Foam Stability Research Articles

Stability and rheological properties of firefighting foams with sodium carboxymethyl cellulose and hydrocarbon/silicone surfactants

Firefighting foam stability is a critical aspect in determining fire extinguishing efficiency. The stability and rheological properties of environmental firefighting foams are investigated in this work. The results show that increasing NaCMC polymer concentration decreases the equilibrium surface tension of CTAB/501W surfactants, while enhancing the viscosity and conductivity. And the yield strain and yield stress of the foam improve. However, when the NaCMC concentration is enhanced to 0.8 wt.%, the NaCMC concentration has less effect on the change in rheological characteristics. Slight NaCMC concentration addition enhances the foaming ability of CTAB/501W surfactant solutions, but further addition of NaCMC concentration decreases its foaming ability. Increasing NaCMC cocentration reduces the height decay of the foam produced by CTAB/501W surfactant solutions, retarding the bubble coarsening and liquid drainage process, and leading to foam stability enahncement, where the stability turning point occurs when NaCMC concentration is 0.5 wt.%. This is mainly attributed to the interaction between NaCMC polymer and CTAB molecule, and the increased viscosity.

The Effect of Foaming Agent and Whipping Time on Albedo Characteristics of Watermelon Flour

The fruit's rind is one of the natural waste products that can support nutrients like carbohydrates, proteins, and lipids. Reused rind contributes significantly to the effective handling of this fruit's byproduct. Processing watermelon rind into powder by lowering water content and activity is an alternative to extending its shelf life. The flour form is easier and more suitable for application in multiple processing food products such as snacks, beverages, bread, and pasta. The focus of this study was to verify the optimal state for foam mat drying using concentrations of foaming agent in the form of egg albumin (10, 15, 20), foam stabilizer in the form of CMC with concentrations of 1 %, and whipping time (5, 7, and 9 minutes) on for bulk density, WSI, hygroscopicity, L* value, a* value, b* value of total phenol and antioxidants activity, and the maximum antioxidant content of watermelon albedo flour using Completely Randomized design with 2 factorial resulting in nine treatments with three replications. Based on the outcome, the bulk density of the resulting watermelon albedo flour ranges from 0.25-0.50 gr/cm-3 and 0.11-0.26%. of WSI; 22.63-28.18g/100g-1 of hygroscopicity; 1.92-4.67% of total phenol; 47.48-66.59% of L*value; 3.74-6.11% of a*value; 5.47-16.08% of b*values and 6.80- 25.26% RSa of antioxidants activity. The best treatment for antioxidant activity was 25.26% RSA using 10% of the foaming agent and whipping time in 9 minutes. These findings have implications for the food industry because they suggest that treatments can influence the properties of watermelon albedo flour.

Enhanced hydrogen sequestration through hydrogen foam generation with SDS Co-Injection in subsurface formations

Inherent obstacles of hydrogen injection encompass preferential channeling through larger pathways, hampering microscopic sweep efficiency within porous media. This work pioneers hydrogen foam generation via co-injection with aqueous sodium dodecyl sulfate (SDS) to enhance subsurface hydrogen trapping capacity. SDS addition substantially reduces hydrogen-water interfacial tension and enhances viscoelasticity. In contrast to robust hydrogen foam stability, CO2 foams display compromised integrity, stemming from surfactant aggregation and ensuing infiltration channels that permit aqueous phase CO2 dissolution. Hydrogen foams conversely sustain uniform surfactant films, forestalling hydrogen ingress into water. Synergetic microfluidic and pore-scale simulations visualise mechanisms underlying intensified hydrogen capture and mobility reduction attributed to deformation and flow resistance when traversing constricted pores. Introducing the amphiphilic SDS surfactant considerably promotes hydrogen adsorption through its hydrophobic tail. Core flooding experiments quantify over 2 times hydrogen storage ratio amplification with SDS relative to sole hydrogen injection. The framework contributes pragmatic strategies to harness high-efficiency hydrogen foam for scalable clean energy storage.

Sustainable technologies of leather wastes: An eco-friendly oily acrylic resin-reinforced collagen polypeptide-based foaming agent for the green processing of foam concrete

In this research, a new eco-friendly collagen polypeptide-based foaming agent (CPFC) was prepared via a hydrophobic grafting modification of the collagen polypeptide (CP) from oily acrylic resin, in which the CP was extracted from leather wastes. The molecular structure of CPFC was characterized by FTIR, TGA, XRD, and laser particle size tests. The results showed that the oily acrylic acid molecular chain segments effectively copolymerized with CP, improving the hydrophilic and lipophilic balance of CP molecules. Given the increase of acrylic resin content in CPFC, the surface tension of CPFC at the critical micelle concentration gradually decreased and tended toward the critical value (30 mN/m). The foamability and foaming multiple of 9-CPFC and 7-CPFC reached 535 mL and 31.89 times, respectively. Next, we prepared a CPFC-based foam concrete with a dry density of 500 Kg/m3 via compounding a mechanical CPFC foaming agent with concrete. The resulting CPFC-based foam concrete was then characterized using a universal testing machine, thermal conductivity meter, ultra-depth 3d microscope, and other analysis techniques. The results indicated that the 5-CPFC foam concrete possessed good foam stability and compatibility. Moreover, the pore size distribution of the foam concrete was uniform, showing a less stringy pore structure, and the water absorption rate and thermal conductivity levels were 27.8 % and 0.61 W/m·K, respectively. The compressive strength of cement was approximately 1.37 MPa, which exceeds the national standard. Moreover, it was found that the CPFC possessed certain retarding properties that were similar to cement, suggesting that it is a highly suitable additive for foam concrete.

Study on the effects of carbon nanotubes and xanthan gum on the performance of hydrocarbon surfactant foam systems

To improve the foam stability and fire extinguishing efficiency of fluorine-free foam (FFF) extinguishing agents, xanthan gum (XG) and carbon nanotubes (CNTs) were selected as the foam stabilizer agents in this study to investigate their effects on the physical and chemical properties and fire suppression effectiveness of FFF system. The effects of the four factors, namely, α-olefin sulfonic acid Salts (AOS), YT-Y762, XG, and CNTs, on the foam performance were investigated using L9(34) orthogonal design method. Three FFF solutions were selected based on the following four parameters: surface tension, 25% drainage time, foam height, and viscosity. The physicochemical properties, fire extinguishing, and burnback performance of the three FFF solutions were investigated. The results indicated that XG and CNTs have synergistic effects on the FFF solution, and the addition of 0.2 wt% of XG and 0.4 wt% of CNTs increased the 25 % drainage time by 81.40 %. The fire extinguishing performance was effectively improved. The results show the potential application of carbon nanotubes in FFF extinguishing agents.

Incorporating soy protein hydrolysate and temperature-induced gelling polysaccharide as partial egg replacements for enhanced texture in sponge cake

This study investigated the influence of hydroxypropyl methylcelluloses (HPMCs), temperature-induced gelling polysaccharides, on the air-water interfacial properties and foaming properties of four soy proteins. Two HPMCs with different relative molecular masses (F50, lower; K4M, higher) were combined with soy protein isolate (SPI), 7S fraction (7S), selective hydrolysate by pepsin (SPSH), and limited hydrolysate by papain (SPLH). Interfacial shear rheology results showed that SPSH exhibited the highest G′ value, indicating a multi-layer adsorption structure formed due to both 7S and low molecule peptides in SPSH. Adding F50 to SPI, 7S, and SPSH improved foam expansion and stability while adding K4M negatively impacted the foam expansion of SPI, 7S, and SPSH but enhanced the foam expansion of SPLH. These findings suggest that the interfacial interaction patterns between HPMCs and soy proteins are significantly influenced by the molecular weight of HPMCs and the structure of the protein. Incorporating F50 into SPSH decreased the gelation temperature, shortened the formation time of a solid-like layer at the air-water interface, and expanded the linear viscoelastic region during structural fraction testing. Consequently, the combination of F50 and SPSH was chosen as the viable alternative to egg protein in sponge cake, as it facilitated rapid foam stability and prevented collapse during baking. The resulting sponge cake, with half of the egg replaced by the SPSH/F50 mix, exhibited a similar appearance and texture to that with only the egg added. Thus, combining temperature-induced gelling polysaccharides with SPSH offers a promising alternative to egg protein in aerated bakery food production.

Synthesis and properties of secondary alkyl sulfonates

Currently, there are totaling two production methods of sodium secondary alkyl sulfonate SAS 60, that are, sulfochlorination and sulfoxidation. These two methods involving SO2 (the first one is also Cl2) may be harmful to the enviroment. In this paper, sodium secondary alkyl sulfonate was synthesized by NaHSO3 addition reaction of dodecane’s double bond with azodiisobutyronitrile as catalyst, and the yield was over 95% within 2 h. The reaction avoids SO2 and Cl2, with the preparation process being simple and safe. The target product β-sodium dodecyl sulfonate was characterized by FT-IR, mass spectrometry,1H NMR and 13C NMR characterization. The physical and chemical properties such as surface tension, contact angle, wettability, emulsification, foaming and decontamination were tested. The results demonstrate that the Krafft point for the determination of sodium secondary alkyl sulfonate is 26 °C, with its CMC of 2.93 × 10−3 mol/L and γcmc of 27.06 mN/m according to γ-lgc curve. The results of thermodynamic parameters depict that micellization of the surfactants is a spontaneous process. Compared with conventional SAS 60, sodium secondary alkyl sulfonate is an anionic surfactant with excellent emulsifying ability, favorable foam performance and corking foam stability.

Исследование абсорбции углеводородов пеной

Purpose. To explain the behavior of air-mechanical foam in contact with hydrocarbons, there are generally accepted ideas about the thermodynamic stability of asymmetric aqueous films. These concepts cannot be considered exhaustive, since they do not explain the phenomena of foam stabilization in contact with hydrocarbons. The reason for the stabilization of foam films is the formation of stable emulsions in them caused by mass transfer. The aim of the work is to investigate the phenomenon of mass transfer between the aqueous phase and hydrocarbons on the stability of foam in contact with hydrocarbons. The following tasks are set in the work: to experimentally establish the effect of mass transfer on the contact destruction/stabilization of foam and the effect of hydrocarbon emulsification on the stability of foam from foaming agents of various nature. Methods. The experimental research method consists in measuring the rate of destruction of foam in contact with liquid and gaseous hydrocarbons, as well as with hydrocarbon vapors. Findings. The paper presents the results of experimental studies, on the basis of which it is shown that the intensification of the mass transfer process between the aqueous phase and the hydrocarbon contributes to the stabilization of foam in contact with hydrocarbons. A study of the destruction of foam in hydrocarbon vapors has shown that, regardless of the nature of foaming agents, the greatest stabilization of foam is observed in the case of compliance of the foaming agent with the optimal hydrophilic-lipophilic balance of hydrocarbon, at which its emulsification occurs. Research application field. In this paper, the effect of mass transfer between the aqueous and hydrocarbon phases on the stabilization of foam in contact with hydrocarbons is investigated. The results obtained can be used for scientific and educational purposes in the study of the physicochemical properties of air-mechanical foam. Conclusions. It has been experimentally shown that the intensification of mass transfer contributes to a more intensive formation of an emulsion, which helps to stabilize the foam. All other things being equal, a foam more stable on the surface of hydrocarbons makes it possible to obtain a foaming agent, which in this system exhibits the properties not only of a foaming agent, but also of a good emulsifier.

Effect of hydroxypropyl methylcellulose as foam stabilizers on the stability of foam and properties of foamed concrete

The stability of precast foam was a key factor in the performance of situ casted foam concrete (FC). Enhancing the viscosity of the foaming liquid is considered to be an effective way to improve the stability of precast foams. hydroxypropyl methylcellulose (HPMC) has become a widely used thickener in cement and concrete. In this study, HPMC was used to enhance the basic properties of foamed concrete (FC). The properties of foam and FC were tested using different test methods. The findings indicated that HPMC could improve the stability of foam by adsorbing on the surface of gas-liquid film; the mechanism of this stabilization process was explained as follows: the gas-liquid interfacial tension of foam was reduced, thus enhancing the strength and toughness of liquid film; HPMC enhanced the viscosity of the foaming solutions, thus affecting the drainage rate of foam and the diffusion rate of inter-bubble gas. HPMC could also effectively optimize the bubble size and uniformity of the bubble size distribution, delaying the foam disproportionation and coalescence. Furthermore, HPMC refined and homogenized the pore structure of FC, contributing to the enhancement of its strength.

ВЛИЯНИЕ УЛЬТРАЗВУКА НА ИЗМЕНЕНИЕ ОБЪЕМНОЙ ДОЛИ ЖИДКОСТИ В ПЕНЕ И ЕЕ УСТОЙЧИВОСТЬ

The wave method of acting on matter has long been an area of intense scientific and technological research. Ultrasonic dispersion allows to obtain various highly dispersed, homogeneous and chemically pure suspensions of solid particles in liquids; accelerates and improves the process of obtaining emulsions; ultrasonic waves of different frequencies create conditions for obtaining the structure of the material, which is characterized by isotropy of certain properties. However, the mechanism of action on dispersed systems, namely Pickering systems, is not well understood today. In this work, a study of the variation of liquid volume fraction and stability of bulk foam and its monolayers has been carried out to determine the features and nature of the failure of this dispersed system under wave action. The dependence of foam multiplicity and stability on the concentration and nature of solid particle-stabilizers (titanium oxide, clay, Aerosil-380), as well as on the content and structure of surfactant modifiers (anionic surfactant sodium decyl sulfate, cationic surfactants hexylamine and cetyltrimethylammonium bromide) is shown. The increase in the concentration of solid particles led to an increase in the resistance of foams to ultrasonic actions. The study of changes in the multiplicity and stability of foams at changing the concentration of water repellent showed that the maximum resistance to ultrasonic impacts was observed not at the maximum concentration of surfactant, but at the concentration corresponding to the maximum contact angle of wetting of particles (540). The use of long-chain surfactant cetyltrimethylammonium bromide increased the stability of the foam layer to ultrasonic action at a lower (compared to hexylamine) degree of modification of the surface of solid particles.

Effect of Maltodextrin and Egg White Powder on Physical Characteristics of Sorghum Powdered Drink

Sorghum, a cereal plant variety, is cultivated in Indonesia as a promising food alternative with notable nutritional and bioactive properties. It presents opportunities for innovative food products like powdered beverages. To make sorghum powdered beverages, adding filler (maltodextrin) and foaming agent (egg white powder) is an essential factor that may impact the physical characteristics of the final products. This study aimed to explore how varying concentrations of maltodextrin and egg white powder affect the physical attributes of sorghum powder beverages. Maltodextrin (10%, 20%, 30%) and egg white powder (4%, 4.5%, 5%) were the independent variables examined. The analysis encompassed solubility, bulk density, foam stability, foam density, and drying rate. Findings indicated that incorporating maltodextrin and egg white flour increased foam density (0.69 g/mL) and stability (94%). Higher maltodextrin concentration enhanced beverage solubility, while bulk density remained relatively unaffected (0.79 to 0.87 g/mL). Notably, the blend with 30% maltodextrin and 4.5% egg white powder exhibited the swiftest drying rate (0.0456 g water/g dry solid min). These outcomes provide a foundation for advancing sorghum-based powdered beverages.

The bubbly life and death of animal and plant milk foams.

Milk foams are fragile objects, readily prepared for frothy cappuccinos and lattes using bovine milk. However, evolving consumer preferences driven by health, climate change, veganism, and sustainability have created a substantial demand for creating frothy beverages using plant-based milk alternatives or plant milks. In this contribution, we characterize maximum foam volume and half-lifetime as metrics for foamability and foam stability and drainage kinetics of two animal milks (cow and goat) and compared them to those of the six most popular, commercially available plant milks: almond, oat, soy, pea, coconut, and rice. We used three set-ups: an electric frother with cold (10 °C) and hot (65 °C) settings to emulate the real-life application of creating foam for cappuccinos, a commercial device called a dynamic foam analyzer or DFA and fizzics-scope, a bespoke device we built. Fizzics-scope visualizes foam creation, evolution, and destruction using an extended prism-based imaging system facilitating the capture of spatiotemporal variation in foam microstructure over a broader range of heights and liquid fractions. Among the chosen eight milks, oat produces the longest-lasting foams, and rice has the lowest amount and stability of foam. Using the hot settings, animal milks produce more foam volume using an electric frother than the top three plant milks in terms of foamability (oat, pea, and soy). Using the cold settings, oat, soy, and almond outperform cow milk in terms of foam volume and lifetime for foams made with the frother and sparging. Most plant milks have higher viscosity due to added polysaccharide thickeners, and in some, lecithin and saponin can supplement globular proteins as emulsifiers. Our studies combining foam creation by frothing or sparging with imaging protocols to track global foam volume and local bubble size changes present opportunities for contrasting the physicochemical properties and functional attributes of animal and plant-based milk and ingredients for engineering better alternatives.

Microwave-Assisted Foam Mat Drying of Kumquat Puree and Investigation of Some Parameters

Kumquat puree, with the initial moisture content of 81.02 ± 0.8 (w.b) on wet basis, were dried using two different methods, microwave, and fan assisted hot air microwave (FAHA) until moisture content of kumquat was reduced to average of 25.02 ± 1.35 w.b. Microwave trials at 1.8 , 3.6, and 5.4 Wg- 1 lasted for 51, 19 and 12 min, respectively. FAHA trials at 1.8 , 3.6, and 5.4 Wg-1 at 100 ˚C and 1.8 , 3.6, and 5.4 Wg-1 at 150 ˚C lasted for 46, 38, 18, 15, 12 and 11 minutes, respectively. For foam drying, Soy protein (1%) and Maltodextrin (1%) were used as foaming agent, and carboxymethyl cellulose (1%) was used as foam stabilizer. After the drying trials, dried kumquat was powdered. The scope of this study, a comparison of measurements of moisture with that of the predicted results was made, obtained from 13 thin layer drying models. Further, drying rate, color parameter and energy consumptions were compared for each trial. For all microwave and FAHA trials, Kulcu et. al. (Unpublished) model was the most appropriate models depending on lowest chi-square value and root mean square error values. Color and specific energy consumptions (SEC) were categorized by using (SPSS) and chosen the best efficiency condition for drying as considering SEC. The most suitable drying process according to SEC was obtained at 5.4 Wg-1 microwave drying method with value of 1.94 Whg-1.

Impact of covalent binding with p-coumaric acid on pea protein's structural and functional properties

This study investigated the effects of covalent bonding between pea protein isolate (PPI) and p-coumaric acid (p-CA) on the protein's secondary structure, solubility, foaming properties, emulsifying capacity, and thermal stability. Binding p-CA led to alterations in the secondary structure of PPI, including an increase in α-helix and random coil and a decrease in β-turn. Additionally, it resulted in a reduction in SH and NH groups, as well as a decrease in particle size. As the amount of bound p-CA increased, an increased tendency for aggregation was proposed, resulting in the formation of soluble aggregates through hydrophobic interactions, which was confirmed by a reduction in particle size of proteins after being dissolved in SDS. These structural modifications influenced the protein's functional properties, with the conjugates showing enhanced solubility and emulsifying capacity across various pH levels, but weaker foaming capacity and foam stability. Furthermore, the conjugates exhibited a lower initial denaturation temperature but a higher thermal denaturation enthalpy change (ΔH) compared to PPI, which may be attributed to protein unfolding and the formation of new covalent bonds. This study highlights the potential of p-CA covalent modification of PPI to enhance its functional properties, making it more suitable for food industry applications.

Production, stability and properties of ultrafine MgAl2O4 (spinel) particles stabilized Mg–3Ca alloy foams

The present work reports the synthesis and mechanical behavior studies of Mg–3Ca alloy foams stabilized by ultrafine MgAl2O4 (spinel) particles. The MgAl2O4 particles was created in-situ in the Mg–3Ca alloy melt through the reaction of Mg, Al and O. Foaming was done by adding dolomite (CaMg(CO3)2) as blowing agent in the melt. The foaming behaviour was studied for different MgAl2O4 content in Mg–3Ca and holding times (10 and 15 min). The study reveals that the presence of MgAl2O4 significantly influences the foaming behavior of Mg–3Ca alloy resulting in equiaxed cell structure, uniform cell size distribution, higher expansion in comparison to the Mg–3Ca alloy foam which contains only MgO and CaO. An in-depth phase and microstructural analysis were performed to investigate the particles present in the gas-solid interface of the foam that contributes to foam stabilization. The quasi-static compression studies of foams exhibited better compressive strength (≈3–11 MPa) and energy absorption capacity (≈1.3–5.7 MJ/m3) in comparison to the Mg foams reported in the literature. The ductility of the Mg foams was also measured and compared with that of existing aluminium foams.

Effect of a novel drying method based on supercritical carbon dioxide on the physicochemical properties of sorghum proteins.

The aim of this research was to use supercritical carbon dioxide (SC-CO2) drying as a novel approach for generating sorghum protein concentrates/isolates with enhanced functional properties. Sorghum protein extracts were obtained from white whole-grain sorghum flour and were dried by two methods, namely, freeze-drying and SC-CO2 drying. The collected proteins were characterized for their morphology, color, crystallinity, surface hydrophobicity, emulsifying activity index (EAI), creaming index (CI), foaming capacity (FC), foaming stability (FS), protein solubility, chemical interactions, and viscosity. The SC-CO2-dried proteins exhibited higher porosity compared to the freeze-dried ones with smaller particle sizes (∼5.1 vs. 0.4 μm, respectively). The XRD patterns indicated that the SC-CO2-dried proteins had a lower crystallinity than the freeze-dried proteins. However, the surface hydrophobicities of the freeze-dried and SC-CO2-dried proteins were similar. The EAI results showed that the emulsifying activity of freeze-dried protein powder (40.6) was better than that of SC-CO2-dried protein powder (29.8). Nevertheless, the solubility of SC-CO2-dried proteins was higher than that of freeze-dried proteins in most of the pHs investigated. Overall, the proposed SC-CO2 drying method has the potential to generate porous protein powders with improved solubility that can be used in developing functional foods.

A Comparison of Laboratory and Field Measurements of Whitecap Foam Evolution From Breaking Waves

AbstractSufficiently energetic breaking ocean waves produce distinctive visible foam signatures on the water surface called whitecaps. The mixture of surface whitecap foam cells, and sub‐surface bubbles, results in the broad‐band scattering of light that allow whitecaps to be measured with optical cameras. In this paper the temporal evolution of whitecap foam area from laboratory and oceanic breaking waves is compared. When appropriately scaled, the foam area time series for both laboratory and oceanic breaking waves follow similar trends, despite occurring in vastly different settings. Distinct similarities of the signature of foam stabilization due to the presence of surfactants in the controlled laboratory experiments are also found in the field suggesting foam stabilization may be a means to remotely sense the presence/absence or concentration of surfactants in the ocean. In addition, probability density distributions of key whitecap variables such as foam area growth and decay timescales and maximum foam area are compared between laboratory and oceanic whitecaps. The oceanic whitecaps are much larger in scale than the laboratory breaking waves, whereas the whitecap growth and decay timescales are similar in magnitude, the latter suggesting that the depths to which bubbles are injected during active air entrainment in the field are relatively shallow. The aggregated whitecap statistics are used to estimate the energy dissipation of individual whitecaps in a novel manner.

Comparison of ancestral and traditional methods for elaborating sparkling wines

This work compares the ancestral method for elaborating sparkling wines with the most widely used traditional method. Ancestral method is a single fermentation procedure in which the fermenting grape must is bottled before the end of alcoholic fermentation whereas traditional method involves a second fermentation of a base wine inside a bottle. Macabeo grapes were used to elaborate a traditional sparkling wine and two ancestral sparkling wines, one with a low yeast population and one with a high yeast population. The findings indicate that ancestral sparkling wines have lower ethanol content and can be elaborated using lower sulphur dioxide levels. In general, ancestral sparkling wines showed similar protein concentration, higher polysaccharide content, similar or better foamability (HM) than the traditional sparkling wine., No differences were found in the foam stability (HS). In addition, the sensory analysis indicated that ancestral sparkling wines have smaller bubble size, lower CO2 aggressivity, they seemed to have longer ageing time and were scored better than the traditional sparkling wine. These results therefore indicate that the ancestral method is of great interest for the elaboration of high-quality sparkling wines.

Foam stability and thermo-mechanical properties of micro/nano filler loaded castor oil based flexible polyurethane foam

Use of fillers in polymers is to improve thermo-mechanical properties of the resulting material. Fillers are also used in polymeric foam as cell openers. Flexible polyurethane (PU) foams undergo major loss in structural stability when synthetic polyol is replaced with castor oil in the formulation as an alternate polyol. This study probes the effect of various micro and nano-fillers on PU foams prepared using blend polyol containing castor oil and synthetic polyol at a ratio of 1:1. Physical and cellular properties such as foam height, cell diameter, strut thickness and cell number density were evaluated to probe the structural stability of the foam. All foams prepared were found stable while it was found that the densities of the PU foams synthesized were greater than that of the conventional PU foams. Addition of fillers found to enhance thermal and mechanical properties of the foam. Moreover, all foam samples were found to observe thermal stability over and above 258 °C. Minimum glass transition temperature was recorded for 15% HG samples (i.e., −35.5 °C). Highest tensile strength was observed for 15% Si samples whereas, highest elongation was observed for 10% NC.

A liquid metal-embedded 3D interconnected-porous TPU/MXene composite with improved capacitive sensitivity and pressure detection range.

Flexible capacitive sensors are widely deployed in wearable smart electronics. Substantial studies have been devoted to constructing characteristic material architectures to improve their electromechanical sensing performance by facilitating the change of the electrode layer spacing. However, the air gaps introduced by the designed material architectures are easily squeezed when subjected to high-pressure loads, resulting in a limited increase in sensitivity over a wide range. To overcome this limitation, in this work, we embed the liquid metal (LM) in the internally interconnected porous structure of a flexible composite foam to fabricate a flexible and high-performance capacitive sensor. Different from the conventional conductive elements filled composite, the incompressible feature of the embedded fluidic LM leads to significantly improved mechanical stability of the composite foam to withstand high pressure loadings, resulting in a wider pressure sensing range from 10 Pa to 260 kPa for our capacitive composite sensor. Simultaneously, the metal conductivity and liquid ductility of the embedded LM endow the as-fabricated capacitive sensor with outstanding mechanical flexibility and pressure sensitivity (up to 1.91 kPa-1). Meanwhile, the LM-embedded interconnected-porous thermoplastic polyurethane/MXene composite sensor also shows excellent reliability over 4000 long-period load cycles, and the response times are merely 60 ms and 110 ms for the loading and unloading processes, respectively. To highlight their advantages in various applications, the as-proposed capacitive sensors are demonstrated to detect human movements and monitor biophysical heart-rate signals. It is believed that our finding could extend the material framework of flexible capacitive sensors and offer new possibilities and solutions in the development of the next-generation wearable electronics.