, which has an impact on the effectiveness of the foam. Therefore, it is crucial to investigate the effect of temperature on the stability of foam. The prior research related with the foam stability has mainly concentrated on foam drainage and foam height. However, the details of foam stability have only been the subject of a few studies. In this paper, the effect of temperature on stability and film thinning behavior of aqueous film forming foam (AFFF) were studied using microscope and the optical interferometry device, where the bubble sizes, the film thinning rate and the film lifetime were measured and analysed. The results show that the foam stability and foam film stability were influenced by temperature significantly. The foam lifetime was discovered to decrease exponentially with the increase of temperature, showing that the foam became unstable under higher temperature. Besides, the predicted model of film thickness under different temperature was induced and verified, where the effect of temperature on the foam fluid viscosity was considered. It was also found that the film lifetime decreased exponentially with the increase of temperature, which was caused by the changing in the film surface adsorption, film surface elasticity and the film viscosity under higher temperature.

Assessing the mixed foam stability of different foam extinguishing agents under room temperature and thermal radiation: An experimental study

Foam extinguishing agents have suggested to be an important method for suppressing fires, where the foam stability is one of the key factors in determining the efficiency of fire-fighting. Thus, lots of studies related to foam stability have been carried out. However, the influence of mixed types and ratios of foam agents on foam stability is still unclear. This study focuses on the foam stability of mixed foam extinguishing agents, especially under the condition of the thermal environment. The effects of mixed type and ratio on foaming ability, drainage, coarsening and insulation characteristics are studied systematically. Results indicate that the stability of mixed foam with S addition would decrease when compared with pure AFFF and FP foam, as evidenced by the decrease in foaming height, drainage time, and foam lifetime. The addition of S to AFFF or FP would produce the wetter mixed foam. In addition, the stability of AFFF/FP mixed foam first increases and then decreases with the increase of FP addition, and reaches the peak value at the mixed ratio of 3:1. The variations of foam stability could be explained by the change of viscosity and structure of adsorption layer at gas–liquid interface. The results provide helpful guidance for achieving high effective utilization of different foam extinguishing agents.

Effect of bentonite clay particles on the behavior of foam stabilized by SDS–PVA complexes

In recent years, there has been increased interest in studies on the stabilization of liquid foams with solid particles. In such three-phase foams it is possible to completely stop the process of syneresis, reduce coalescence and diffusion of gas. The presence of particles can significantly increase the time of the foam lifetime. Such Pickering foams are also of interest as an environmentally friendly and cost-effective alternative to surfactants and high molecular weight foaming agents. This work is focused on investigating the foam stability containing SDS–PVA composite foaming agents with bentonite clay particles. Foam stability was characterized via the time of foam drainage. The stability of foams of SDS–PVA mixtures in absence and presence of bentonite particles was determined. The stability of foams is higher in presence of the bentonite particles.

Shape Memory Polymer Foams with Phenolic Acid-Based Antioxidant Properties.

Phenolic acids (PAs) are natural antioxidant agents in the plant kingdom that are part of the human diet. The introduction of naturally occurring PAs into the network of synthetic shape memory polymer (SMP) polyurethane (PU) foams during foam fabrication can impart antioxidant properties to the resulting scaffolds. In previous work, PA-containing SMP foams were synthesized to provide materials that retained the desirable shape memory properties of SMP PU foams with additional antimicrobial properties that were derived from PAs. Here, we explore the impact of PA incorporation on SMP foam antioxidant properties. We investigated the antioxidant effects of PA-containing SMP foams in terms of in vitro oxidative degradation resistance and cellular antioxidant activity. The PA foams showed surprising variability; p-coumaric acid (PCA)-based SMP foams exhibited the most potent antioxidant properties in terms of slowing oxidative degradation in H2O2. However, PCA foams did not effectively reduce reactive oxygen species (ROS) in short-term cellular assays. Vanillic acid (VA)- and ferulic acid (FA)-based SMP foams slowed oxidative degradation in H2O2 to lesser extents than the PCA foams, but they demonstrated higher capabilities for scavenging ROS to alter cellular activity. All PA foams exhibited a continuous release of PAs over two weeks. Based on these results, we hypothesize that PAs must be released from SMP foams to provide adequate antioxidant properties; slower release may enable higher resistance to long-term oxidative degradation, and faster release may result in higher cellular antioxidant effects. Overall, PCA, VA, and FA foams provide a new tool for tuning oxidative degradation rates and extending potential foam lifetime in the wound. VA and FA foams induced cellular antioxidant activity that could help promote wound healing by scavenging ROS and protecting cells. This work could contribute a wound dressing material that safely releases antimicrobial and antioxidant PAs into the wound at a continuous rate to ideally improve healing outcomes. Furthermore, this methodology could be applied to other oxidatively degradable biomaterial systems to enhance control over degradation rates and to provide multifunctional scaffolds for healing.

Stabilization of CO2 aqueous foams at high temperature and high pressure: Small-angle neutron scattering and rheological studies

Understanding the evolution process including foaming, stabilization and ageing of CO2 aqueous foams in high-temperature, high-pressure (HTHP) environments is crucial for their rational use. However, most studies have been conducted through macroscopic observation, leading to inadequate insight into their intrinsic mechanisms. Herein we attempt to gain a comprehensive understanding of the evolution process by a combination of HTHP visualization foam meter, HTHP small-angle neutron scattering (SANS) and HTHP rheometry. A C22-tailed tertiary amine, N-erucamidopropyl-N,N-dimethylamine (UC22AMPM), was first employed to develop robust CO2 foams. It was found the volume and lifetime of UC22AMPM-CO2 foams decrease by 24.7% and 56.9%, respectively, as the temperature rises from 25 °C to 100 °C at 8.5 MPa. The deterioration of the foam properties at elevated temperatures is attributed to the continuous phase viscosity reduced from 70 mPa·s to 25 mPa·s, leading to faster drainage, as well as to the coarsening rate increased from 6.60 × 104 to 9.83 × 105 μm3 min−1, concomitant with the change in bubbles shapes, indicting enhanced coarsening and coalescence. Likewise, the decrease in pressure from 10 MPa to 1 MPa also resulted in a 46.7% and 91.0% reduction in foam volume and lifetime at 100 °C, respectively. The impairment in foam properties with decreasing pressure also comes from the decrement of continuous phase viscosity, accelerating the drainage and thereby lowering the liquid fraction in the foam. The decreasing liquid fraction entails the liquid film to thin and weak, enhancing coarsening of bubbles and their merging. Our findings improve the insights into the foam evolution and pave a new road towards more comprehensive characterization of CO2 foam under HTHP conditions.

Copolymer Reversible Addition-Fragmentation Chain Transfer Synthesis of Polyethylene Glycol (PEG) Functionalized with Hydrophobic Acrylates: A Study of Surface and Foam Properties

A series of amphiphilic statistical copolymers involving poly(ethylene glycol) monomethacrylate (PEGMA, -OH terminated, average Mn 200 molecular weight) and various hydrophobic acrylates were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. The gradient copolymers were characterized by gel-permeation chromatography (GPC), 1H nuclear magnetic resonance (NMR), and attenuated total reflection Fourier transform infrared spectroscopy (FTIR-ATR). Solution properties of the copolymers were investigated utilizing surface tension measurement, dynamic light-scattering (DLS), as well as foam analysis using a dynamic foam analyzer (DFA). The PEG-functionalized copolymers showed a systematic trend depending on the hydrophobic moiety in properties including surface tension, critical micelle concentration (CMC), foam lifetime, and liquid drainage from the foam. Copolymers with alkyl-acrylates exhibited the best foam lifetime, demonstrating that the choice of hydrophobic moiety is crucial for foam stability. The PEG-functionalized materials described are considered promising additives for foam-stability purposes.

Superstable Wet Foams and Lightweight Solid Composites from Nanocellulose and Hydrophobic Particles.

Colloids are suitable options to replace surfactants in the formation of multiphase systems while simultaneously achieving performance benefits. We introduce synergetic combination of colloids for the interfacial stabilization of complex fluids that can be converted into lightweight materials. The strong interactions between high aspect ratio and hydrophilic fibrillated cellulose (CNF) with low aspect ratio hydrophobic particles afford superstable Pickering foams. The foams were used as a scaffolding precursor of porous, solid materials. Compared to foams stabilized by the hydrophobic particles alone, the introduction of CNF significantly increased the foamability (by up to 350%) and foam lifetime. These effects are ascribed to the fibrillar network formed by CNF. The CNF solid fraction regulated the interparticle interactions in the wet foam, delaying or preventing drainage, coarsening, and bubble coalescence. Upon drying, such a complex fluid was transformed into lightweight and strong architectures, which displayed properties that depended on the surface energy of the CNF precursor. We show that CNF combined with hydrophobic particles universally forms superstable complex fluids that can be used as a processing route to synthesize strong composites and lightweight structures.

Duality of foam stabilization

Can a foam reversibly switch between particle and surfactant stabilization? Sure it can. Foams are usually stabilized either by surfactant or hydrophobic nanoparticles. Their combination often worsens the stability of the foam, with surfactant adsorption on the particles causing complex behavior. A system that shifts between both types of mechanisms is desirable both for fundamental studies and practical applications. We developed a system consisting of aqueous solution of 0.1 mM sodium dodecyl sulfate (SDS) + 1.23 mM ZnSO4 + 1.23 mM ethylenediaminetetraacetic acid (EDTA). The foam was produced by the shaking method. The pH value was varied by small additions of HCl or NaOH. We tuned the foam lifetime from 15 to 20 s (at pH = 3 – 6), to ~ 180 s near pH = 1, and increase it in the pH range of 6–12, reaching 15 min at pH = 12. This is due to emerging of nanoparticles of Zn(OH)2 at pH > 8, smoothly switching between the stabilization mechanisms. The nano-suspension of Zn(OH)2 is transparent due to the low aggregation. The low cost (<$1 per metric ton) and the large timescales makes our system desirable for industrial applications.

Amino acid-driven hydrophobization of alumina and production of particle stabilised foams in a broader pH range

The interaction of amino acids (glycine, L-valine, L-isoleucine and L-leucine) with alumina surface was studied to induce partial hydrophobization and production of ultrastable particle-stabilised foams. The evaluation of these amino acids was carried out by mechano-quantum simulations followed by experimental tests (foamability, zeta potential, contact angle and foam lifetime measurements). The experimental results agreed with the trends pointed out by the simulations. The selected amino acids interact with alumina particles in aqueous media and in a broader pH range, leading to hydrophobization of surfaces, which was more intense for amino acids with higher molecular mass (isoleucine and leucine). As a consequence, ultrastable foams with a longer lifetime (> 100 h) were produced and the foam microstructure was preserved from ageing phenomena. Moreover, the attained foams were stable at different pH, opening up new possibilities to develop macroporous multiphasic ceramics, which can result in novel materials for thermal insulation at high temperatures.

Finite Element Model for Coupled Diffusion and Elastoplastic Deformation during High-Temperature Oxidation of Fe to FeO

Solid-oxide iron-air batteries are an emerging technology for large-scale energy storage, but mechanical degradation of Fe-based storage materials limits battery lifetime. Experimental studies have revealed cycling degradation due to large volume changes during oxidation/reduction (via H2O/H2 at 800 °C), but degradation has not yet been correlated with the microstructural stress and strain evolution. Here, we implement a finite element model for oxidation of a Fe lamella to FeO (74% volumetric expansion), in a lamellar Fe foam designed for battery applications. Growth of FeO at the Fe/gas interface is coupled, via an oxidation reaction and solid-state diffusion, with the shrinkage rate of the Fe lamellar core. Using isotropic linear elasticity and plastic hardening, the model simulates deformation of a continuously growing FeO layer by dynamically switching “gas” elements into new “FeO” elements along a sharp FeO/gas interface. As oxidation progresses, the effective plastic strain and von Mises stress increase in FeO. Distribution of tensile and compressive stresses along the Fe/FeO interface are validated by oxidation theory and explain interface delamination, as observed during in operando X-ray tomography experiments. The model explains the superior stability of lamellar vs dendritic foam architectures and the improved redox lifetime of Fe-Ni foams.

Characterizing the Role of Fluorocarbon and Hydrocarbon Surfactants in Firefighting-Foam Formulations for Fire-Suppression

Fluorosurfactants used in current firefighting foams must be replaced with environmentally-friendly surfactants; however, current fluorine-free surfactants have subpar fire performance. Understanding how a surfactant affects fire performance of a foam is essential for developing new fluorine-free replacement surfactants. Surfactants can affect foam’s ability to form a barrier between a fuel-pool surface and the fire, thereby suppressing the fuel feeding the fire and extinguishing it. Fire extinction performance, fuel transport through a foam layer, foam degradation by fuel, and foam spread rate were measured for solutions containing a mixture of fluorocarbon and hydrocarbon surfactants as well as the individual surfactants to understand the roles of different surfactants in a formulation. The measurements show that surfactants that reduce foam degradation and fuel transport rates through a foam covering a hot pool result in faster fire extinction. Small synergistic effects for the mixture of fluorocarbon surfactant (Cap) and a hydrocarbon surfactant (Gluc215) were found in foam degradation rate and fuel transport rate through a foam layer. The surfactant mixture had 30% longer foam lifetime and two times smaller fuel flux through a foam layer than the fluorosurfactant solution (Cap) and differed with the hydrocarbon surfactant solution (Gluc215) by more than a factor of ten. However, when a different hydrocarbon surfactant (TX100) was used in a mixture with the fluorocarbon surfactant, synergism was not observed. The small synergistic effects resulted in a small reduction in fire extinction times at high foam application rates (> 500 mL/min) at which foams can cover the pool surface quickly. At small foam application rates (< 500 mL/min), effects of foam spread rates become significant; the fluorocarbon surfactant (Cap) formed wetter foam that spread quicker and led to smaller fire extinction times than those for the surfactant mixture (Cap + Gluc215). Thus, the foam spread, foam degradation, and fuel transport rate measurements performed in the presence of a hot fuel but in the absence of a fire can explain the surfactant’s effects on fire extinction times for the six formulations. Traditionally, aqueous film formation was thought to be the key for fire suppression of AFFF rather than the foam dynamics described in this work. Additional properties such as static and dynamic surface tensions, foam bubble diameter distribution, bubble coarsening, liquid drainage from the foam, and expansion ratio were also measured in the absence of a fuel, but did not explain the trends in extinction performance among the surfactants.

Spinodal stratification in ultrathin micellar foam films

We report the discovery and analysis of spinodal stratification, a hitherto unreported mechanism underlying drainage and rupture of micellar foam films, that presents unexplored opportunities for understanding and controlling the stability, lifetime and properties of ubiquitous foams.

Thickness-Dependent Phase Transition Drives Nanoridge-to-Mesa Instability in Micellar Freestanding Films.

Understanding fluxes and instabilities within freestanding ultrathin films is necessary for a better understanding of, and control over, the stability and lifetime of foams and emulsions. In micellar foam films, confinement-induced layering of micelles leads to stepwise thinning or stratification that occurs by the expansion of thinner, darker domains. Often, because of a nanoridge-to-mesa instability, one or more brighter white spots or "mesas" appear at the circular moving front between thinner domains and the thicker (less dark) surrounding film. Previous studies assume that the instability and the appearance of white spots are similar to the capillarity-driven Rayleigh instability that leads to the breakup of a coherent liquid jet. Using the IDIOM (interferometry digital imaging optical microscopy) protocols we recently developed, we characterize the nanoridge-to-mesa instability with exquisite spatiotemporal resolution (thickness <1 nm, time <1 ms). The instability could be classified as a Rayleigh instability if a similar sequence of thick and thin undulations is visualized around the expanding domains. However, quantitative analysis reveals that only mesas grow in size after the instability, whereas the rest of the nanoridge preserves its shape. By analogy to the phase separation into compositionally distinct regions, we show that the spontaneous nucleation of thicker mesas in stratifying films is a phase transition driven by the oscillatory nature of the free-energy functional.

On the energy required to maintain an ocean mirror using the reflectance of foam

Among the various interventions proposed to remediate the health and security effects of climate change by solar radiation protection is the proposal to enhance natural ocean whitecap formation. Compared to other solar protection interventions, this is technically simple and quickly terminated. However, it has a drawback: even if the energy be obtained from wind or wave, the power demand to maintain a foam raft determines the capitalization of equipment. The average power demand is inversely related to foam lifetime which can be prolonged by surfactants preferably derived from ingenerate resources. Here, we estimate the associated energy and power demands by identifying the parameters that can be adjusted to moderate the capital cost of implementation. Before dividing by efficiency factors, the range of power demand for an intermediate areal energy requirement of 5 MJ/km2 of ocean varies from 6 to 30 W/km2 for foam lifetime of 10–2 days. The most likely route to deployment is through merchant ship lubrication using bubbly liquids which both reduces fuel consumption and creates an extended wake and is perhaps an example of technical symbiosis.

Influence of fuel on foam degradation for fluorinated and fluorine-free foams

We performed experiments to quantify fuel-induced foam degradation by applying foams onto liquid fuels and water (for comparison) and measuring foam thickness over time. Our investigation included two firefighting foams, one fluorine-free (RF6-ICAO) and the other fluorinated (AFFF), and a foam made with a common surfactant, SDS. We applied a roughly 2cm thick foam layer onto three liquid fuels (n-heptane, methylcyclohexane, and isooctane) at room and elevated temperatures. Foam lifetime was reduced by 50 and 75% for AFFF and RF6 respectively for foams on fuels compared to foams on water at room temperature. For all experiments, the fluorine-free foams (RF6 and SDS) degraded much faster than AFFF. Further, the effect of fuel temperature was significant when the foams were placed over hot fuel: the lifetime of the firefighting foams decreased by 1–2 orders of magnitude between experiments conducted with fuel at room temperature and 50°C. Prior to the onset of foam degradation over fuels, the firefighting foams experienced a preliminary expansion (by up to 50% in volume). Video recordings of degradation show that expansion results primarily from bubbles near the interface increasing in size with accelerated coarsening by coalescence. We propose and discuss a mechanism for fuel-induced foam degradation based on our observations. Our results show that fluorine-free RF6 degrades faster than AFFF (by a factor of 3 at room temperature and 12 at elevated temperatures over fuel), which may contribute to differences in their firefighting performance.

Ultra-stable aqueous foams with multilayer films stabilized by 1-dodecanol, sodium dodecyl sulfonate and polyvinyl alcohol

Ultra-stable foams with multilayer films which were similar to the structure of colloidal gas aphrons (CGA) were designed in view of increasing the amount of bound water molecules at molecular level. This liquid films in foams were spontaneously assembled by 1-dodecanol (DDA), sodium dodecyl sulfonate (SDS) and polyvinyl alcohol (PVA) molecular with a remarkable long-term foam lifetime. The states of water molecules as well as the molecular arrangement in foam films have been analyzed via molecular dynamics (MD) simulation to explore microscopic character and stable mechanism of multilayer films of foams. We found that the combined effect of SDS, DDA and PVA could not only increase distribution intensity of bound water molecules in foam film layers, but it also could decrease the gas diffusion rate to balance the pressure of the air layers of the multilayer foams.