Gas-liquid Interface Research Articles

Considerations on the use of microsensors to profile dissolved H2 concentrations in microbial electrochemical reactors.

Measuring the distribution and dynamics of H2 in microbial electrochemical reactors is valuable to gain insights into the processes behind novel bioelectrochemical technologies, such as microbial electrosynthesis. Here, a microsensor method to measure and profile dissolved H2 concentrations in standard H-cell reactors is described. Graphite cathodes were oriented horizontally to enable the use of a motorized microprofiling system and a stereomicroscope was used to place the H2 microsensor precisely on the cathode surface. Profiling was performed towards the gas-liquid interface, while preserving the electric connections and flushing the headspace (to maintain anoxic conditions) and under strict temperature control (to overcome the temperature sensitivity of the microsensors). This method was tested by profiling six reactors, with and without inoculation of the acetogen Sporomusa ovata, at three different time points. H2 accumulated over time in the abiotic controls, while S. ovata maintained low H2 concentrations throughout the liquid phase (< 4 μM) during the whole experimental period. These results demonstrate that this setup generated insightful H2 profiles. However, various limitations of this microsensor method were identified, as headspace flushing lowered the dissolved H2 concentrations over time. Moreover, microsensors can likely not accurately measure H2 in the immediate vicinity of the solid cathode, because the solids cathode surface obstructs H2 diffusion into the microsensor. Finally, the reactors had to be discarded after microsensor profiling. Interested users should bear these considerations in mind when applying microsensors to characterize microbial electrochemical reactors.

Disinfection Efficacy of Electrohydraulic Discharge Plasma against Xanthomonas campestris pv campestris: A Sustainable Seed Treatment Approach.

The prevalence of plant diseases caused by pathogens such as Xanthomonas campestris pv campestris (Xcc) poses a significant challenge to sustainable agriculture, necessitating the development of effective and eco-friendly disinfection methods. In this study, we investigated the efficacy of electrohydraulic discharge plasma (EHDP) as a promising alternative for disinfection against Xcc, a pathogen responsible for black rot in cruciferous vegetables. Unlike conventional gas-phase plasma, EHDP introduces two pivotal components: gas-liquid interface plasma (GLIP) and its consequential byproduct, plasma-activated water (PAW). While GLIP enables dual-phase production of reactive oxygen and nitrogen species (RONS), PAW is a reservoir of liquid-phase long-lived RONS, thereby enhancing its bactericidal efficacy. In our evaluations, we tested EHDP-induced GLIP and EHDP-induced PAW against Xcc cells in both in vitro (Xcc suspension) and in vivo (Xcc-inoculated cabbage seeds) settings, achieving noteworthy results. Within 15 min, these methods eliminated ∼98% of the Xcc cells in suspension. For in vivo assessments, nontreated seeds exhibited an infection rate of 98%. In contrast, both EHDP treatments showed a significant reduction, with ∼60% fewer seeds infected while maintaining ∼90% germination rate. In addition, the liquid-phase RONS in EHDP-PAW may enhance seed vigor with a faster germination rate within the initial 5 days. Remarkably, around 90% of EHDP-PAW-treated seeds yielded healthy seedlings, indicating dual benefits in bacterial suppression and seed growth stimulation. In contrast, the percentage of healthy seedlings from nontreated, Xcc-inoculated seeds was approximately 70%. Our research demonstrates the feasibility of using eco-friendly EHDP in the seed disinfection process.

Vapor flow resistance characteristics in ultra-thin flat heat pipes

Miniaturization of electronic devices drives the development of ultra-thin flat heat pipes (UTFHPs). As the thickness of the heat pipe decreases, the microchannel effect of the vapor chamber exacerbates the deterioration of the vapor flow. The vapor flow resistance, which is often neglected in the conventional theory due to its extremely small effect compared to the liquid flow resistance, becomes a critical factor limiting the heat transfer performance of UTFHPs. To provide a theoretical basis for the design and performance evaluation of UTFHPs, this paper investigated the effect of the wicking structure and working fluid on the vapor flow resistance of UTFHPs based on theoretical derivations, numerical calculations, and experiments. The results indicate that conventional analytical theory is no longer applicable when the total thickness of the heat pipe is less than 1 mm. The vapor flow resistance obtained using the conventional equation is significantly smaller than the experimental results, which is caused by several factors: raised momentum loss at the wall and gas-liquid interface due to the growth of the velocity gradient in the boundary layer; exacerbated flow instability due to the mixing between the vapor and the working fluid; enhanced fluctuating effects on the flow channel due to the presence of the wicking structure. A dimensionless flow resistance coefficient equation was proposed with the vapor Reynolds number and the channel height ratio. An experiment was conducted to verify the accuracy of the flow resistance correlation. The difference between the experimental results and the calculated values is less than 15 %.

Charged Microdroplets as Microelectrochemical Cells for CO2 Reduction and C-C Coupling.

Charged microdroplets offer novel electrochemical environments, distinct from traditional solid-liquid or solid-liquid-gas interfaces, due to the intense electric fields at liquid-gas interfaces. In this study, we propose that charged microdroplets serve as microelectrochemical cells (MECs), enabling unique electrochemical reactions at the gas-liquid interface. Using electrospray-generated microdroplets, we achieved multielectron CO2 reduction and C-C coupling to synthesize ethanol using molecular catalysts. These catalysts effectively harness and relay electrons, enhancing the longevity of solvated electrons and enabling multielectron reactions. Importantly, we revealed the intrinsic relationship between the size and charge density of a MEC and its reaction selectivity. Employing in situ mass spectrometry, we identified reaction intermediates (molecular catalyst adducts with HCOO) and oxidation products, elucidating the CO2 reduction mechanism and the comprehensive reaction procedure. Our research underscores the promising role of charged microdroplets in pioneering new electrochemical systems.

Superhydrophilic floral foam-based solar evaporator with excellent salt-resistance and cost advantages for high efficient desalination

Effective management of heat, outstanding water transport paths, and hydrophilicity are essential for porous solar-driven interfacial evaporators, but their development still faces many difficulties. Here, by cross-linking a carbon black (CB)/polyvinyl alcohol (PVA) slurry with three-dimensional super hydrophilic floral foam (FF) to form a CB/PVA functional film on the surface of FF, a CB/PVA-decorated floral foam based (FFCP) solar-driven water evaporator with low-cost was finally obtained. Owing to the super hydrophilicity, sufficient water supply, self-holding water capacity and the special porous structure of the FF, as well as efficient light absorption and photothermal conversion efficiency of the dispersed CB. The gas-liquid interface temperature of the optimized FFCP-1.0 evaporator reaches 41.2 °C under single solar irradiation, obtaining an evaporation rate of up to 1.69 kg m−2 h−1. In addition, the FFCP-1.0 evaporator had a great evaporation rate in highly concentrated salt solutions. Moreover, the FFCP-1.0 evaporator has remarkable purification effects on seawater, organic dye effluents and heavy metal ion solutions. The evaporator has low cost, excessive evaporation rate and excellent durability. Therefore, this work is of considerable scientific significance for promoting the practical application of porous-based solar evaporators and clean water production.

Mixtures of phase transforming fluids and gases: Phase field model and stabilized isogeometric discretization

Liquid–vapor phase change in the presence of non-condensable gases is a classical problem, which continues to challenge continuum modeling. Here, we propose a new model based on the phase field method, which describes the dynamics of the non-condensable gas, phase change and flow simultaneously. The model is built by extending van der Waals and Korteweg’s theory for phase-transforming mixtures. The model equations have fourth order partial differential operators which presents significant challenges to standard spatial discretization methods. In addition, the isentropic form of the model equations is not hyperbolic at the liquid–gas interface, which inhibits the direct application of most algorithms for systems of hyperbolic equations. We propose a novel numerical scheme based on Isogeometric Analysis and the Taylor–Galerkin method, and also implement a residual based discontinuity capturing scheme. We show numerical results, at micron length scales, to study the accuracy and stability of our algorithm. We study the flow of a shock wave past a liquid droplet to test our numerical algorithm when steep gradients are present in the solution. Finally, we use the model to study the problem of gas–vapor bubble collapse near a solid wall.

Estimation of mean shear rate in a vessel of a planetary centrifugal mixer based on the heat balance equation

A planetary centrifugal mixer can fluidize highly viscous fluids in a vessel by strong centrifugal force. Although high rotation and revolution speeds are effective for particle dispersion, experimental observation and numerical simulation of the fluid flow in a moving vessel with a gas-liquid interface have not been well studied. Therefore, we propose the methodology to estimate a shear rate, which dominates the particle dispersion, by the temperature measurement. We measured the fluid and gas temperatures during mixing and used the heat balance equation to estimate the viscous dissipation rate, providing the mean shear rate. As a result, the effect of the mixing conditions on the mean shear rate has been clarified. Furthermore, the ratio of centrifugal force to viscous force helps identify the viscous dominant region, where not only a considerable rotation speed of the vessel but also a large revolution radius is helpful to increase the shear rate.

Mass transfer of chemical specie in acoustic cavitation bubble

In the present study, mass transfer across a gas–liquid interface of acoustic cavitation bubble is numerically investigated. The volume of fluid (VOF) method coupled with a continuous species transfer model is used to simulate the bubble dynamic behavior and mass transfer across the gas–liquid interface. Chemical species are transferred repeatedly between the liquid and gas phases in accordance with the bubble oscillation, and the chemical species are transferred into the bubble during an expansion phase, and to the outside of bubble during a compression phase. Time-averaged Sherwood number is decreased with an amplitude of sound pressure due to area effect and shell effect, and its value becomes negative at high pressure amplitude. Meanwhile, the instantaneous Sherwood number is largely varied in accordance with the bubble oscillation, and its magnitude becomes O(101) ∼ O(102).

Characterization of Commercially Available Human Primary Alveolar Epithelial Cells.

In vitro lung research requires appropriate cell culture models that adequately mimic invivo structure and function. Previously, researchers extensively used commercially available and easily expandable A549 and NCI-H441 cells, which replicate some but not all features of alveolar epithelial cells. Specifically, these cells are often restricted by terminally altered expression while lacking important alveolar epithelial characteristics. Of late, human primary alveolar epithelial cells (hPAEpCs) have become commercially available but are so far poorly specified. Here, we applied a comprehensive set of technologies to characterize their morphology, surface marker expression, transcriptomic profile, and functional properties. At optimized seeding numbers of 7,500 cells per square centimeter and growth at a gas-liquid interface, hPAEpCs formed regular monolayers with tight junctions and amiloride-sensitive transepithelial ion transport. Electron microscopy revealed lamellar body and microvilli formation characteristic for alveolar type II cells. Protein and single-cell transcriptomic analyses revealed expression of alveolar type I and type II cell markers; yet, transcriptomic data failed to detect NKX2-1, an important transcriptional regulator of alveolar cell differentiation. With increasing passage number, hPAEpCs transdifferentiated toward alveolar-basal intermediates characterized as SFTPC-, KRT8high, and KRT5- cells. In spite of marked changes in the transcriptome as a function of passaging, Uniform Manifold Approximation and Projection plots did not reveal major shifts in cell clusters, and epithelial permeability was unaffected. The present work delineates optimized culture conditions, cellular characteristics, and functional properties of commercially available hPAEpCs. hPAEpCs may provide a useful model system for studies on drug delivery, barrier function, and transepithelial ion transport invitro.

Bubble deformation and breakup in a non-uniform electric field

In this paper, the deformation and breakup characteristics of bubbles during their growth in a mixture solution were studied with a needle-ring electrode configuration in the presence of an electric field by using high-speed photography. Four distinct bubble breakup behaviors of the dripping, cone, kink and branch regimes were achieved successfully by manipulating the electric field. By considering the applied voltage and gas flow rate, the cone structure, interface stability, departure frequency and bubble size distribution in different regimes were quantitatively discussed. It is found that the cone height, interface curvature, departure frequency and bubble size distribution are evidently associated with bubble breakup regimes. As the electric field strength increased, the bubbles stretched in the direction of the electric field and the instability of the gas–liquid interface became stronger, which eventually resulted in the bubbles fragmenting into a cluster of microbubbles. This work provides a fundamental understanding of bubble deformation and breakup mechanisms under the electric field.

Eco-friendly hydrogels serving as water carriers for improving methane hydrate formation and dissociation processes

Natural gas hydrate (NGH) technology has broad application prospects in natural gas storage & transportation. In this work, we used eco-friendly hydrogel-based carriers to retain water, which were biodegradable polyvinyl alcohol-co-acrylic acid (PVA-co-PAA) hydrogels, and to react with methane gas and form hydrates. Results indicated that hydrates could be formed in the presences of swollen hydrogel particles without stirring or promoter adding, which might be owed to the enlarged gas–liquid interface and bulk of hydrophilic groups. When mass ratio of PVA to AA (MR) was 1:4, hydrogels performed best in improving hydrate formation kinetics: the induction time was 20.3 ± 20 min, the hydrate growth rate was 0.0217 ± 0.0088 mmol mL−1 min−1, and the methane uptake was 75.84 ± 4.70 mmol mol−1 water. Morphological observations revealed the water retaining behaviors of hydrogels during hydrate formation-dissociation process: a part of absorbed water transferred out of hydrogels to participate in hydrate formation, and afterwards hydrogels could re-absorbed the water released by hydrate dissociation. Moreover, hydrogels kept excellent recycling performance in seven repeated hydrate formation-dissociation cycles. Collectively, PVA-co-PAA hydrogels used as water carriers could improve mass transfer and reaction efficiency during methane hydrate formation-dissociation process, and meanwhile exhibited good recyclability due to strong water absorbing and retaining capabilities, implying that PVA-co-PAA hydrogels might potentially have broad applications in NGH technology.

Fine Particle Migration in a Gas Hydrate Sand: Single- and Two-Phase Fluid Using a Device for Observation at the Pore Scale

The production of natural gas hydrates will change the cementation strength, porosity, and effective stress in the stratum, which may lead to engineering and geological disasters. Sand production is a phenomenon where sand particles are carried out of the reservoir along with fluids during gas extraction, posing challenges to safe and sustainable production. This study explored the mechanism of fine particle migration in multiphase flow by a microscopic visualization test device. The device can inject a gas–liquid–solid phase at the same time and allow real-time observation. Experimental tests on fine particle migration of single- and two-phase fluid flow were carried out considering different conditions, i.e., fine particle concentration, fine particle size, fluid flow rate, and gas–liquid ratio. The results show that in single-phase fluid flow, the original gas will gradually dissolve in the liquid phase, and finally stay in the test device as bubbles, which can change the pore structures, resulting in the accumulation of fine particles at the gas–liquid interface. In two-phase fluid flow with mixed gas–water fluids, there are two flow modes of gas–liquid flow: mixed flow and separated flow. The interfacial tension at the gas–liquid interface can effectively migrate fine particles when the gas–liquid flows alternately and the sand production rate further increases as the gas–liquid ratio increases. In addition, changes in the concentration of fine particles, particle size, fluid flow rate, and the gas–liquid ratio will affect the migration of fine particles, leading to differences in the final sand production.

Highly efficient molecular film for inhibiting volatilization of hazardous nitric acid

Nitric acid, an important basic chemical raw material, plays an important role in promoting the development of national economy. However, such liquid hazardous chemicals are easy to cause accidental leakage during production, transportation, storage and use. The high concentration and corrosive toxic gas generated from decomposition shows tremendous harm to the surrounding environment and human life safety. Therefore, how to inhibit the volatilization of nitric acid and effectively control and block the generation of the toxic gas in the first time are the key to deal with the nitric acid leakage accident. Herein, a new method of molecular film obstruction is proposed to inhibit the nitric acid volatilization. The molecular film inhibitor spontaneously spread and form an insoluble molecular film on the gas-liquid interface, changing the state of nitric acid liquid surface and inhibiting the volatilization on the molecular scale. The inhibition rate up to 96% can be achieved below 45 °C within 400 min. Cluster structure simulation and energy barrier calculation is performed to elucidate the inhibition mechanism. Theoretical analysis of energy barrier shows that the specific resistance of the inhibitor significantly increased to 460 s·cm−1 at 45 °C, and the generated energy barrier is about 17,000 kJ·mol−1, which is much higher than the maximum energy required for nitric acid volatilization of 107.97 kJ·mol−1. The molecular film obstruction strategy can effectively inhibit the volatilization of nitric acid. This strategy paves the way for preventing the volatilization of liquid hazardous chemicals in accidental leakage treatment.

Spray characteristics of a self-aspirating ethanol ejector for high-efficiency combustion-based thermoelectric micropower generators

Low energy conversion efficiencies of combustion-based micropower generators due to parasitic losses continue to be a persistent challenge that limits their practical applications. This work puts forward a new strategy of liquid fuel delivery to a microcombustor with improved atomization and minimum parasitic energy losses. In the present work, a self-aspirating spray ejector is developed, providing continuous ethanol fuel delivery in the form of micron-sized droplets to a microcombustor of ∼ 1.0 kW thermal input capacity. The ejector base- setup works on the principle of generating a low-pressure zone through gas expansion, leading to the self-aspiration of ethanol fuel. The emerging annular ethanol film manifests a shear instability at the liquid–gas interface and subsequently breaks up into ligaments and fine micron-sized droplets. It was observed that the hydrostatic pressure of gas and aerodynamic Weber number (We) significantly affect the ethanol ejection rate, Sauter mean diameter (SMD), droplet distribution, and entrainment ratio of the spray ejector. The spray characteristics, such as mean droplet velocity and spray angle, are mainly governed by a single parameter, i.e., aerodynamic Weber number (We). The maximum power consumption for the spray ejector during the steady-state operation is ∼ 0.1 % of thermal input. The present investigation suggests that the self-aspirating spray ejector generates a fine and fully developed spray with ultra-low energy input requirement and, thus, can be a potential candidate for the development of high-efficiency combustion-based thermoelectric micropower generators.

Equilibrium shape of a bubble in a liquid-filled horizontal capillary

Bubbles in liquid-filled capillaries feature in many practical applications. Here we describe the equilibrium shape of a bubble in a horizontal capillary of circular cross section, as computed numerically using energy minimization. The data is presented in terms of empirical scaling laws for bubble extension along and perpendicular to the tube axis, over a large range of the ratio of cylinder to bubble diameter and Bond numbers. We also consider the limiting case of a bubble underneath a flat plate, for which approximate analytical solutions are available. Furthermore, we deduce an equation for the mean curvature of the gas–liquid interface and the variation of contact areas, the latter being of relevance also for estimating the wall drag forces of flowing bubbles.

Mechanistic insights into Criegee intermediates with benzoic acid at gas-phase and air-water interface and nucleation of product

Since the detection of Criegee intermediates (CIs) in the gas phase, there has been a growing recognition of the important role of CIs in atmospheric oxidation. Further mechanistic investigations are required to fully elucidate the bimolecular reactions of CIs with atmospheric species, such as OH, RO2/HO2, and carbonyl compounds. This article primarily focused on the reactions of benzoic acid (BA) with syn- and anti- CH3CHOO (C2 CI) in both gas-phase and gas-liquid interface using density functional theory (DFT) and Born-Oppenheimer molecular dynamics (BOMD). The results reveal that the reactions of BA and anti-CH3CHOO represent the principal pathway in the gaseous phase, whereas the water-involved reactions exhibit higher energy barriers, rendering them less favorable for occurrence. At the air-water interface, the reactions of BA and C2 CI as well as the BA-catalyzed hydration take place on the picosecond scale, conforming to the stepwise mechanism. The direct reaction between BA and anti- CH3CHOO remains the main sink for C2 CI at the gas-liquid interface. Molecular dynamics (MD) simulations were employed to investigate the nucleation capabilities of the reaction products and to examine the influence of temperature and water molecules on the nucleation process. The product molecules of BA and C2 CI can form stable clusters with sulfuric acid, ammonia molecules, and water molecules within 30 ns? The nucleation ability tends to decrease with the elevation of temperature, and the presence of water molecules promotes the new particles formation (NPF). Therefore, BA and its products with CIs affect the budget of secondary organic aerosols (SOAs).

Modelling and Simulation of a Wet Scrubber System

Shipping is a relatively clean transport method with low emissions per ton-mile compared with road transport. However, harmful emissions emitted in coastal areas are a concern, as these affect local air quality and health. To reduce sulphur oxide (SOX ) emissions, the International Maritime Organization (IMO) implemented a global sulphur cap of 0.5 wt% and the 0.1 wt% limit in emission control areas (ECAs). Ship owners can opt for either low sulphur fuels or wet scrubber systems. Wet scrubber systems are a reliable method for reducing SOX emissions with capture rates of up to 98%. These systems may use seawater alkalinity or caustic soda (e.g. closed-loop systems) to neutralise the SOX emissions. However, the dynamic loading of engines can cause large fluctuations in the exhaust flow conditions, and it is unknown how these affect the effectiveness of the scrubber. This study explores the impact of dynamic loads on the SOX removal efficiency of closed-loop wet scrubbers. A dynamic model of a closed-loop wet scrubber utilising fresh water and caustic soda is developed and verified using publicly available data. The model applies the two-film theory to model the gas-liquid interface. Billet and Schultes liquid hold-up theory is used to model the liquid film thickness in the packed bed. Maintaining scrubber efficiency with large load fluctuations or high-frequency fluctuations requires an increased liquid flow. The scrubber control system used a set-point of 75% of the equivalent compliance limit to ensure compliance with the 0.1% ECA limit during load fluctuations. The model and results can be used to develop a more advanced control system for improved scrubber operation and integration with a selective catalytic reduction (SCR) system to demonstrate compliance with the IMO NOX Tier III limit when using high-sulphur heavy fuel oil (HFO).

Measurement of jet gas–liquid interface fluctuations based on ultrasonic scattering

Aero-engines and rocket engines regularly experience unstable combustion phenomena. In order to reveal the complex mechanism, it is necessary to measure the jet gas–liquid interface disturbances. However, most techniques require optical access and the measurement fails when the optical access is limited. Ultrasonic method can be considered as an alternative under this condition. The present work proposes an acoustic approach for measuring the jet gas–liquid interface based on ultrasonic scattering. A thorough investigation of the scattered acoustic field by the jet is conducted by experimental methods. The high-speed imaging technology is used to verify the ultrasonic measurement results. The direct measurement results demonstrate a relationship between the perturbation of the jet gas–liquid interface and that of ultrasonic scattered acoustic pressure in certain measuring direction. The scattered acoustic field is analyzed theoretically and practically by using customized metal scatters of certain size. Based on these findings, a database is created to enable the ultrasonic measurements to be calibrated. The corrected result shows that the correlation of ultrasonic measurements and results from cameras has been greatly improved, and the maximum relative error of the ultrasonic measurements is 30.9%, the average relative measurement error is 2.1%. It is proved that the method of determining the gas–liquid interface of jet by ultrasonic scattering wave is feasible. The method may also be used for the measurement of the overall jet fluctuations and breakup.

Experimental study of the wave characteristics in annular gas-liquid flow along the wide side of a rectangular microchannel

The characteristics of gas-liquid annular flow in a microchannel with a rectangular cross-section were experimentally studied for the flow of a mixture of viscous silicone oil PMS 200 - nitrogen. To form the required annular flow regime, a side T-mixer was used at the microchannel inlet. High-speed visualization allowed to register flow areas of menisci and waves at the gas-liquid interface, located on short and wide microchannel sides for the given range of superficial gas and liquid velocities Jgas=24.3-93.8 m/s, Jliq=0.01 m/s. The processing of the flow images was performed using the Python program and areas, which are covered by various size waves on the microchannel wide side between the menisci, were identified and summarized. The dependence of the dimensionless wave area on Regas was obtained and compared with the dynamics of liquid layer thickness for the flow of the PMS200-nitrogen mixture, which was obtained previously.

Exploring Energy-Efficient Techniques and Chemical Kinetics in Reactive Nitrogen Species Production with Ambient Air Plasma for Plasma-Activated Water

This study investigates the chemical kinetics of reactive nitrogen species (RNS) formation in plasma-activated water (PAW) using ambient air and evaluates its energy efficiency. A singular plasma/liquid reactor was used, with discharge power varying between 16.4 to 27.0 W and agitation speeds set at 0 and 750 rpm. The reactor operated across temperatures from 5.00 to 35.00 °C during a consistent 180 min plasma activation. Analysis encompassed ionic strength effects, hydronium activity, conductivity, and iodide as a redox indicator for PAW. Findings suggest reactive species predominantly originate in the gas phase and move to the liquid through the gas-liquid interface. HNO3 production in PAW demonstrated a zero-order, temperature-dependent reaction. From activation parameters and ionic strength effects, a determinant step for aqueous HNO3 production was proposed. The influence of discharge power and agitation on HNO3 and HNO2 production was associated with heightened rates due to increased mass transfer and electric field strength. The research highlighted a trade-off between energy efficiency and nitrate production rate, emphasizing an efficiency peak of 72.33 nmol J-1 in PAW, pivotal for cost and operational benefits.