Bubble Film Research Articles

Hydrodynamics of gas–liquid flow in a capillary at elevated pressure: Implications for the fixation of CO2 into propylene carbonate in microreaction systems

It is an effective method to capture and utilize carbon dioxide (CO2) by using a microreactor to fix CO2 into propylene carbonate (PC). It is important to investigate the hydrodynamics of gas–liquid flow in order to ensure that the CO2 fixation in microreactor is operated under suitable conditions to obtain efficient mass transfer performance. However, few studies have been concerned with PC-CO2 two-phase flow in microreactors, especially at elevated pressures. Therefore, a high-pressure microreactor observation platform is established in this work to explore the hydrodynamics of PC-CO2 two-phase flow at a pressure range of 0.5–3.0 MPa. Three typical patterns such as bubbly flow, slug flow, and slug-annular flow were observed, and the flow pattern diagram at different pressures was plotted using WeG and WeL. The influence of the pressure on flow pattern transition lines was further discussed. Moreover, the effects of pressure on bubble size, liquid film thickness, bubble frequency, and gas volume fraction were investigated, as well as the corresponding correlation models. This work may provide guidance for the operation of gas–liquid microreactor at elevated pressure, especially for the fixation of CO2 into PC.

Effects of Salinity Beyond Coalescence on Submicron Aerosol Distributions

AbstractBubbles entrained by ocean waves rise to the surface and burst, creating a shower of droplets which contribute to sea spray aerosols. Submicron‐sized droplets, of which an estimated 60%–80% come from a bursting bubble film cap, play a key role in global climate atmospheric processes. However, many aspects of predicting the number and size of submicron drops emitted from a bursting bubble remain unknown. It is well‐documented that higher salinity increases submicron droplet production, which has been attributed to the role of salt in the suppression of bubble coalescence. We experimentally show that submicron drop production increases with salinity despite using a salt that does not affect bubble coalescence, indicating that salinity plays a role in the physics of submicron aerosol formation beyond coalescence. Laboratory experiments are conducted using sodium acetate solutions of salinity S = 0.001–0.1 M with millimeter‐sized bubbles generated via a needle. Unlike previous studies, the measured droplet size distributions are converted to formation diameter, revealing that the peak aerosol formation diameter decreases with higher salinity. Applying this diameter conversion to past studies, we find the peak formation diameter exhibits a scaling of Dform ∼ S−0.32 across three orders of magnitude in salinity and for a variety of salts, bubble coalescence behaviors, and bubble generation mechanisms. This result suggests that salinity has a systematic effect on the length scale of the rupturing bubble film which generates the aerosols. Consequently, salinity likely impacts the submicron aerosol production in oceanic environments even if bubble coalescence is negligible.

Effect of polyvinyl alcohol on the performance of carbon fixation foam concrete

The introduction of carbon fixation components (calcium hydroxide) can improve the carbon fixation performance of foam concrete, however, it will reduce its mechanical properties. In order to solve the adverse effects of carbon fixation components on the mechanical properties and optimize carbon absorption properties, a specific amount of polyvinyl alcohol (PVA) is incorporated. In this study, a serial of test was employed to study the production process of nano-modified foam, and some basic properties of the foam concrete of interest. Further, the micromorphology and pore structure of the samples were analyzed by scanning electron microscope and mercury injection test. The evolution of mineral phases was examined by X-ray diffraction and thermogravimetric analysis. The results show that the carbon sequestration efficiency of foam concrete increases significantly as the amount of calcium hydroxide is increased, however, the cementitious properties of materials deteriorate, resulting in a decrease in strength. When PVA is added as a water-soluble admixture, it can enhance the mechanical properties of test blocks. It can also strengthen the bubble film of nano stabilized foam, increase bubble stability, and refine the internal pore structure of carbon fixation foam concrete. In addition, PVA can modify the calcium hydroxide morphology from a laminar structure to a smaller volume edge corrosion structure, which improves the reaction activity of calcium hydroxide with CO2 and boosts the efficiency of carbon sequestration.

Influence of slurry characteristics on the bubble stability in foamed concrete

Foamed concrete is a porous material widely used in construction, but its production remains challenging due to bubble stability issues. To address these problems, this study analyzed the influence of bubble characteristics and paste viscosity on bubble stability in foamed concrete, specifically examining pore diameter distribution, collapse depth, and density deviation. Results showed that increasing the strength of the bubble film led to a decrease in the collapse depth of foamed concrete, along with a more uniform distribution of pores and a smaller average pore diameter, indicating improved bubble stability. Adequate viscosity in the cement paste was crucial for limiting the collapse and decreasing density deviation of foamed concrete, as well as ensuring a uniform distribution of early strains. Furthermore, the bubble duration remained unruptured for longer than the setting time of foamed concrete, which was necessary for bubbles to stay stable. A theoretical model of paste viscosity and bubble film strength, based on force analysis of bubbles in foamed concrete, was developed and can predict the required paste viscosity for bubble stability. This study provides important insights into the production of stable foamed concrete and offers a theoretical basis for future research in this area.

Role of the Polar Proportion of Compound Collectors in Low-Rank Coal Flotation Upgrading: Insights from the Molecular Scale

The compound collector, composed of polar and nonpolar parts, is a current research hotspot in low-rank coal (LRC) flotation because of its outstanding flotation efficiency and cost-effectiveness. However, the selection of compound proportions lacks theoretical guidance. In this study, we comprehensively investigate the internal mechanism underlying the influence of the compound proportion (oleic acid (OA):dodecane (DD)) on LRC flotation. The molecular dynamics (MD) simulation between bubble and oil film was simultaneously employed to study the surface interactions between three phases in contact. The compound proportion of OA:DD = 1:3 was found to be the most conductive for Zhuanlongwan coal flotation, which provides a robust collecting capacity while minimizing excessive interaction with water. Thus, the spreading area is the largest and the interaction energy between collectors and LRC is the highest. Additionally, the oil film formed at this ratio has the best hydrophobicity, resulting in the fastest adhesion speed of the bubble and the most stable bubble adhesion. Proportions of OA:DD above 1:3 promote self-aggregation of polar collectors, leading to a worsened spreading of the oil film. Conversely, proportions below 1:3 result in weak interaction with LRC, leading to a smaller spreading area and reduced hydrophobicity. Ultimately, we propose the LRC–compound collector matching principle, which suggests that the best proportion of compound collectors fits well with the oxygen-containing sites on the LRC surface.

Bubble floatation, burst, drainage, and droplet release characteristics on a free surface: A review

The phenomenon of a bubble bursting to generate droplets exists in industrial and environmental systems and has a subtle impact on our daily lives. A bubble generated by gas injection or heating rises to the free surface and undergoes floating, drainage, and eventually bursting processes to produce film and jet droplets. The interrelated processes make it difficult to understand the characteristics of a bubble burst. Thus, a summary of the individual stages of a single bubble from generation to burst is necessary. First, we describe the calculation method and simple expressions for the shape of a bubble floating on a free surface. Next, we discuss the bubble drainage model and its influencing factors as this directly determines the time evolution of the film thickness. As an essential factor that affects the film thickness, the bubble drainage time is defined as the bubble lifetime. We compare the bubble lifetime distributions in the published literature and explore the associated influencing factors. Then, we investigate the bubble bursting dynamics and focus on the bubble film opening process once a hole appears on its surface. As a legacy of bubble burst, we explore the production process, size, and number of film and jet droplets. Finally, we discuss the enrichment phenomenon and the enrichment factors of film and jet droplets when releasing particles entrained by droplets. This review considers a series of processes for bubble burst to generate droplets and concentrates on the mechanism and experimental correlations with a summary and future prospects.

A green aqueous foam stabilized by cellulose nanofibrils and camellia saponin for geological CO2 sequestration

Geological carbon dioxide (CO2) sequestration in deep formations involves displacing resident fluids from porous media, and is considered an unstable process due to the unfavorable mobility ratio between the resident and displacing fluid. Although various chemicals have been added to improve the CO2 sweep efficiency, this often results in pollution to the environment and the formation. Here, a novel green foam was prepared to enhance CO2 sequestration. The stabilization mechanisms and rheological properties of the CO2 foam with cellulose nanofibrils (CNFs) and camellia oleifera saponin (COS) were explored. Furthermore, the pore-scale behaviors of the CO2 foams were investigated using a micromodel. The results demonstrated that the abundant hydrogen bonds between CNFs and COS molecules led to the formation of tight bubble film which could stably encapsulate CO2 and inhibit its diffusion. The interlacement and entanglement of CNFs endowed liquid phase with high viscosity, which restrained liquid drainage and improved the interfacial viscoelasticity. The pore-scale behaviors showed that the higher viscoelasticity and stability caused large numbers of small bubbles to group together, providing greater flow resistance to control the mobility of the foam. The CNF/COS foam significantly improved the CO2 saturation in aquifers and oil reservoirs.

Enrichment of saccharides at the air–water interface: a quantitative comparison of sea surface microlayer and foam

Environmental context Saccharides contribute substantially to dissolved organic carbon in the ocean and are enriched at the ocean surface. In this study, we demonstrate that saccharides are more enriched in persistent whitecap foam compared to the sea surface. The maturation of bubbles at the air–water interface is thus expected to enhance the enrichment of organic matter at the ocean surface and ultimately in the sea spray aerosol that forms when bubbles burst at the ocean surface. Rationale Organic matter accumulates at the ocean surface. Herein, we provide the first quantitative assessment of the enrichment of dissolved saccharides in persistent whitecap foam and compare this enrichment to the sea surface microlayer (SSML) during a 9 day mesocosm experiment involving a phytoplankton bloom generated in a Marine Aerosol Reference Tank (MART). Methodology Free monosaccharides were quantified directly, total saccharides were determined following mild acid hydrolysis and the oligo/polysaccharide component was determined as the difference between total and free monosaccharides. Results Total saccharides contributed a significant fraction of dissolved organic carbon (DOC), accounting for 13% of DOC in seawater, 27% in SSML and 31% in foam. Median enrichment factors (EFs), calculated as the ratio of the concentrations of saccharides relative to sodium in SSML or foam to that of seawater, ranged from 1.7 to 6.4 in SSML and 2.1–12.1 in foam. Based on median EFs, xylitol, mannitol, glucose, galactose, mannose, xylose, fucose, rhamnose and ribose were more enriched in foam than SSML. Discussion The greatest EFs for saccharides coincided with high chlorophyll levels, indicating increasing ocean surface enrichment of saccharides during phytoplankton blooms. Higher enrichments of organic matter in sea foam over the SSML indicate that surface active organic compounds become increasingly enriched on persistent bubble film surfaces. These findings help to explain how marine organic matter becomes highly enriched in sea spray aerosol that is generated by bursting bubbles at the ocean surface.

Numerical analysis of flow characteristics and bubble behavior inside a two-phase closed thermosyphon under various temperature difference

In the present work, a two-dimensional axisymmetric two-phase closed thermosyphon model (TPCT) filled with refrigerant R-134a is developed based on the volume of fluid method, Lee model, and continuum surface force model in the Fluent to simulate the two-phase flow characteristics and bubble behaviors inside the TPCT. The effects of increasing the temperature difference (ΔT) between the evaporator and condenser on the inner wall temperature, liquid film distribution and bubble growth process are highlighted. The results show that at different ΔT and FR, the inner wall temperature of the evaporator section increases and then decreases in inhomogeneity with increasing temperature difference at 0.5 and 0.75 FR, while at 0.05 and 0.25 FR, almost uniform inhomogeneity is observed. Bubble behavior is varied and affected by different ΔT at 0.75 FR, the comprehensive bubble incubation and growth is achieved by observing video.

Utilization of ovalbumin-propylene glycol alginate complex system for superior foam: The effect of pH-driven phase behavior

The complex system of ovalbumin (OVA) was constructed using propylene glycol alginate (PGA), and the mechanism linking pH-driven phase behavior of the complex system to foam performance was studied in this work. According to the pH-driven phase behavior of the OVA-PGA complex system determined by turbidity, three pH levels (pH 3.0: insoluble complex, pH 4.0: soluble complex and pH 7.0: co-soluble) were selected. The OVA-PGA complex system possessed better foaming properties compared with the OVA system at the same pH level. It exhibited excellent foaming ability (160.0%) and foaming stability (80.8%) at pH 3.0 and pH 4.0, respectively. At pH 3.0, the insoluble complexes enhanced the thermodynamic activity of the protein and interfacial stability to create abundant foams. The smaller OVA-PGA soluble complexes with moderate charge at pH 4.0, possessed network-like structures capable of forming thicker and moister bubble films to avoid foam instability. The FTIR confirmed the presence of electrostatic repulsion and hydrophobic interaction in the system of pH 7.0, while electrostatic attraction was predominant at pH 3.0. Besides that, the rheological and surface tension results further indicated that insoluble complexes diffused rapidly to the air/water interface at pH 3.0, and soluble complexes with lower interfacial tension presented viscoelasticity that stabilized the interface. However, the co-soluble state of the system at pH 7.0 was detrimental to the foaming properties due to competitive adsorption and unfolding of the OVA. This work provides the theoretical support for producing superior foam and regulating foam-based food systems precisely. • Foam performance was enhanced at various pH levels by adding polysaccharides. • The pH-driven phase behavior caused differences in aerated OVA-PGA complex system. • Soluble complexes formed network-like interface and greatly raised foam stability. • Excellent foaming ability was showed at pH 3.0 due to OVA-PGA insoluble complexes.

Machining Characteristics of Diamond Grit-Coated Tools in Grinding-Aided ECDM

ABSTRACT This study investigated the machining characteristics of two shapes of diamond grit – coated tools used in grinding-aided electrochemical discharge machining for optical glass drilling. The effects of roundness, machining accuracy, surface roughness, and tool wear under various machining depths and electrode rotating speeds were explored, and the bubble generation mechanism and its effects on the discharge current waveform and electrochemical discharge reaction were observed. This study revealed that the bubbles generated in the gaps between diamond particles combine with each other to form a stable bubble film. Owing to differences in bubble generation mechanisms, the discharging waveform of the diamond grit – coated tool was more stable than that of tools without diamond grit – coated. It is known from the experimental results that the diamond grit – coated ball end mill in this method minimized shape roundness errors, machining time and the fracture of the exit edge.

Foam coarsening under a steady shear: interplay between bubble rearrangement and film thinning dynamics.

Aqueous foams are unstable and age by drainage and coarsening. Today, these effects are well described, as also their impact on foam properties. In that respect, the foam viscoelastic properties evolve in time as a consequence of coarsening which tends to increase the mean bubble size. Here, we investigate the reverse coupling, and study if and how the continuous flow of a foam can impact its dynamics of coarsening. We introduce a new protocol where brief oscillatory measurements are inserted during a constant steady shear, allowing us to monitor the relative variation of the bubble size with time (obtained from the one of the elastic modulus G') as a function of the applied shear rate. It turns out that the coarsening rate is strongly impacted by the applied shear: this rate is continuously reduced above a critical shear rate, which itself decreases with the bubble size. This coarsening-rate reduction is interpreted as the result of out-of-equilibrium and shear-dependent film thicknesses, being higher than at rest. The critical shear rate, above which films are dynamically sustained at higher thickness than at equilibrium, emerges from the competition between the rate of rearrangements and the time required to drain the thick film created during the rearrangement. We thus report here a first experimental proof and measurements of out-of-equilibrium film thicknesses within a sheared foam, and of the impact this has on coarsening.

A Scalable Heat Pump Film with Zero Energy Consumption.

Radiative cooling is an effective technology with zero energy consumption to alleviate climate warming and combat the urban heat island effect. At present, researchers often use foam boxes to isolate non-radiant heat exchange between the cooler and the environment through experiments, so as to achieve maximum cooling power. In practice, however, there are challenges in setting up foam boxes on a large scale, resulting in coolers that can be cooled below ambient only under low convection conditions. Based on polymer materials and nano-zinc oxide (nano-ZnO, refractive index > 2, the peak equivalent spherical diameter 500 nm), the manufacturing process of heat pump film (HPF) was proposed. The HPF (4.1 mm thick) consists of polyethylene (PE) bubble film (heat transfer coefficient 0.04 W/m/K, 4 mm thick) and Ethylene-1-octene copolymer (POE) cured nano-ZnO (solar reflectance ≈94% at 0.075 mm thick). Covering with HPF, the object achieves 7.15 °C decreasing in normal natural environment and 3.68 °C even under certain circumstances with high surface convective heat transfer (56.9 W/m2/K). HPF has advantages of cooling the covered object, certain strength (1.45 Mpa), scalable manufacturing with low cost, hydrophobic characteristics (the water contact angle, 150.6°), and meeting the basic requirements of various application scenarios.

On the physicochemical properties and foaming characteristics of proteins in cement environment

This paper investigates the physicochemical properties of proteins and their foaming characteristics in a synthetic pore solution (SPS) of cementitious materials to aid in understanding their air-entraining performance. A total of 13 proteins with different molecular structures and physicochemical properties were studied. Hydrodynamic size, charge, viscosity, surface tension, and foam capacity and stability of the proteins at different concentrations were characterized. Optical microscopy was employed to study the foam bubble size and structure. It was shown that proteins gained an increased negative charge and their hydrodynamic size generally decreased at high pH relevant to cementitious materials. The effect of SPS on surface tension of the proteins did not show a specific trend. The proteins demonstrated improved foaming in SPS compared to in deionized water (DI) due to the change in their physicochemical properties. The bubble size distributions of the foams were analyzed. Optical microscopy showed increased roughness and thickness of the bubble film in the foam in SPS, indicating enhanced stability compared to the foam in DI. The effect of surface tension of the proteins on their foaming behavior was shown to be less in SPS than in DI.

Modelling the Electrochemical Reduction of CO2 Under Gas-Liquid Channel Flow

Electrochemical reduction of CO2 using renewable energy sources is a promising avenue for sustainable production of bulk chemicals. The feasibility of electrochemical CO2 conversion has been demonstrated at lab scale, leading researchers to propose several reactor designs. However, CO2 electrolysis in aqueous systems is severely limited by mass transfer leading to performance parameters insufficient for industrial application. A strategy to enhance mass transfer is to actively introduce CO2 as a gas flow, while keeping diffusion paths between reactants and catalyst short, by separating the gas flow from the liquid electrolyte through a gas diffusion electrode. However, flooding, salt formation, or drying out of the gas diffusion electrode are common problems and present a major obstacle towards commercialisation. A promising reactor concept that enhances mass transfer without the complexity of a gas diffusion electrode is a structured reactor operated under gas-liquid Taylor flow: gas–liquid flow consisting of elongated bubbles in micro- or milli-channels. While this is a proven concept to enhance mass transfer for heterogeneous catalysis in flow cells, literature on this approach for electrochemical processes is scarce and the effect of process conditions on the reactor performance are poorly understood.In this work, we propose a tubular cell design, inspired from the field of fuel cells, for CO2 electrolysis with a zero-gap membrane electrode assembly. We further, develop a simple analytical model revealing how reactor performance in terms of faradaic efficiency and current density is governed by the key features of Taylor flow. These features film thickness, bubble velocity, and volume fraction of CO2 bubbles over aqueous electrolyte, for a given cathode potential.We find that the film thickness and the volume ratio of CO2/electrolyte fed to the reactor significantly affect the limiting current density and the faradaic efficiency. Additionally, we find industrially relevant performance with faradaic efficiencies (> 90 %) at current densities of up to 500 mA cm-2, when operating the reactor at elevated pressure beyond 5 bar. This demonstrates the general potential of this reactor concept to overcome mass transfer limitations in the field of electrolysis. We compare our predictions with numerical simulations, showing good agreement for a large window of operation conditions, illustrating when the simple predictive expressions for the current density and faradaic efficiency can be applied. We will discuss the importance of including interdependencies on the reactor scale to assess the economic viability of a reactor design and presents a tool to evaluate reactor design choices. We expect that the simple predictive expressions are instrumental in guiding experimental studies and reactor design choices, taking into account both technical and economic considerations.

Conjugate heat transfer effects on flow boiling in microchannels

• Heat distributes nonuniformly around the perimeter of microchannels in evaporators. • In single-phase flow, low aspect-ratio channels yield best heat transfer. • In two-phase flow, the Nusselt number trends vs. aspect-ratio are mixed. • High aspect-ratio channels exhibit larger thermal resistance and higher evaporator temperatures. • Conjugate heat transfer should be included in design models for evaporators. This article presents a computational study of saturated flow boiling in non-circular microchannels. The unit channel of a multi-microchannel evaporator, consisting of the fluidic channel and surrounding evaporator walls, is simulated and the conjugate heat transfer problem is solved. Simulations are performed using OpenFOAM v2106 and the built-in geometric Volume Of Fluid method, augmented with self-developed libraries to include liquid-vapour phase-change and improve the surface tension force calculation. A systematic study is conducted by employing water at atmospheric pressure, a channel hydraulic diameter of D h = 229 µm, a uniform base heat flux of q b = 100 kW / m 2 , and by varying the channel width-to-height aspect-ratio and channel fin thickness in the range ϵ = 0.25 –4 and W f = D h / 8 − D h , respectively. The effects of conjugate heat transfer and channel aspect-ratio on the bubble and evaporative film dynamics, heat transfer, and evaporator temperature are investigated in detail. This study reveals that, when the flow is single-phase, higher Nusselt numbers and lower evaporator base temperatures are achieved for smaller channel aspect-ratios, from Nu ≃ 4 and T b − T s a t ≃ 9 K when ϵ = 4 , to Nu ≃ 6 and T b − T s a t ≃ 2 K when ϵ = 0.25 , for same fin thickness W f = D h / 8 . In the two-phase flow regime, Nusselt numbers in the range Nu = 12 − 36 are achieved. The trends of the Nusselt number versus the aspect-ratio are non-monotonic and exhibit a marked dependence on the channel fin thickness. For small fin thicknesses, W f = D h / 8 and W f = D h / 4 , an overall ascending trend of Nu for increasing aspect-ratios is apparent, although in the narrower range ϵ = 0.5 –2 the Nusselt number appears weakly dependent on ϵ . For thicker fins, W f = D h / 2 and W f = D h , the Nusselt number decreases slightly when increasing the aspect-ratio in the range ϵ = 0.5 –2, although this trend is not monotonic when considering the entire range of aspect-ratios investigated. Nonetheless, due to conjugate heat transfer, Nusselt numbers and evaporator base temperatures follow different trends when varying the aspect-ratio, and channels with ϵ < 1 seem to promote lower evaporator temperatures than higher aspect-ratio conduits.

Ultralight, super thermal insulation, and fire-resistant cellular cement fabricated with Janus nanoparticle stabilized ultra-stable aqueous foam

Bubble-templated advanced cellular materials with ultralight and superior thermal insulation properties have shown great potential in energy-saving related applications. However, insufficient bubble stability severely limits the generation of such highly porous structures. Herein, we present an efficient method to produce ultra-stable aqueous foams by combing the amphiphilic Janus nanoparticle (Janus-SiO2) with the surface-active polymer (hydroxypropyl methylcellulose). The life of aqueous foams is dramatically increased from 4 h to over 5 days with 5 wt% Janus-SiO2 concentration. The dense adsorbed layer formed by the efficient adsorption of Janus-SiO2 on the bubble film is the main factor for the outstanding foam stability. Owing to the greatly enhanced foam stability, we successfully produce ultralight cellular cement with hierarchical cellular structures and a density as low as 80 kg/m3. The ultralight cellular cement exhibits superior mechanical strength, robust fire resistance and excellent thermal insulation with a thermal conductivity as low as 0.038 W·m−1·K−1. This work provides a practical approach for fabricating various advanced porous materials that employ bubbles as templates.

Microstructural Characteristics of Graphite Microcrystals in Graphitized Coal: Insights from Petrology, Mineralogy and Spectroscopy

Graphite microcrystals are the product of coal graphitization and widely exist in the graphitized coal of Yongan Coalfield, Fujian Province, China, which is direct mineralogical evidence for the transformation of coal to graphite. Optical microscopy, scanning electron microscopy (SEM) and micro-Raman spectroscopy were used to detect the morphology and microstructure of the in situ graphite microcrystals. The results show that the volume proportion of graphite microcrystals in graphitized coal samples is between 2.39% and 7.32%, and the optical anisotropy of graphite microcrystals is stronger than that of coal macerals. Graphite microcrystals show the occurrence of attached microcrack inner walls or infilling the cell cavity, with several forms of flakes, needles or aggregates. Under optical microscopy of polarized light and with a retarder plate of 1λ, graphite microcrystals show the color of primary yellow and secondary blue, and the two kinds of colors appear alternately when the microscope is rotating. Additionally, flake-like graphite microcrystals with an isochromatic zone diameter of 10−50 μm are the most widely distributed in graphitized coal. Under SEM, graphite microcrystals show a rough and irregular edge and are characterized by flow or bubble film structures along with several pores, which is the product of cooling crystallization after the softening and melting of carbon-containing substances. Moreover, flake-like graphite microcrystals developed interlayer pores with a clear outline of loose stacking and were almost entirely composed of pure carbon; a small amount of oxygen is related to oxygen-containing functional groups or structural defects. The micro-Raman spectra of graphite microcrystals in the first-order region are characterized by low-intensity D1 and D2 bands and a high-intensity G band, and the parameters R1 and R2 vary from 0.21–0.39 and 0.60–0.74, respectively. The second-order micro-Raman spectra of graphite microcrystals are characterized by a higher intensity of the 2D1 band and a lower intensity of the other three bands. The parameter R3, derived from the area ratio of the 2D1 band to all the bands in the second-order region, was proposed. The value of R3 ranges between 0.78 and 0.86, and both of them indicate a higher percentage of graphene plane with a highly internal crystallographic structure. Similar to the parameters R1 and R2 in the first-order micro-Raman spectrum, the parameter R3 is an effective parameter to characterize the ordering degree of the microstructure, which may be used to evaluate the graphitization degree of graphitization coal.

Plasma-enabled electrochemical jet micromachining of chemically inert and passivating material

Electrochemical jet machining (EJM) encounters significant challenges in the microstructuring of chemically inert and passivating materials because an oxide layer is easily formed on the material surface, preventing the progress of electrochemical dissolution. This research demonstrates for the first time a jet-electrolytic plasma micromachining (Jet-EPM) method to overcome this problem. Specifically, an electrolytic plasma is intentionally induced at the jet-material contact area by applying a potential high enough to surmount the surface boundary layer (such as a passive film or gas bubble) and enable material removal. Compared to traditional EJM, introducing plasma in the electrochemical jet system leads to considerable differences in machining performance due to the inclusion of plasma reactions. In this work, the implementation of Jet-EPM for fabricating microstructures in the semiconductor material 4H-SiC is demonstrated, and the machining principle and characteristics of Jet-EPM, including critical parameters and process windows, are comprehensively investigated. Theoretical modeling and experiments have elucidated the mechanisms of plasma ignition/evolution and the corresponding material removal, showing the strong potential of Jet-EPM for micromachining chemically resistant materials. The present study considerably augments the range of materials available for processing by the electrochemical jet technique.

Innovative water-based dynamic liquid bubble membrane generation device for gas/vapour separation

Gas and vapour separation by membrane technology has become a promising method because of its high energy efficiency and low capital investment. Here, we demonstrate a proof of concept of the design of a new type of liquid membrane called the “dynamic liquid bubble membrane” (DLBM) connected by plateau borders. The DLBM is composed of a network of bubbles formed by passing a gaseous mixture through water containing a surfactant. The bubble network serves as a membrane to selectively separate the gases/vapours of mixtures according to the different solubilities of the gases in the bubble liquid. To the best of our knowledge, this type of liquid foam has not been described previously. The effects of key experimental factors, such as the surfactant concentration in the water of the membrane solution and the bubble velocity (controlled by the slope of the device) of the DLBM, on the CO2/CH4 separation performance were investigated. The retentate gas (CH4) remained over 99.9 % for the high CH4 concentration injection mixture (>66.7 % CH4). Then, O2/CO2 was used to simulate the fluid of the alveolar lining during the full in-breath process. We found that carbon dioxide transferred outside the bubble film, while oxygen was retained inside the bubble. This finding supports a further understanding of the CO2 transport mechanism of alveoli. We further demonstrated that the thickness of the bubble film was closely correlated to mass transfer resistance theory, while the gas selectivity was completely unaffected. This innovative DLBM generation device is a highly attractive and promising prototype for designing a green and highly efficient gas/vapour separation membrane technology.