Foams: From nature to industry

Study on plant-based proteins and their complexes in improving food foam system: A review

The unique hollow structure of foam confers appealing sensory qualities and visual appeal to food products while delivering a desirable texture. Recently, the development of green and healthy food-grade foam improvers has become a research priority for food foam systems. Plant-based proteins with typical amphiphilic structures have attracted the attention of an increasing number of researchers owing to their excellent surface activity, naturalness and sustainability. Current strategies frequently employ polysaccharides and polyphenols to interact with plant-based proteins, forming binary or ternary complexes that markedly enhance foam stability and functional performance. Therefore, this paper reviewed the mode of formation, classification criteria, destabilization mechanisms and application scope of food foams. Additionally, we also summarized the types of interaction between the components of plant-based protein complexes, as well as the mechanisms and commonly used characterization methods for improving foam stability. Finally, the research deficiencies and future prospects of plant-based protein complexes in improving food foam systems are noted. This review provides theoretical basis and practical guidance for the development of efficient and green food foam stabilizers.

Mimicking the properties of commercial chocolate mousses using plant proteins as foaming stabilisers. Texture, rheology, color and proton mobility

The growing demand for plant-based food has led to explore plant proteins as substitutes for animal ingredients in aerated foods like chocolate mousse. This study examines how three plant proteins-pea protein isolate, soy protein concentrate, and kidney bean flour-affect the texture, structure, consistency, and water mobility of chocolate mousse. Sixteen different chocolate mousse recipes were prepared using these three plant proteins, with variations in foaming time, rotor speed, and ingredient proportions. The prepared mousses were evaluated for color, texture, rheology, pH, and water mobility and compartmentalization using time domain 1H nuclear magnetic resonance relaxometry (TD-NMR). Commercial samples were included for comparison. Multivariate analysis showed that mousses made with pea and soy proteins were the most similar in texture and structure to commercial products when the recipe was properly adjusted. This study highlights the potential of plant-based proteins for creating plant-based chocolate mousses. The use of TD-NMR in combination with rheology, and texture analysis provided insights into how plant proteins interact with other ingredients, which can help in optimizing the processing methods for better texture, consistency, and quality.

Identification of foam susceptible locations in the Delhi Reach of the Yamuna River

Recently, the occurrence of foam formation across many rivers has been a cause of concern for the global scientific community. The primary reasons behind foam formation include anionic surfactants, nutrients, organic and inorganic substances, and pathogens, which have been widely studied in the past. However, the issue of foam formation on water surfaces and identifying foam-susceptible locations has not been addressed comprehensively in the past literature. To address this, the present study, for the first time in river management literature, proposes a unified framework to investigate the foam formation issue and identify foam-susceptible locations over Delhi's reach of the Yamuna River, a stretch known for witnessing extensive pollution and excessive foam formation. The foam-related parameters were initially identified, and efficiency scores for four locations-Wazirabad Barrage (u/s), ITO Bridge, Nizamuddin Bridge, and Okhla Barrage (d/s)-were evaluated using the data envelopment analysis model. It was observed that three locations demonstrated low-efficiency scores in comparison to Wazirabad (u/s), indicating a high susceptibility to foam formation, which is critical from an environmental perspective, characterized by elevated levels of nutrients, surfactants, and organic pollutants. The reduced freshwater availability, lack of dissolved oxygen, discharge of untreated or partially treated effluents from multiple drains, and high concentrations of surfactants were noticed, which necessitate focused interventions in this area. In response, the study recommends remedial measures, including ensuring adequate environmental flow, pollutant oxidation, phytoremediation, stringent regulations, and public awareness to address foam formation issues in the Yamuna River.

Another Possible Rationale for Foam Stability: The Quantity and Strength of Hydrogen Bonds at the Gas-Liquid Interface

This study examines the foams generated by three surfactants: sodium dodecyl sulfate (SDS), sodium dodecylbenzenesulfonate (SDBS), and sodium lauryl polyoxyethylene ether sulfate (AES). By analyzing the hydrogen bond at the gas-liquid interface, the research provides novel insights into the mechanisms by which surfactants stabilize foams. Surfactants adsorb at the gas-liquid interface, establishing hydrogen bonds with water molecules while simultaneously retarding the structural relaxation of water-water hydrogen bonds within the hydration layer. This phenomenon can impede drainage during the surface tension drainage phase. Surfactants that readily form hydrogen bonds with water are more likely to adsorb at the gas-liquid interface, thereby enhancing the foam stability. The presence of robust hydrogen bonds and the frequent reconstruction of these bonds contribute to the establishment of a stable hydrogen bond network, which can reinforce the gas-liquid film and potentially augment its elasticity, enabling it to better withstand external perturbations. Although the Marangoni effect typically promotes bubble coalescence, a stable hydrogen bond network may mitigate this process.

Unveiling the organic nature of phosphogypsum foam: Insights into formation dynamics, pollution load, and contribution to marine pollution in the Southern Mediterranean Sea

The foamability of dissolved phosphogypsum from the phosphate fertilizer factories of Gabes (SE Tunisia) is a spectacular phenomenon that has not yet been thoroughly studied. The main objective of this research was to investigate the organic properties of phosphogypsum foam (PGF) to understand its formation process, determine the origin of its enhanced radiochemical contaminants load, and identify its role in pollutants dispersion in marine environment of the Southern Mediterranean Sea. This study identified PGF as an unnatural, surfactant-stabilized, and ephemeral aqueous foam. PGF-forming process comprises three main steps: (i) formation (through phosphogypsum dissolution), (ii) stabilization (facilitated by organic surfactants and gypsum crystals), and (iii) destabilization (geochemical (involving the dissolution of the PGF skeleton gypsum) and/or mechanical (influenced by wind and wave action)). The amphiphilic nature of PGF organic matter and the presence of specific organic groups are responsible for its high toxic contaminants load. PGF contributes, through its elevated pollutants content and its ability to migrate far from its source, to the marine dispersion of industrial toxic radiochemical contaminants. It is therefore recommended to mitigate the environmental and health risks associated with PGF, including banning the discharge of untreated phosphogypsum and other industrial wastes into the coastal environment of Gabes.

Study of the Influence of Bamboo Suspension Water-Removal Processes on the Properties of Bamboo-Based Molding Materials

Bamboo is a fast-growing lignocellulosic plant in nature. It is an abundant and renewable resource with wide applications. The processing of bamboo results in a large amount of residue. In this paper, we developed a method to utilize bamboo residue to prepare a novel lightweight porous molding material. A hydrated thermochemical grinding process was proposed to disintegrate bamboo fibers and activate bamboo's own binding components. The influence of the water removal by pressure from bamboo suspension and subsequent different drying methods on the product's properties was evaluated. The two-step drying ensured a low production cost and high product quality. The bamboo molding material was characterized based on thermal stability, morphology, functional groups, particle size distribution, crystallinity, and mechanical strength. A lightweight porous material was obtained with a density of 0.23-0.35 g/cm3 by freeze-drying. A high mechanical strength was obtained with a tensile strength of 0.62 MPa and a compressive strength of 10.31 MPa by oven drying. The auto-adhesive mechanisms, including fiber anchorage, polymerization, water plasticization, and heat plasticization, were discussed. The bamboo molding material is a reconstruction of bamboo cell wall components and is easy to recycle. It has potential applications in construction and buildings, packaging, and indoor furnishings.

Sea foam contains hemoglycin from cosmic dust

In-falling cosmic dust has left evidence of meteoritic polymer amide in stromatolites, both fossil and modern. In search of evidence for continued present day in-fall, sea foam was collected from two beaches in Rhode Island and subjected to Folch extraction to concentrate amphiphilic components in a chloroform water-methanol interphase layer. Hemoglycin polymer amide molecules previously characterized by MALDI mass spectrometry in meteorites and stromatolites were identified in sea foam either directly, or via their fragmentation patterns. Residual isotope enrichment pointed to an extra-terrestrial origin. The unique resiliency of sea foam may be due to the formation of extended hemoglycin lattices that stabilize its closed-cell structure and its lightness can potentially be explained by photolytic hydrogen production.

Bubble dynamics manipulation in polymeric foaming

The release of pressure from a high-pressure-stable polymer/gas solution is a common method for creating gas bubbles and forming foam with a typical polyhedral cell structure. We propose a new approach to control the foaming process by pausing the bubble growth at intermediate pressure before reaching ambient pressure. This allows us to control the growth of the bubbles and investigate various physical phenomena involved in polymer foaming, such as Ostwald ripening, bubble interactions, coalescence, and different bubble growth regimes. We conducted these studies in a model system PP/N2 by subjecting the solution to non trivial pressure histories. Our method will have an impact on the study of fundamental phenomena involved in foaming and their application in creating new materials.

A review of cellulose nanomaterial-stabilized Pickering foam: Formation, properties, and emerging oilfield applications

The rapid development of the petroleum industry has led to increasing demands for high-performance oilfield working fluids, such as drilling fluids, fracturing fluids, and fluids for enhanced oil recovery. Liquid foam is widely utilized as the oilfield working fluids due to its advantages, including low density, high mobility, superior cutting suspending ability, excellent fluid diversion capacity, and outstanding sweep efficiency. However, the short lifespan of foam limits its broad application in the oilfield. Considering the advantages of environmental protection, renewability, high specific surface area, tailorable surface chemistry, and excellent rheological properties of cellulose nanomaterials (CNMs), Pickering foams stabilized by CNMs offer improved eco-friendliness and foam stability. In this review, the classification and preparation methods of CNMs are briefly introduced. Subsequently, the preparation methods, properties, and application prospects of CNM-stabilized Pickering foams as oilfield working fluids are summarized. Finally, the challenges and prospects of CNM-stabilized Pickering foam are outlined, aiming to pave the way for the development of petroleum industry in an eco-friendlier manner.

Feasibility analysis of continuous extraction of biomaterials from flocculant sludge and potential applications in the fire protection field

The sludge contains many high-value biological materials. However, current extraction methods focus only on individual materials, neglecting the further extraction potential of the residual after extraction. This study used continuous extraction to extract extracellular polymeric substances (EPS) and proteins (PN) from sludge, verified the flame retardancy of EPS and the foaming properties of PN and finally analyzed the economic feasibility of continuous extraction. The results showed that continuous extraction increased the protein extraction from 857.11 mg/L to 1089.41 mg/L. EPS reduced the heat release rate of linen fabric from 379.2 (J/g·K) to 38.3 (J/g·K), and PN achieved foaming capacity and stability reaching 770% and 71%, meeting the standards of foam extinguishing agents. The binding form of EPS with linen fabric and the peptide content in PN are crucial factors affecting their application effectiveness. Economic analysis showed that continuous extraction reduced processing costs by 37.64% compared to traditional sludge disposal methods.

Cassava leaves as an alternative protein source: Effect of alkaline parameters and precipitation conditions on protein extraction and recovery

Alternative protein sources have been required to meet the significant plant protein demand. Agro-industrial by-products such as leaves have considerable potential as a source of macromolecules once they are mostly discarded as waste. The current study evaluated dried cassava leaves as a protein source. First, alkaline extraction parameters (solid-liquid ratio, pH, and temperature) were optimized and the run that result in the highest protein yield were acidified at pH 2.5 or 4. The influence of carbohydrate solubilized on protein precipitation was also evaluated by removing it via alcoholic extraction prior to precipitation. The experimental design showed that high pH and temperature conditions associated with a low solid-liquid ratio led to increased protein yields. The presence of carbohydrates in the supernatant significantly influenced protein precipitation. The protein concentrate had around 17.51% protein when it was obtained from a supernatant with carbohydrates, while protein content increased to 26.88% when it was obtained from carbohydrate-free supernatant. The precipitation pH also influenced protein content, whereas protein content significantly decreased when pH increased from 2.5 to 4. The natural interaction between carbohydrates and proteins from cassava leaves positively influenced the emulsion stability index and the foaming capacity and stability. Thus, the presented results bring insights into challenges in extracting and precipitation proteins from agro-industrial by-products.

Application of soy protein isolate-naringenin complexes as fat replacers in low-fat cream: Based on protein conformational changes, aggregation states and interfacial adsorption behavior

In this study, changes in the structural and functional properties of soybean protein isolate (SPI)-naringenin (NG) complexes under different amounts of naringenin treatments were explored, elucidating the effect of the complexes as fat replacers at the 15 % substitution level on the properties of low-fat cream. Finally, the correlation between the structure and function of the complex and the properties of low-fat cream was further analyzed. The addition of NG promotes the increase of SPI aggregation and particle size, and reduces the interfacial tension of the complex. Meanwhile, at the mass ratio of 48:3, NG and SPI formed a dendritic network structure suitable for stabilizing cream. The fat properties of cream indicate that low-fat creams stabilized by appropriate proportions of SPI-NG complexes displayed small and dense fat crystal network structures. In addition, low-fat cream stabilized by the SPI-NG complexes have improved whipping time, overrun, firmness, storage stability and rheological properties compared to natural SPI. It is worth noting that the overall quality of the cream stabilized by the SPI-NG complex with a mass ratio of 48:3 was almost close to that of full-fat cream. Therefore, this study promotes the potential applications of protein-polyphenol complexes as fat replacers in the food industry.

The peak viscosity of decaying foam with natural drainage and coarsening

Studying the change in foam viscosity during foam decay, a spontaneous and inevitable process, is of fundamental and practical interest across many applications, ranging from the froth in a cup of coffee to the carbon sequestration in deep geological reservoirs. However, standard rheological measurements impose several experimental constraints, such as the narrow sample confinement and the long initial setup time, interfering with the natural conditions for foam decay. Here, we perform fast and in situ measurements on decaying foam immediately after its generation in a wide column, measuring the viscosity by vibrational probes and measuring the foam structure by optical imaging. We successfully capture the changes during the transition from the drainage-dominated stage to the coarsening-dominated stage. The viscosity reaches its maximum at the crossover point, elucidating the competing effects of drainage and coarsening. The viscosity peaks magnitude and position are influenced by the gas solubility and diffusion coefficient. The phenomena are quantitatively explained by the film-shearing model. Our findings provide the foundation for enhancing foam stability and performance, improving the efficiency of foam-based applications.

Spatiotemporal mapping of nanotopography and thickness transitions of ultrathin foam films

Freshly formed soap films, soap bubbles, or foam films display iridescent colors due to thin film interference that changes as squeeze flow drives drainage and a progressive decrease in film thickness. Ultrathin (thickness <100 nm) freestanding films of soft matter containing micelles, particles, polyelectrolyte-surfactant complexes, or other supramolecular structures or liquid crystalline phases display drainage via stratification. A fascinating array of thickness variations and transitions, including stepwise thinning and coexistence of thick-thin flat regions, arise in micellar foam films that undergo drainage via stratification. In this tutorial, we describe the IDIOM (interferometry digital imaging optical microscopy) protocols that combine the conventional interferometry principle with digital filtration and image analysis to obtain nanometer accuracy for thickness determination while having high spatial and temporal resolution. We provide fully executable image analysis codes and algorithms for the analysis of nanotopography and summarize some of the unique insights obtained for stratified micellar foam films.

Structure of Human Serum Albumin at a Foam Surface

Proteins can be adsorbed on the air-water interface (AWI), and the structural changes in proteins at the AWI are closely related to the foaming properties of foods and beverages. However, how these structural changes in proteins at the AWI occur is not well understood. We developed a method for the structural assessment of proteins in the foam state using hydrogen/deuterium exchange mass spectrometry. Adsorption sites and structural changes in human serum albumin (HSA) were identified in situ at the peptide-level resolution. The N-terminus and the loop (E492-T506), which contains hydrophobic amino acids, were identified as adsorption sites. Both the structural flexibility and hydrophobicity were considered to be critical factors for the adsorption of HSA at the AWI. Structural changes in HSA were observed after more than one minute of foaming and were spread widely throughout the structure. These structural changes at the foam AWI were reversible.

Using cryo-SEM and EDS to investigate the stabilisation of oil-water interfaces in mixed aqueous-and-oil foams

For multi-phase soft matter systems, optical microscopy is frequently employed to distinguish the different phases. Unfortunately, optical microscopy does not succeed in all cases. Consequently, researchers sometimes require more advanced imaging techniques with superior resolution or sample penetration capabilities. One such complex system is a mixed aqueous-and-oil foam stabilised by colloidal particles, which is composed of two immiscible foams organised as the dispersed and continuous phases of an emulsion. While its morphology has been extensively studied using fluorescence confocal microscopy, not all questions have been answered. While the aqueous phase bubble interfaces are stabilised by silica particles and the oil phase bubble interfaces are stabilised by fluorinated particles, it remains to be seen how the aqueous-oil interfaces are stabilised. Hence, to gain insights into the role of the different particles at the interfaces, we employ cryogenic scanning electron microscopy (Cryo-SEM) and energy-dispersive X-ray spectroscopy (EDS). We find that the hydrophobic silica particles reside at both the aqueous-air and aqueous-oil interfaces. In contrast, the fluorinated particles, which exhibit hydrophobic and oleophobic properties simultaneously, are exclusively found at the oil-air interfaces.

Liquid foams: Properties, structures, prevailing phenomena and their applications in chemical/biochemical processes

Liquid foams are gas-liquid dispersions with flexible structures that provide high gas-liquid interfaces. This property nominates liquid foams as excellent gas-liquid contactors, systems that are widely used in the chemical and biochemical industries. However, challenges such as a lack of comprehensive understanding and foam instability have historically hindered their widespread industrial use in most applications. It was not until the recent development of nanofluidics, nanotechnology, surface science, and other related fields that the understanding, analysis, and control of foam phenomena improved. This led to the development of innovative stabilization techniques and foam-based unit operations in chemical and biochemical processes, each of which requires in-depth and exclusive reviews to fully comprehend their potential and limitations and to identify areas for further improvement and innovation. This paper reviews the foams, the common phenomena in them, the characteristics that make them suitable for chemical/biochemical engineering, reports on their current applications and recent developments in this field.

An overview of edible foams in food and modern cuisine: Destabilization and stabilization mechanisms and applications

Foam, as a structured multi-scale colloidal system, is becoming increasingly popular in food because it gives a series of unique textures, structures, and appearances to foods while maintaining clean labels. Recently, developing green and healthy food-grade foaming agents, improving the stability of edible foams, and exploring the application of foam structures and new foaming agents have been the focus of foam systems. This review comprehensively introduces the destabilization mechanisms of foam and summarizes the main mechanisms controlling the foam stability and progress of different food-grade materials (small-molecular surfactants, biopolymers, and edible Pickering particles). Furthermore, the classic foam systems in food and modern cuisine, their applications, developments, and challenges are also underlined. Natural small-molecular surfactants, novel plant/microalgae proteins, and edible colloidal particles are the research hotspots of high-efficiency food-grade foam stabilizers. They have apparent differences in foam stability mechanisms, and each exerts its advantages. However, the development of foam stabilizers remains to be enriched compared with emulsions. Food foams are diverse and widely used, bringing unique enjoyment and benefit to consumers regarding sense, innovation, and health attributes. In addition to industrial inflatable foods, the foam foods in molecular gastronomy are also worthy of exploration. Moreover, edible foams may have greater potential in structured food design, 3D/4D printing, and controlled flavor release in the future. This review will provide a reference for the efficient development of functional inflatable foods and the advancement of foam technologies in modern cuisine.

Adhesive properties of Aphrophoridae spittlebug foam

Aphrophora alni spittlebug nymphs produce a wet foam from anal excrement fluid, covering and protecting themselves against numerous impacts. Foam fluid contact angles on normal (26°) and silanized glass (37°) suggest that the foam wets various substrates, including plant and arthropod surfaces. The pull-off force depends on the hydration state and is higher the more dry the fluid. Because the foam desiccates as fast as water, predators once captured struggle to free from drying foam, becoming stickier. The present study confirms that adhesion is one of the numerous foam characteristics resulting in multifunctional effects, which promote spittlebugs' survival and render the foam a smart, biocompatible material of biological, biomimetic and biomedical interest. The sustainable 'reuse' of large amounts of excrement for foam production and protection of the thin nymph integument suggests energetic and evolutionary advantages. Probably, that is why foam nests have evolved in different groups of organisms, such as spittlebugs, frogs and fish.

Unveiling the origin, fate, and remedial approaches for surfactants in sewage-fed foaming urban (Bellandur) Lake

Foam formation in surface water bodies has become a global phenomenon, but the solutions to this crisis are often insufficient. Foam formation in water bodies is attributed to surfactants and requires a comprehensive assessment of various sources of surfactants to evolve mitigation strategies. The study is focused on thoroughly analyzing surfactants in the water and foam fractions of a large waterbody in Bangalore (India) spanning around 1000 acres (400 ha), which has been foaming for two decades. Results revealed that the key surfactants originate predominantly from anthropogenic sources with a small component emerging from naturogenic sources. Anthropogenic surfactants were found to be predominant (96.5%), with linear alkylbenzene sulphonates (LAS) of various C-chain lengths 12-20 being the most prevalent. Naturogenic surfactants derived from bacterial genera Pseudomonas exhibited significant microbial diversity, accounting for over 19% of total bacterial population in both the water and organic sediments of the lake. Modelling studies and field validation efforts were carried out to understand the fate of LAS in the foaming lake. The results indicated that these surfactants donot degrade under the prevailing conditions and timeframe as wastewater traverses through the lake, and their presence was also observed in the organic sludge sediment. Modeling the underlying processes revealed that a minimum dissolved oxygen (DO) concentration of 3.5 mg/l enables the degradation of over 90% of surfactants within the residence time of 8-10 days in Lake. Additionally, the process of desludging could contribute to an additional increase to the overall efficiency of surfactant removal, simultaneously removing legacy sorbed surfactants to sediments.

Characterizing Aqueous Foams by In Situ Viscosity Measurement in a Foam Column

Foam characterization is essential in many applications of foams, such as cleaning, food processing, cosmetics, and oil production, due to these applications' diversified requirements. The standard characterization method, the foam column test, cannot provide sufficient information for in-depth studies. Hence, there have been many studies that incorporated different characterization methods into a standard test. It should be enlightening and feasible to measure the foam viscosity, which is both of practical and fundamental interest during the foam column test, but it has never been done before. Here, we demonstrate a method to characterize aqueous foams and their aging behaviors with the simultaneous measurement of foam viscosity and foam height. Using a vibration viscometer, we integrate foam column experiments with in situ foam viscosity measurements. We studied the correlation among the foam structure, foam height, and foam viscosity during the foam decay process. We found a drastic decrease in foam viscosity in the early foam decay, while the foam height remained unchanged, which is explained by coarsening. This method is much more sensitive and time-efficient than conventional foam-height-based methods by comparing the half-life. This method successfully characterizes the stability of foams made of various combinations of surfactants and gases.

Evaporation-Induced Temperature Gradient in a Foam Column

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

Perspectives on Saponins: Food Functionality and Applications

Saponins are a diverse group of naturally occurring plant secondary metabolites present in a wide range of foods ranging from grains, pulses, and green leaves to sea creatures. They consist of a hydrophilic sugar moiety linked to a lipophilic aglycone, resulting in an amphiphilic nature and unique functional properties. Their amphiphilic structures enable saponins to exhibit surface-active properties, resulting in stable foams and complexes with various molecules. In the context of food applications, saponins are utilized as natural emulsifiers, foaming agents, and stabilizers. They contribute to texture and stability in food products and have potential health benefits, including cholesterol-lowering and anticancer effects. Saponins possess additional bioactivities that make them valuable in the pharmaceutical industry as anti-inflammatory, antimicrobial, antiviral, and antiparasitic agents to name a few. Saponins can demonstrate cytotoxic activity against cancer cell lines and can also act as adjuvants, enhancing the immune response to vaccines. Their ability to form stable complexes with drugs further expands their potential in drug delivery systems. However, challenges such as bitterness, cytotoxicity, and instability under certain conditions need to be addressed for effective utilization of saponins in foods and related applications. In this paper, we have reviewed the chemistry, functionality, and application aspects of saponins from various plant sources, and have summarized the regulatory aspects of the food-based application of quillaja saponins. Further research to explore the full potential of saponins in improving food quality and human health has been suggested. It is expected that this article will be a useful resource for researchers in food, feed, pharmaceuticals, and material science.

Effect of airflow rate and drainage on the properties of 2D smectic liquid crystal foams

As a two-phase system, foams are widely applied in the industry and exist ubiquitously in our daily lives. For this reason, studying them and investigating the parameters that affect their properties is crucial for the development of new and improved foam-based products. In this paper, we create 2D foam using an ordered fluid, the smectic liquid crystal (LC), and discuss the experimental parameters that affect their fabrication, including temperature and confining conditions. Then, we examine the influence of the injected airflow rate and drainage on their structure, size, liquid fraction, and stability. Finally, we compare their behavior to that of low-viscosity liquid foams and discuss the difference between them. Our findings indicate that surface tension is the dominant parameter in LC foam systems. Despite the strong elasticity of LCs, surface tension plays a crucial role in determining the properties of elastic foams. These results provide valuable insights that can be applied to different industrial applications. For instance, they may find relevance in the fields of cosmetics, thermal insulation, oil recovery, and sensing, where the fabrication of foams with high-viscosity fluids is required.

Mixed aqueous-and-oil foams in bulk

HYPOTHESIS

Because particle-stabilised foams are extremely stable and have a yield stress, a particle-stabilised aqueous foam and a particle-stabilised oil foam can be mixed together to give a stable composite foam which brings together two immiscible liquids.

EXPERIMENTS

We have developed a mixed foam system comprised of an olive oil foam with bubbles stabilised using partially fluorinated particles and an aqueous foam with bubbles stabilised using hydrophobic silica particles. The aqueous phase is a mixture of water and propylene glycol. We have studied this system using bulk observations, confocal microscopy and rheology as we vary the proportions of the two foams, the silica particles and the propylene glycol, and the sample age.

FINDINGS

The composite foam resembles an emulsion of one foam within another and is stable for a week or more. The structure and flow properties depend on the proportions of the two phases and the quantities of both silica particles and propylene glycol. Inversion between water-in-oil and oil-in-water is observed, where both phases are foams, driven both by silica wettability and by adding increasing quantities of the dispersed foam. Composites formed at the inversion point are the least stable, showing significant phase separation in less than one week.

A Comparative Study on CO2-Switchable Foams Stabilized by C22- or C18-Tailed Tertiary Amines

The CO2 aqueous foams stabilized by bioresource-derived ultra-long chain surfactants have demonstrated considerable promising application potential owing to their remarkable longevity. Nevertheless, existing research is still inadequate to establish the relationships among surfactant architecture, environmental factors, and foam properties. Herein, two cases of ultra-long chain tertiary amines with different tail lengths, N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) and N-oleicamidopropyl-N,N-dimethylamine (UC18AMPM), were employed to fabricate CO2 foams. The effect of temperature, pressure and salinity on the properties of two foam systems (i.e., foamability and foam stability) was compared using a high-temperature, high-pressure visualization foam meter. The continuous phase viscosity and liquid content for both samples were characterized using rheometry and FoamScan. The results showed that the increased concentrations or pressure enhanced the properties of both foam samples, but the increased scope for UC22AMPM was more pronounced. By contrast, the foam stability for both cases was impaired with increasing salinity or temperature, but the UC18AMPM sample is more sensitive to temperature and salinity, indicating the salt and temperature resistance of UC18AMPM-CO2 foams is weaker than those of the UC22AMPM counterpart. These differences are associated with the longer hydrophobic chain of UC22AMPM, which imparts a higher viscosity and lower surface tension to foams, resisting the adverse effects of temperature and salinity.

When Bulk Matters: Disentanglement of the Role of Polyelectrolyte/Surfactant Complexes at Surfaces and in the Bulk of Foam Films

Foam films display exciting systems as on one hand they dictate the performance of macroscopic foams and on the other hand they allow studies of surface forces. With regard to surface forces, we attempt to disentangle the effect of the foam film surfaces and the foam film bulk. For that, we study the influence of salt (LiBr) on foam films formed by mixtures of oppositely charged polyelectrolyte and surfactant: anionic monosulfonated polyphenylene sulfone (sPSO₂-220) and cationic tetradecyltrimethylammonium bromide (C₁₄TAB). Adding a small amount of salt (≤10^-3 M) decreases the foam film stability due to a weakened electrostatic net repulsion. In contrast, a large amount of salt (10^-2 M) unexpectedly increases the foam film stability. Disjoining pressure isotherms reveal that the increased stability is due to an additional steric stabilization, which is attributed to sPSO₂-220/C₁₄TAB complexes in the film bulk. These bulk complexes also contribute to the measured apparent surface potential between the two air/water interfaces. We find, for the first time, the formation of Newton black films for mixtures of anionic polyelectrolytes and cationic surfactants.

Nanocellulose interface enhanced all-cellulose foam with controllable strength via a facile liquid phase exchange route

The development of sustainable, biodegradable, non-toxic biomass foams with outstanding physical properties to replace traditional petroleum-based foams is urgent. In this work, we proposed a simple, efficient, and scalable approach to fabricate nanocellulose (NC) interface enhanced all-cellulose foam through ethanol liquid phase exchange and subsequent ambient drying. In this process, NCs served as reinforcer and binder were integrated with pulp fiber to improve cellulose interfibrillar bonding and interface adhesion between NCs and pulp microfibrils. The resultant all-cellulose foam displayed stable microcellular structure (porosity of 91.7-94.5 %), low apparent density (0.08-0.12 g/cm³), and high compression modulus (0.49-2.96 MPa) by regulating the content and size of NCs. Further, the strengthening mechanism of the structure and property of all-cellulose foam were investigated in detail. This proposed process enabled ambient drying, and is simple and feasible for low-cost, practicable, and scalable production of biodegradable, green bio-based foam without special apparatuses and other chemicals.

Application of Nanotechnology in Extinguishing Agents

Extinguishing agents are a very important tool in the field of security, both in terms of private and social aspects. Depending on the type of burning substance and place of fire, appropriately prepared and developed solutions should be used. We can distinguish, among others, materials, powders or foaming agents. Modifications introduced into them, including ones based on the achievements in the field of nanotechnology, can improve their safety of use and extend their service life. Such amendments also reduce the costs of production and neutralization of the area after a fire, and increase the fire extinguishing effectiveness. The introduction of nanoparticles allows, e.g., shortening of the fire extinguishing time, reduction of the risk of smoke emission and the toxic substances contained in it, and an increase in the specific surface of particles and thus increasing the sorption of pollutants. The elaborations use metal nanoparticles, e.g., NP-Ag, metal oxides such as NP-SiO₂, as well as particles of substances already present in extinguishing agents but treated and reduced to nanosize. It should be noted, however, that all changes must lead to obtaining a tool that meets the relevant legal requirements and has appropriate approvals.

Probing foams from the nanometer to the millimeter scale by coupling small-angle neutron scattering, imaging, and electrical conductivity measurements

Liquid foams are multi-scale structures whose structural characterization requires the combination of very different techniques. This inherently complex task is made more difficult by the fact that foams are also intrinsically unstable systems and that their properties are highly dependent on the production protocol and sample container. To tackle these issues, a new device has been developed that enables the simultaneous time-resolved investigation of foams by small-angle neutron scattering (SANS), electrical conductivity, and bubbles imaging. This device allows the characterization of the foam and its aging from nanometer up to centimeter scale in a single experiment. A specific SANS model was developed to quantitatively adjust the scattering intensity from the dry foam. Structural features such as the liquid fraction, specific surface area of the Plateau borders and inter-bubble films, and thin film thickness were deduced from this analysis, and some of these values were compared with values extracted from the other applied techniques. This approach has been applied to a surfactant-stabilized liquid foam under free drainage and the underlying foam destabilization mechanisms were discussed with unprecedented detail. For example, the information extracted from the image analysis and SANS data allows for the first time to determine the disjoining pressure vs. thickness isotherm in a real, draining foam.

Preparation and Application of Foaming Agent Based on the Compound System of Short-Chain Fluorocarbon and Soybean Residue Protein

This study provides a new idea for the design of an advanced foaming agent with soybean residue protein (SRP) as a potential protein source. In order to achieve the most effective foaming performance, we employed the novel approach of response surface methodology (RSM) to improve important process parameters in a hot-alkali experiment. The experimental results showed that the optimum reaction parameters of pH and temperature were pH 10.2 and 50.5 °C, respectively, which, when continued for 3 h, led to the highest foaming property of the SRP foaming agent (486 mL). Based on the scheme, we also designed an experiment whereby we incorporated 1.0g/L FS-50 into the SRP foaming agent (SRP-50) to achieve higher foaming capacity compared with the commercial foaming agent. This foaming agent was cheaper than commercial vegetable protein foaming agents (12 USD/L) at 0.258 USD/L. Meanwhile, the properties of foam concrete prepared using SRP-50 were studied in comparison with a commercial vegetable protein foaming agent (PS). The results demonstrated that the foam prepared using SRP-50 had better stability, and the displacement of the foam decreased by 10% after 10 min. During the curing period, the foam concrete possesseda compressive strength of 5.72 MPa after 28 days, which was an increase from 2.95 MPa before. The aperture of the foam ranged from 100 to 500 μm with the percentage increasing up to 71.5%, which indicated narrower pore-size distribution and finer pore size. In addition, the shrinkage of the foam concrete was also improved. These findings not only achieve the utilization of waste but also provide a new source for protein foaming agents.

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.

Improving foam performance using colloidal protein-polyphenol complexes: Lactoferrin and tannic acid

In this study, colloidal complexes were prepared from bovine lactoferrin (BLF) and tannic acid (TA) and then their ability to form and stabilize foams was characterized. The molecular interactions between BLF and TA were studied using fluorescence and molecular docking analysis, which suggested that hydrophobic forces were primarily involved in holding the complexes together. The production of colloidal BLF-TA complexes was supported by increases in turbidity and mean particle diameter, quenching of intrinsic fluorescence, decrease in surface hydrophobicity, and change in conformation. When used alone, BLF exhibited good foam formation but poor foam stability properties. In contrast, BLF-TA complexes exhibited good foam stability but poor foamability properties. The change in foaming properties of the proteins was closely related to their interactions with the polyphenols. These findings may be useful for the development of novel functional ingredients to construct food foams with good physicochemical and nutritional attributes.

A novel food-based negative oral contrast agent compared with two conventional oral contrast agents in abdominal CT: a three-arm parallel blinded randomised controlled single-centre trial

Background

A negative oral contrast agent (OCA) has been long sought for, to better delineate the bowel and visualise surrounding structures. Lumentin® 44 (L44) is a new OCA formulated to fill the entire small bowel. The aim of this study was to compare L44 with positive and neutral conventional OCA in abdominal computed tomography (CT).

Methods

Forty-five oncologic patients were randomised to receive either L44 or one of the two comparators (MoviPrep® or diluted Omnipaque®). Abdominal CT examinations with intravenous contrast agent were acquired according to standard protocols. The studies were read independently by two senior radiologists.

Results

The mean intraluminal Hounsfield units (HU)-values of regions-of-interest (ROIs) for each subsegment of small bowel and treatment group were -404.0 HU for L44, 166.1 HU for Omnipaque®, and 16.7 HU for MoviPrep® (L44 versus Omnipaque, p < 0.001: L44 versus MoviPrep p < 0.001; Omnipaque versus MoviPrep, p = 0.003). Adverse events, only mild, using L44 were numerically fewer than for using conventional oral contrast agents. Visualisation of abdominal structures beyond the small bowel was similar to the comparators.

Conclusions

L44 is a negative OCA with luminal radiodensity at approximately -400 HU creating a unique small bowel appearance on CT scans. The high bowel wall-to-lumen contrast may enable improved visualisation in a range of pathologic conditions. L44 showed a good safety profile and was well accepted by patients studied.

Trial registration

EudraCT (2017-002368-42) and in ClinicalTrials.gov (NCT03326518).

Supplementary Information

The online version contains supplementary material available at 10.1186/s41747-022-00267-z.

Pickering foams and parameters influencing their characteristics

Pickering foams are available in many applications and have been continually gaining interest in the last two decades. Pickering foams are multifaceted, and their characteristics are highly dependent on many factors, such as particle size, charge, hydrophobicity and concentration as well as the charge and concentration of surfactants and salts available in the system. A literature review of these individual studies at first might seem confusing and somewhat contradictory, particularly in multi-component systems with particles and surfactants with different charges in the presence of salts. This paper provides a comprehensive overview of particle-stabilized foams, also known as Pickering foams and froths. Underlying mechanisms of foam stabilization by particles with different morphology, surface chemistry, size and type are reviewed and clarified. This paper also outlines the role of salts and different factors such as pH, temperature and gas type on Pickering foams. Further, we highlight recent developments in Pickering foams in different applications such as food, mining, oil and gas, and wastewater treatment industries, where Pickering foams are abundant. We conclude this overview by presenting important research avenues based on the gaps identified here. The focus of this review is limited to Pickering foams of surfactants with added salts and does not include studies on polymers, proteins, or other macromolecules.

A biological nanofoam: The wall of coniferous bisaccate pollen

The outer layer of the pollen grain, the exine, plays a key role in the survival of terrestrial plant life. However, the exine structure in different groups of plants remains enigmatic. Here, modern and fossil coniferous bisaccate pollen were examined to investigate the detailed three-dimensional structure and properties of the pollen wall. X-ray nanotomography and volume electron microscopy are used to provide high-resolution imagery, revealing a solid nanofoam structure. Atomic force microscopy measurements were used to compare the pollen wall with other natural and synthetic foams and to demonstrate that the mechanical properties of the wall in this type of pollen are retained for millions of years in fossil specimens. The microscopic structure of this robust biological material has potential applications in materials sciences and also contributes to our understanding of the evolutionary success of conifers and other plants over geological time.