Wet Foams Research Articles

Microalgae Chlorella vulgaris and kraft lignin stabilized cellulosic wet foams for camouflage.

Plants, animals, and humans use camouflage to blend in with their surroundings. The camouflage is achieved with different combinations of colors, patterns, and morphologies. In stealth applications, the simplest camouflage uses textiles colored similarly to the environment to create an illusion. However, often, visible light range camouflage is not enough since the multispectral detection technologies of today are readily utilized for identification. Foams can be created with a straightforward fabricating process, and lightweight material exhibits good thermal insulation properties, providing stealth in the infrared light region. Herein, we produce cellulosic wet foams from surfactant and bleached pulp or cellulose nanofibrils. The visible light camouflage is created with green microalgae, Chlorella vulgaris, and brown kraft lignin, which also stabilized the foams. The thermal and spectral camouflage performance of foams was influenced by the cellulose content as well as the stability and water content of foams. Overall, these results give insight into how stability impacts the thermal and spectral properties of wet foams and provide a solid base for further material development to improve camouflage performance. While there is plenty of data on dry foams, the functional behavior of wet foams is currently not well known. Our method, using plant-based components can be exploited in a variety of other applications where simplicity and scalability are important.

Self‐Assembly of Strongly Amphiphilic Janus Nanoparticles into Freestanding Membranes

AbstractAmphiphilic nanoparticles can adsorb at interfaces between immiscible phases and form membranes. The ability to handle nanoparticle monolayers/bilayers as freestanding membranes independently from the interfaces on which they form can be of significant practical interest. Janus nanoparticles (JNPs) could generate membranes that have tunable pore size and are amphiphilic. Here it is shown how freestanding membranes from JNPs can be generated. First, strongly amphiphilic Janus nanoparticles that generate foams in surfactant‐free conditions are synthesized. Upon drying the foam lamellae remain as freestanding membranes consisting of perfectly oriented bilayers or monolayers of JNPs, thus providing direct evidence of structural organization of amphiphiles in foam lamellae. The wet foam lamellae can be picked up on grids and upon drying generate freestanding membranes. Sturdy freestanding membranes by transferring monolayers of JNPs from the air–water interfaces onto grids with fine openings are also prepared.

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.

Insights into interfacial behaviours of surfactant and polymer: A molecular dynamics simulation

Surfactants and polymers are two common chemicals used to stabilize the wet foam, but molecular-level insights into the structures, dynamics and hydrogen bonds of these agents and water at the surface are still lack. Herein a series of all-atom molecular dynamics simulations were conducted to explore the behaviors of surfactants/polymers at liquid films. Simulations confirmed that OPC water model can better reproduce the surface tension, and surface tension decreases while interface thickness increases as the increase of surfactant surface concentration. Our findings provide molecular insights on the morphology transitions of surfactant monolayers, which is dependent on the van der Waals energy among surfactants. Surfactant monolayers greatly retard the mobility of the interfacial water molecules, but such capability only exists nearby interfacial area. By adding polymers inside, water dynamics can be retarded further through the hydrogen bonding effects. In other words, polymers in the bulk solution produce a “bolt-like” effect which constrains the water around the chains.

Harnessing Ostwald ripening to fabricate hierarchically structured mullite-based cellular architecture via the gelation network-triggered morphology-regulation method

Gas-in-liquid foam templating is considered as a promising method for commercial-scale production of porous materials. However, Ostwald ripening occurs frequently during the preparation, which could bring about inhomogeneous bubbles with large sizes and deteriorate the performance of resultant products. To tackle this challenging task, we report a gelation network-triggered morphology-regulation strategy during the foaming process. As an exemplar, boehmite sol was incorporated into the andalusite suspension to stabilize the foam framework. After casting, the wet foam was able to maintain its overall shape, since andalusite particles were anchored onto the gas-liquid interface by the gelation network during mechanical frothing. The samples calcined at 1400°C feature homogeneous bubble with hierarchical porous architecture, smaller pore size with a mean value of 5.32 μm and robust compressive strength of 11.97MPa at a porosity level of 84 %. Our approach substantially broadens the design philosophy of high performance porous ceramics.

Coarsening of foam in two model fractures with different roughness

Foam coarsening by diffusion (Ostwald ripening) has been well studied in bulk foams. However, it is less well understood in porous media. In particular, the mechanisms that may slow or stop coarsening have not been fully investigated. In this paper, we report an experimental study of foam coarsening in two 1-m-long and 15-cm-wide model fractures. The model fractures, Model 1 and Model 2, are made of glass plates and have different roughness. Model 1 has a regular roughness with hydraulic aperture of 46 μm. Model 2 has an irregular roughness with hydraulic aperture of 78 μm. The two model fractures are transparent, which allows direct investigation of foam in the fractures. We characterize the fracture geometries by studying the aperture distribution in the two model fractures. Both model fractures are then represented by a 2D network of pore bodies and pore throats. To study coarsening, we inject pre-generated foam at different foam qualities (ratio of gas volumetric rate to total rate) into the model fractures. After foam reaches steady-state, we shut the inlet and outlet valves of the fractures for 24 h. Foam coarsens by gas diffusion during this period. We use a high-speed camera to record images of the static foam during coarsening at two fixed locations in the fracture: 19 and 73 cm from the inlet, separately. We then use ImageJ software to process the images to study foam texture and quantify coarsening process. By correlating the aperture histogram of model fractures and water-occupied area fraction, we estimate the local aperture at water-gas interfaces at each specific coarsening time. Using the local aperture, we further estimate the height of lamellae available for gas diffusion at the end of the coarsening experiments. Based on this information, we discuss whether coarsening stops at the end of the coarsening experiments because bubbles are in equilibrium in pressure, or slows nearly to a stop because bubbles lose contact through lamellae. Coarsening studies in bulk and microfluidics assume coarsening slows and stops when lamella curvature is zero. We show in our model fractures that the lack of lamellae in wet foams can also play a part. In addition, we adopt a novel technique to calculate water saturation and capillary pressure of foam in our model fractures. We then explain how these foam properties affect its coarsening behavior.

Numerical Modeling of Combustion and Detonation in Aqueous Foams

Combustible aqueous foams and foamed emulsions represent prospective energy carriers. This paper is devoted to the overview of model assumptions required for numerical simulations of combustion and detonation processes in aqueous foams. The basic mathematical model is proposed and used for the analysis of the combustion development in the wet aqueous foam containing bubbles filled with reactive gas. The numerical results agree with the recent experimental data on combustion and detonation in aqueous foams containing premixed hydrogen–oxygen. The obtained results allowed for distinguishing the mechanisms of flame acceleration, transition to detonation, detonation propagation, and decay.

Super liquid repellent surfaces for anti-foaming and froth management

Wet and dry foams are prevalent in many industries, ranging from the food processing and commercial cosmetic sectors to industries such as chemical and oil-refining. Uncontrolled foaming results in product losses, equipment downtime or damage and cleanup costs. To speed up defoaming or enable anti-foaming, liquid oil or hydrophobic particles are usually added. However, such additives may need to be later separated and removed for environmental reasons and product quality. Here, we show that passive defoaming or active anti-foaming is possible simply by the interaction of foam with chemically or morphologically modified surfaces, of which the superamphiphobic variant exhibits superior performance. They significantly improve retraction of highly stable wet foams and prevention of growing dry foams, as quantified for beer and aqueous soap solution as model systems. Microscopic imaging reveals that amphiphobic nano-protrusions directly destabilize contacting foam bubbles, which can favorably vent through air gaps warranted by a Cassie wetting state. This mode of interfacial destabilization offers untapped potential for developing efficient, low-power and sustainable foam and froth management.

Structural properties of cellulose nanofibril foam depending on wet foaming conditions in Pickering stabilization

Porous cellulose nanofibril (CNF) foam was prepared by stabilizing bubbles with CNF and a surfactant and then drying the stabilized wet foam in a convection oven. The consistency of carboxymethylated CNF (CMCNF) and the addition amount of the surfactant were controlled and the effects of these factors on the CNF wet foam and dry foam properties were investigated. An adequate amount of the surfactant (0.02–0.04 wt%) with CMCNF consistency higher than 0.5 wt% yielded wet foams with excellent stability. When the wet foam was dried at 60 °C in an oven, dry CNF foam with over 97% porosity was generated. The stable wet foams resulted in dry CNF foam with a sphere-like pore structure and low levels of shrinkage during drying. In contrast, unstable wet foams generated dry foam with severe shrinkage and large cavities. The pore size and the porosity of the dried foam were determined by the shape of bubbles in the wet foam and the degree of shrinkage during drying, which, in turn, affected the mechanical strength. In addition, the compressive strength of the oven-dried foam was 83% higher than that of the freeze-dried foam. Therefore, the preparation of a stable wet porous CMCNF foam by controlling the CMCNF consistency and the amount of surfactant was essential for obtaining a porous CMCNF foam with a uniform pore structure and good mechanical strength by oven drying.Graphic abstract

“Bottom-Up” Assembly of Nanocellulose Microgels as Stabilizer for Pickering Foam Forming

Microgels assembled from bio-based nanomaterials are a promising soft stabilizer for a Pickering system. In this study, nanocellulose microgels with foaming properties were constructed by electrostatic assembly between nisin and 2,2,6,6-tetramethyl piperidine-1-oxyl-oxidized cellulose nanocrystals (TOCNC). Pickering wet foam was prepared by using the microgels as a foaming stabilizer. Nanocellulose microgels exhibited better foaming ability and foam stability than TOCNCs. Quartz crystal microbalance with dissipation and transmission electron microscopy analyses confirmed that the nanocellulose microgels prepared under different nisin concentrations demonstrated significant differences in morphology, conformation, and structural strength. Microgel particles prepared at 0.03 and 0.06 wt % nisin concentrations had a unique dendritic microstructure. Microgels containing 0.06 wt % nisin displayed better foaming ability and foam stability. It was possible that the soft dendritic structure of the microgels could endow bubbles with sufficient thickness and strength to prevent coalescence. This novelty nanocellulose microgel is expected to be used for expanding the application of nanocellulose in the functional interfacial design of Pickering foams.

PICKERING EMULSION TECHNOLOGY IN FABRICATE CELLULOSE FOAM FROM OIL PALM EMPTY FRUIT BUNCH WASTE

PICKERING EMULSION TECHNOLOGY IN FABRICATE CELLULOSE FOAM FROM OILPALM EMPTY FRUIT BUNCH WASTE. Cellulose from the oil palm empty fruit bunch (OPEFB) waste can make a porous material. This study aims to make cellulose foam with Pickering emulsion technology used cellulose nanofiber as a Pickering agent. The mechanism of Pickering emulsion is learned from foamability and stability of foam in the presence of various concentrations of surfactant. The result showed that using Pickering emulsion technology only needed surfactant with a small concentration to improve foamability and stability. The addition of CNF indeed improved the stability and foamability with the Pickering effect. The stability test shows that the foam stabilized with CNF appeared to be relatively stable. In contrast to the CNF free system, the foams were collapse in three days tested. Structures of foam was characterized using an optical microscope and showed that the foam was composed into two- or three dimensional microstructures formed by gas bubble of wet foam in random orientations. This process generated the lightweight Cellulose foam from OPEFB waste, with a density of 0.07 g/cm3. Using Pickering emulsion technology to make cellulose foam can be one way to overcome OPEFB waste and this foam is potential for various applications.

Effect of finite contact angle on the structure and shear behavior of two-dimensional wet foam

We present simulations of a 2D liquid foam with finite contact angles, over a wide range of liquid fraction. It is shown that, with the increase of liquid fraction, the variation of average coordination number may be characterized by three different regimes (the “flat” regime, the approximate linear regime and the gentle decreasing regime). The detailed distribution of the coordination number is displayed; the fraction of bubbles with the same number of neighbours, as a function of the average coordination number, follows the Gaussian distribution. In addition, we present the variation of the length of the gas-liquid-gas interfaces (films) with the liquid fraction, as well as the distribution of the Plateau border sides. Furthermore, in response to shear, the stress level of the wet foam rises with the increase of contact angle. We find a linear relationship between the yield stress and the cosine of the contact angle.

An Investigation of Mechanistic Foam Modeling for Optimum Field Development of CO2 Foam EOR Application

SummaryWhile there are a number of mechanistic foam models available in the literature, it still is not clear how such models can be used to guide actual field development planning in enhanced oil recovery (EOR) applications. This study aims to develop the framework to determine the optimum injection condition during foam EOR processes by using a mechanistic foam model. The end product of this study is presented in a graphical manner, based on the sweep-efficiency contours (from reservoir simulations) and the reduction in gas mobility (from mechanistic modeling of foams with bubble population balance).The main outcome of this study can be summarized as follows: First, compared to gas/water injection with no foams, injection of foams can improve cumulative oil recovery and sweep efficiency significantly. Such a tendency is observed consistently in a range of total injection rates tested (low, intermediate, and high total injection rates Qt). Second, the sweep efficiency is more sensitive to the injection foam quality fg for dry foams, compared to wet foams. This proves how important bubble-population-balance modeling is to predict gas mobility reduction as a function of Qt and fg. Third, the graphical approach demonstrates how to determine the optimum injection condition and how such an optimum condition changes at different field operating conditions and limitations (i.e., communication through shale layers, limited carbon dioxide (CO2) supply, cost advantage of CO2 compared to surfactant chemicals, etc.). For example, the scenario with noncommunicating shale layers predicts the maximum sweep of 49% at fg = 55% at high Qt, while the scenarios with communicating shale layers (with 0.1-md permeability) predicts the maximum sweep of only 40% at fg = 70% at the same Qt. The use of this graphical method for economic and business decisions is also shown, as an example, to prove the versatility and robustness of this new technique.

High-strength porous alumina ceramics prepared from stable wet foams

Porous ceramics have been widely used in heat insulation, filtration, and as a catalyst carrier. Ceramics with high porosity and high strength are desired; however, this high porosity commonly results in low strength materials. In this study, porous alumina with high porosity and high strength was prepared by a popular direct foaming method based on particle-stabilized wet foam that used ammonium polyacrylate (PAA) and dodecyl trimethyl ammonium chloride (DTAC) as the dispersant and hydrophobic modifier, respectively. The effects of the dispersant and surfactant contents on the rheological properties of alumina slurries, stability of wet foams, and microstructure and mechanical properties of sintered ceramics were investigated. The microstructure of porous ceramics was regulated using wet foams to achieve high strength. For a given PAA content, the wet foams exhibited increasing stability with increasing DTAC content. The most stable wet foam was successfully obtained with 0.40 wt% PAA and 0.02 wt% DTAC. The corresponding porous alumina ceramics had a porosity of 82%, an average grain size of 0.7 µm, and a compressive strength of 39 MPa. However, for a given DTAC content, the wet foams had decreasing stability with increasing PAA content. A possible mechanism to explain these results is analyzed.

Critical role of nanocomposites at air–water interface: From aqueous foams to foam-based lightweight functional materials

The study of aqueous foams has long been an interest of the scientific community. A developing area is the use of foams to prepare lightweight, functional solid foam materials. To design such porous materials, abundant foams should first be generated (good foamability), followed by foam destabilization processes being ceased or significantly retarded (excellent foam stability) to avoid the foam decay upon drying, which can be achieved by finely controlling interfacial behaviour of the foaming agents. The current review considers the interfacial behaviour and foaming potential of emerging nanocomposites such as: surfactant-nanoparticle composites; polymer-nanoparticle composites; and surfactant-graphene oxide (GO) composites. Compared with using only surfactants, polymers or nanoparticles, composite stabilizers are of great interest and importance since they enrich the interfacial behaviour of the bubble-stabilizing film (e.g. mechanical and structural properties), ensuring excellent foamability and long-term foam stability, and novel applications of the solid foam materials. Preparation mechanisms, properties and applications of solid foams prepared via direct foaming methods using the nanocomposite stabilizers are presented and highlighted. For the surfactant-NPs composites, the influences of the property of the foamed suspension and the bubble-stabilizing interface, on the quality of the wet and solid foams, is still elusive. As to the polymer-NPs composites, functional solid foams preparation using their stimuli-responsive performance remains an interesting topic for future research. The fundamental colloidal properties of surfactant-GO composites at the air–water interface and in stabilizing foams remain incompletely understood. Future studies are desired, combining experiments at various scales: interfacial films, Plateau borders, bulk liquid, bubbles, and foams, etc.

Technological aspects of xanthan gum and gum Arabic presence in chicken egg albumin wet foams: Application of nonlinear rheology and nonparametric statistics

This study presents the results of physicochemical and rheological studies of wet foams obtained using the batch method, from chicken egg albumin with the addition of xanthan gum and/or gum Arabic. The physicochemical analysis included determination of foam density, volume fraction of gas phase, overrun, stability and the distribution of gas bubbles suspended in liquid. The study on hydrocolloids effect on the distribution of gas bubbles was based on the standard descriptive parameters estimation and the use of nonparametric Kolmogorov–Smirnov test. The results indicated that foams containing only the gum Arabic addition proved to be bimodal, which was demonstrated by the logarithmic scale. In contrast, the addition of xanthan gum didn't change character of gas bubbles distribution. Nonlinear rheological tests were carried out using the LAOS methodology and the results were analysed with the help of geometric decomposition in combination with Chebyshev harmonics analysis. It allowed to identify xanthan gum as a polysaccharide responsible for viscous nature of foam while gum Arabic as a component increasing plasticity.

Size distribution dependence of collective relaxation dynamics in a two-dimensional wet foam

Foams can be ubiquitously observed in nature and in industrial products. Despite the relevance of their properties to deformation, fluidity, and collapse, all of which are essential for applications, there are few experimental studies of collective relaxation dynamics in a wet foam. Here, we directly observe how the relaxation dynamics changes with increasing liquid fraction in both monodisperse and polydisperse two-dimensional foams. As we increase the liquid fraction, we quantitatively characterize the slowing-down of the relaxation, and the increase of the correlation length. We also find two different relaxation modes which depend on the size distribution of the bubbles. It suggests that the bubbles which are simply near to each other play an important role in large rearrangements, not just those in direct contact. Finally, we confirm the generality of our experimental findings by a numerical simulation for the relaxation process of wet foams.

Experimentally testing a generalized coarsening model for individual bubbles in quasi-two-dimensional wet foams.

We present high-precision data for the time evolution of bubble area A(t) and circularity shape parameter C(t) for several bubbles in a quasi-two-dimensional foams consisting of bubbles squashed between parallel plates. In order to fully compare with earlier predictions, foam wetness is systematically varied by controlling the height of the sample above a liquid reservoir which in turn controls the radius r of the inflation of the Plateau borders. For very dry foams, where the borders are very small, classic von Neumann behavior is observed where a bubble's growth rate depends only on its number n of sides. For wet foams, the inflated borders impede gas exchange and cause deviations from von Neumann's law that are found to be in accord with the generalized coarsening equation. In particular, the overall growth rate varies linearly with the film height, which decrease as surface Plateau borders inflate. More interestingly, the deviation from dA/dt∝(n-6) von Neumann behavior grows in proportion to nCr/sqrt[A]. This is highlighted definitively by data for six-sided bubbles, which are forbidden to grow or shrink except for the existence of this term. It is tested quantitatively by variation of all four relevant quantities: n, C, r, and A.

Evaluation of a Virus Concentration Method Based on Ultrafiltration and Wet Foam Elution for Studying Viruses from Large-Volume Water Samples

Evaluation of a Virus Concentration Method Based on Ultrafiltration and Wet Foam Elution for Studying Viruses from Large-Volume Water Samples

Foam forming of fiber products: a review

Aqueous foam can be used as a transfer medium to form lightweight materials from natural and man-made fibers together with other types of raw materials. This review discusses mechanisms that underlie the forming process and thus influence physical properties of formed fiber networks such as microporous structure, strength behavior, and transport properties. Homogeneous fiber materials can be formed from versatile raw materials, which makes the technology suitable for a vast range of product applications. An intriguing feature of the method is that the wet foam characteristics provide an additional tool to tailor the performance of the dried material. Understanding foam rheology and how that is affected by added fibers is important in developing the forming process. We introduce both fundamental foam properties and practical forming methods, and show how the material properties are affected by the foam–fiber interaction. The basic features of an industrial production process are also described. The potential material properties are compared against key requirements in typical product applications.