Bubble Evolution Research Articles

Understanding Bubble-Induced Overpotential Losses in Multiphase Flow Electrochemical Reactors

Green hydrogen production via water electrolysis can play a significant role in decarbonizing energy and multiple industrial processes. In this electrolysis process, water molecules are oxidized to produce oxygen in the anode, while protons are reduced to hydrogen in the cathode. Both of these electrochemical products are gaseous species that lead to bubble nucleation at the surface of electrodes. This bubble evolution phenomena results in substantial energy losses due to the blockage of ion conduction pathways, reduction of the available electrocatalytic area, and disruption of concentration gradients at the electrode–electrolyte interface. In this study, we implement a microfluidic water electrolyzer to elucidate the impacts of electrochemical reaction conditions and convective flows on bubble-induced overpotential losses. We show that high Reynolds (Re) number flows (i.e., Re > 20) mitigate the formation of large bubbles, resulting in minimal bubble-induced overpotential losses. For flows with smaller Re, periodic evolution of large bubbles leads to overpotential fluctuations on the order of ∼100 mV. Furthermore, to understand the impact of bubbles on concentration overpotentials, we use fluorescence microscopy and pH sensitive dyes to capture the spatiotemporal dynamics of pH gradients and correlate the strength and shape of these gradients to the applied potential and convective forces. We find that the presence of large bubbles at low Re can result in more severe concentration gradients that are affected by the hydrodynamic flows around the bubbles. The findings presented in this work provide insights into the effects of convective flows in mitigating bubble-induced overpotential losses.

Bubble flushing effect in micro EDM drilling and its relation with debris

In micro electrical discharge machining (EDM) drilling with a liquid dielectric, the bubble flushing effect is significant to the dielectric exchange and debris evacuation. Clarifying its mechanism is critical to guild the machining and avoid the deterioration of machining environment, which is a common problem in deep hole drilling with micro EDM. However, the investigation of bubble flushing is always disturbed by debris, which affects bubble production through disturbing the discharge process, because the debris is always conductive as a byproduct of metal materials. In this paper, the bubble flushing effect is systematically investigated from the in situ observation to statistical analysis to clarify the driven force for bubble movement or escape in the narrow gap and assess the evolution of bubble flushing effect with increasing hole depth. Moreover, the influence of debris on the bubble flushing effect is quantitatively analyzed through the contrast experiments in single crystal SiC and stainless steel SUS304 (SS304). It is confirmed in this paper that the debris of SiC has nearly no effect on the discharge due to its special material property. The visible and quantitative investigations reveal the nature bubble flushing effect in micro EDM drilling. The contrast analysis of bubble flushing effect under two cases (with/without influence of debris) clarifies how bubble and debris affect one another and their influences on the drilling process. This study provides new insights into the mechanism of micro EDM drilling and benefits to optimize the actual machining processes for the industry and manufacturer.

Towards an understanding of long gamma-ray burst environments through circumstellar medium population synthesis predictions

ABSTRACT The temporal and spectral evolution of gamma-ray burst (GRB) afterglows can be used to infer the density and density profile of the medium through which the shock is propagating. In long-duration (core-collapse) GRBs, the circumstellar medium (CSM) is expected to resemble a wind-blown bubble, with a termination shock, separating the stellar wind and the interstellar medium (ISM). A long standing problem is that flat density profiles, indicative of the ISM, are often found at lower radii than expected for a massive star progenitor. Furthermore, the presence of both wind-like environments at high radii and ISM-like environments at low radii remains a mystery. In this paper, we perform a ‘CSM population synthesis’ with long GRB progenitor stellar evolution models. Analytic results for the evolution of wind blown bubbles are adjusted through comparison with a grid of 2D hydrodynamical simulations. Predictions for the emission radii, ratio of ISM to wind-like environments, wind, and ISM densities are compared with the largest sample of afterglow derived parameters yet compiled, which we make available for the community. We find that high ISM densities of n ∼ 1000 cm−3 best reproduce observations. If long GRBs instead occur in typical ISM densities of n ∼ 1 cm−3, then the discrepancy between theory and observations is shown to persist at a population level. We discuss possible explanations for the origin of variety in long GRB afterglows, and for the overall trend of CSM modelling to over-predict the termination shock radius.

Single-Entity Electrochemistry of Nano- and Microbubbles in Electrolytic Gas Evolution.

Gas bubbles are found in diverse electrochemical processes, ranging from electrolytic water splitting to chlor-alkali electrolysis, as well as photoelectrochemical processes. Understanding the intricate influence of bubble evolution on the electrode processes and mass transport is key to the rational design of efficient devices for electrolytic energy conversion and thus requires precise measurement and analysis of individual gas bubbles. In this Perspective, we review the latest advances in single-entity measurement of gas bubbles on electrodes, covering the approaches of voltammetric and galvanostatic studies based on nanoelectrodes, probing bubble evolution using scanning probe electrochemistry with spatial information, and monitoring the transient nature of nanobubble formation and dynamics with opto-electrochemical imaging. We emphasize the intrinsic and quantitative physicochemical interpretation of single gas bubbles from electrochemical data, highlighting the fundamental understanding of the heterogeneous nucleation, dynamic state of the three-phase boundary, and the correlation between electrolytic bubble dynamics and nanocatalyst activities. In addition, a brief discussion of future perspectives is presented.

Insight into the bubble-induced overpotential towards high-rate charging of Zn-air batteries

Tremendous efforts are made in designing effective electrocatalysts for rechargeable Zn-air batteries to lower overpotenitals during discharging and charging. When the charging current reaches a critical value, bubble evolution inevitably occurs due to the unreleased oxygen on the air electrode and the possible hydrogen generation on the Zn electrode, whereas the detailed effects on the electrochemical performance are usually overlooked. Herein, the bubble dynamics and the induced overpotential, including activation, ohmic, and concentration overpotentials, during high-rate charging are investigated based on a carbon-free electrode through in-situ observation, mathematical modelling, and numerical simulation. The bubble generation on the air electrode is observed with a three-step process: initial accumulation, transitional growth, and steady evolution. The modelling and simulation results demonstrate that the concentration overpotential is reduced by the bubble-induced convection-disturbance and sink-absorption, while the activation overpotential (4.29 mV at 20 mA cm−2) is an order of magnitude larger than the ohmic component (0.10 mV). Further, a formula to express the whole overpotential is also proposed by the incorporation of potential oscillation from hydrogen bubble evolution on the Zn electrodes. This work elucidates the fundamental relationship between bubble dynamics and electrochemical characteristics, which favors the development of high-performance rechargeable Zn-air batteries.

Blocking of Ge/Si lattice mismatch and fabrication of high-quality SOI-based Ge film by interlayer wafer bonding with polycrystalline Ge bonding layer

We report a comparison of the Ge/SOI wafer bonding by amorphous Ge (a-Ge) and polycrystalline Ge (poly-Ge) interlayers. The Ge layer exfoliation and high-temperature annealing is used to fabricate high-quality SOI-based Ge film. The crystalline state evolution, bubble evolution, and dislocation evolution at the wafer-bonded interface are investigated. The related elimination mechanisms of bubbles and dislocations are revealed. Besides, the repairing of the point defects in the exfoliated Ge film stemmed from the H ion implantation is also demonstrated. For the a-Ge wafer bonding, when the SOI-based exfoliated Ge film is annealed at high temperature to repair the point defects, the bubbles and dislocations form at the bonded interface due to the lattice mismatch and lacking of gas transport channel ascribed to the crystallization of a-Ge (single-crystal Ge). However, for the poly-Ge wafer bonding, the poly-Ge layer not only can serve as a perfect lattice blocking layer due to its inconsistent crystal orientation, but can act as a perfect gas transporter to absorb bubbles thanks to its porous structure. The bubbles and TDs can be eliminated at high temperature during the repairing of the point defects, which in turn improves the Ge film quality.

The mechanisms of underwater wet flux-cored arc welding assisted by ultrasonic frequency pulse high-current

An ultrasonic frequency pulse high-current (UFPHC) was utilized to improve the stability of underwater wet flux-cored arc welding on Q235 steel. Based on the studies of the bubble evolution, arc behavior, droplet geometry, weld formation, and mechanical properties of the joint, the arc extinguishing phenomenon was suppressed effectively by the additional UFPHC, which decreased the amount of spatter on the workpiece and improved the appearance of the weld. The additional UFPHC reduced the average droplet radius by 40% and increased the average transition frequency by 99%, compared with the same value of additional welding current. The UFPHC enabled to decrease the hardness of the heat-affected zone by 15% due to the production of less martensite compared to results obtained by simply increasing the welding current. The vibration effect of the UFPHC in the welding pool was indirectly confirmed by the significantly decreased porosities in the final joint. The morphology of the impact fracture of different welding cases showed that the UFPHC was helpful to decrease the brittle fracture area in the weld joint and increase the value of the impact toughness for the weld joint.

Numerical simulation of hydrogen isotope bubbles evolution and their effect on fuel retention in tungsten

• A high nucleation rate and self-clustering of hydrogen isotope (HI) in the near-surface layer. • The appearance of the densest and largest bubbles in the near-surface layer enhances the hydrogen isotope retention significantly. • The behavior of the bubbles is strongly dependent on the operation regime in a tokamak. The 2D Hydrogen Isotope Inventory Process code with Monte Carlo bubble growth model (HIIPC2D-MC) has been upgraded by adding surface retention and self-clustering bubble nucleation to study the evolution of hydrogen isotope (HI) bubble and its impact on fuel retention. In this code, fuel retention is simulated by HIIPC2D, while the nucleation and growth of the bubble is modeled by 3D Monte Carlo code. Firstly, the dynamic behavior and mechanism of bubble nucleation is studied via self-clustering nucleation module. The effects of irradiation flux, material temperature ( T ) and trap concentration on bubble nucleation are systematically studied. The results show that the low T promotes the self-clustering bubble nucleation, large irradiation flux and low trap density lead to high nucleation rate. The long irradiation time facilitates the formation of bubbles via hydrogen self-clustering. Then, the time evolution of nucleation and growth of the bubbles is demonstrated. After the nucleation, both cascade-coalescence and pressure drive the bubbles to grow. The most and largest bubbles appear in the near-surface layer, and the HI inside the bubble plays important role on the total fuel retention. The simulation indicates the behaviors of the bubbles depend strongly on the operation regime in a tokamak. However, the bubble burst (blistering burst in experiment) is not included in the model, which will affect the bubble performance during long term irradiation.

Research on bubble pulsation and jet loading characteristics during a near-field underwater explosion

<p indent="0mm">To deeply study the bubble jet phenomenon occurring during the near-field underwater explosion, this study performed an underwater explosion test of small equivalent TNT exploding at the bottom of a clamped square plate. The load time–history curve of the bubble jet was successfully measured by the method employing an array wall pressure sensor. The experimental results show that the peak value of the bubble pulsation pressure at the bottom center of the clamped square plate is not significantly different from the peak value of the jet pressure. However, the duration of the jet pressure is 21.36 times that of the bubble pulsation pressure. The bubble pulsation pressure and its duration slightly attenuate, while the jet pressure and its duration attenuate faster with the increasing distance from the sensor to the center, indicating a quite concentrated range of jet load. Further, the numerical simulation of the whole process from the explosion to the bubble jet formation was performed via the Abaqus software. The bubble evolution process and the bubble jet load time-history curve obtained by the simulation are consistent with the experimental results. Finally, the distribution law of the specific impulse of the bubble jet, the evolution law of the bubble jet shape, and the change law of the head speed of the bubble jet during the near-field underwater explosion bubble pulsation process are obtained through numerical simulation. This work can provide a basis for the destructive strike of surface ship structures using bubble jets formed by a near-field underwater explosion.

Research on the effect of asymmetric bubbles on the load characteristics of projectiles during an underwater salvo

This paper analyzes the effect of bubbles on the load characteristic of projectiles launched underwater with transverse-flow under different dimensionless launch time intervals. The realizable k−ε turbulence model and energy equation, as well as the volume of fluid method and overlapping grid technique, were adopted. Additionally, validations of numerical approach and grids independence validation were carried out. Evolution of bubbles at tube outlet, time-domain characteristic and frequency domain characteristic of lateral force, vertical force-torque, and motion characteristics are studied. The results show that spilled gas from the tube outlet fuses and moves in a transverse-flow direction. The pressure pulsation, which is caused by the bubble elastic effect, leads to high-frequency pulsation laws of the load characteristic. In the initial stage of the movement, the motion laws of projectiles under different launch time intervals are similar. At ΔT≤1.2, the flow interference between two projectiles is large, which seriously affects the ballistic stability and safety.

Influence of Surface Tension on Dynamic Characteristics of Single Bubble in Free-Field Exposed to Ultrasound.

The motion of bubbles in an ultrasonic field is a fundamental physical mechanism in most applications of acoustic cavitation. In these applications, surface-active solutes, which could lower the surface tension of the liquid, are always utilized to improve efficiency by reducing the cavitation threshold. This paper examines the influence of liquids’ surface tension on single micro-bubbles motion in an ultrasonic field. A novel experimental system based on high-speed photography has been designed to investigate the temporary evolution of a single bubble in the free-field exposed to a 20.43 kHz ultrasound in liquids with different surface tensions. In addition, the R-P equations in the liquid with different surface tension are solved. It is found that the influences of the surface tension on the bubble dynamics are obvious, which reflect on the changes in the maximum size and speed of the bubble margin during bubble oscillating, as well as the weaker stability of the bubble in the liquid with low surface tension, especially for the oscillating bubble with higher speed. These effects of the surface tension on the bubble dynamics can explain the mechanism of surfactants for promoting acoustic cavitation in numerous application fields.

Experimental and theoretical investigation of the cavity dynamics of underwater launched projectiles

During the underwater launch of a projectile using compressed air, a bubble emerges from the launch tube before the projectile. Upon being pierced by the projectile, this bubble has significant effects on the formation and evolution of the projectile’s shoulder cavity. This paper presents experimental and theoretical analyses of the dynamics of such a projectile’s shoulder cavity launched at low Froude numbers. In the experiments, a dynamic sensor and a camera are used to record the internal pressure and profile of the cavity, respectively. The cavity pressure oscillations explain the cavity ripples observed during the experimental sequence. Accordingly, we establish an independent bubble evolution model to predict the internal cavity pressure based on the assumption of spherical expansion; we also separately construct a shoulder cavity evolution model that treats the cavity as a slender body. The matched asymptotic method is used to solve the shoulder cavity evolution model, and quantitative comparisons between the theoretical and experimental results, including the cavity shape and size, show favourable agreement. Finally, the effects of cavity pressure oscillations and of the nose shape and projectile velocity on the cavity’s behaviour are studied.

Explosive Vaporization of Ethanol on Microheater during Pulse Heating

The article is devoted to the research of transient heat transfer, explosive vaporization and condensation in subcooled ethanol on microheater during pulsed heating. Experiments were performed using high-speed infrared thermography and visualization for various initial liquid temperatures 30–60 °C and heat fluxes in the range of 3.3–7.8 MW/m2. The evolution of temperature field of the microheater and liquid during transient heating and the nucleation temperatures for different heat fluxes and liquid subcooling were determined. It was shown that the nucleation temperature was almost independent on heating power and amounted to 194 °C, which is close to the calculations based on homogeneous nucleation theory. New experimental data were obtained on the times of convection and boiling onset, evolution of vapor bubbles, including the dependence of their maximum sizes on the heat flux and initial liquid temperature. It was shown that the maximum bubble size decreases with an increase in the heat flux and liquid subcooling, varying in the same range as for water. It was demonstrated that at low heat fluxes, during condensation, a liquid funnel was formed at the bubble top, which resembled the formation of a liquid jet during the collapse of cavitation bubbles.

Ocean bubbles under high wind conditions – Part 2: Bubble size distributions and implications for models of bubble dynamics

Abstract. Bubbles formed by breaking waves in the open ocean influence many surface processes but are poorly understood. We report here on detailed bubble size distributions measured during the High Wind Speed Gas Exchange Study (HiWinGS) in the North Atlantic, during four separate storms with hourly averaged wind speeds from 10–27 m s−1. The measurements focus on the deeper plumes formed by advection downwards (at 2 m depth and below), rather than the initial surface distributions. Our results suggest that bubbles reaching a depth of 2 m have already evolved to form a heterogeneous but statistically stable population in the top 1–2 m of the ocean. These shallow bubble populations are carried downwards by coherent near-surface circulations; bubble evolution at greater depths is consistent with control by local gas saturation, surfactant coatings and pressure. We find that at 2 m the maximum bubble radius observed has a very weak wind speed dependence and is too small to be explained by simple buoyancy arguments. For void fractions greater than 10−6, bubble size distributions at 2 m can be fitted by a two-slope power law (with slopes of −0.3 for bubbles of radius &lt;80 µm and −4.4 for larger sizes). If normalised by void fraction, these distributions collapse to a very narrow range, implying that the bubble population is relatively stable and the void fraction is determined by bubbles spreading out in space rather than changing their size over time. In regions with these relatively high void fractions we see no evidence for slow bubble dissolution. When void fractions are below 10−6, the peak volume of the bubble size distribution is more variable and can change systematically across a plume at lower wind speeds, tracking the void fraction. Relatively large bubbles (80 µm in radius) are observed to persist for several hours in some cases, following periods of very high wind. Our results suggest that local gas supersaturation around the bubble plume may have a strong influence on bubble lifetime, but significantly, the gas in the bubbles contained in the deep plumes cannot be responsible for this supersaturation. We propose that the supersaturation is predominately controlled by the dissolution of bubbles in the top metre of the ocean, and that this bulk water is then drawn downwards, surrounding the deep bubble plume and influencing its lifetime. In this scenario, oxygen uptake is associated with deep bubble plumes but is not driven directly by them. We suggest that as bubbles move to depths greater than 2 m, sudden collapse may be more significant as a bubble termination mechanism than slow dissolution, especially in regions of high void fraction. Finally, we present a proposal for the processes and timescales which form and control these deeper bubble plumes.

High-Speed Digital Photography of Gaseous Cavitation in a Narrow Gap Flow

The research of cavitation in narrow gap flows, e.g., lubrication films in journal bearings or squeeze film dampers, is a challenging task due to spatial restrictions combined with a high time-resolution. Typically, the lubrication film thickness is in the range of a few microns and the characteristic time for bubble generation and collapse is less than a few milliseconds. The authors have developed a journal bearing model experiment, which is designed according to similarity laws providing fully similar flow conditions to real journal flows while offering ideal access to the flow by means of optical measurement equipment. This work presents the high-speed photography of bubble evolution and transportation in a Stokes-type flow under the influence of shear and a strong pressure gradient which are typical for lubricant films. A paramount feature of the experiment is the dynamic variation (increase/decrease) of the minimum film thickness which triggers the onset of cavitation in narrow gap flows. Results presented in the work on hand include the time-resolved data of the gas release rate and the transient expansion of gas bubbles. Both parameters are necessary to set up numerical models for the computation of two-phase flows.

Effect of anode channel shape and wettability on CO2 bubble evolution in direct methanol fuel cells

Active direct-methanol fuel cells operate on a liquid supply of reactants to the anode flow channels. Gaseous carbon dioxide is produced during operation forming large bubbles on the top side of diffusion layer, limiting the transport of reactants to the functional layer. This causes a significant drop in the rate of reaction and therefore limits the maximum current density. To collect CO2 bubbles away from the diffusion layer, a new design is proposed. It includes a degassing channel placed at the top of the main trapezoidal anode channel. The wettability of the degassing channel and the dihedral angle of the anode channel are investigated. To assess the effect of these parameters, a three-dimensional, two-phase flow model is developed and numerically simulated. Results show that adding the degassing channel is advantageous in terms of bubble collection. A trapezoidal main channel achieves a significantly higher rate of bubble actuation compared to a rectangular channel. In addition, using a dihedral angle of 20° causes a decrease in the pumping pressure, which reduces pumping losses. Moreover, a contact angle of 100° for the degassing channel provides the best compromise in terms of actuation rate, extraction rate out of the channel, and pressure drop along the channel. However, degassing channels can yield up to three times longer bubbles, which are around 75% slower. These findings create the opportunity to improve the performance of direct-methanol fuel cells by enhancing/optimizing the mass transport of reactants on the anode side.

Molecular Dynamics Simulations of Xe Behaviors at the Grain Boundary in UO2

In this study, we investigated the behavior of xenon (Xe) bubbles in uranium dioxide (UO2) grain boundaries using molecular dynamics simulations and compared it to that in the UO2 bulk. The results show that the formation energy of Xe clusters at the Σ5 grain boundaries (GBs) is much lower than in the bulk. The diffusion activation energy of a single interstitial Xe atom at the GBs was approximately 1 eV lower than that in the bulk. Furthermore, the nucleation and growth of Xe bubbles in the Σ5 GBs at 1000 and 2000 K were simulated. The volume and pressure of bubbles with different numbers of Xe atoms were simulated. The bubble pressure dropped with increasing temperature at low Xe concentrations, whereas the volume increased. The radial distribution function was computed to explore the configuration evolution of Xe bubbles. The bubble structures in the GB and bulk material at the same temperature were also compared. Xe atoms were more regular in the bulk, whereas multiple Xe atoms formed a planar structure at the GBs.

Bubble behavior and evolution characteristics in the RH riser tube-vacuum chamber

Abstract The motion behavior of bubbles in a riser tube is studied in order to analyze the bubble evolution characteristics. Gas distribution and bubble movement in risers and vacuum chambers have important effects on liquid steel flow, mixing and refining process. It is found that the initial diameter of argon bubbles in the riser tube decreased with decreasing vacuum degree. The diameter of argon bubbles in the riser tube increased with increasing gas flow rate. The bubbles could be divided into the single bubble rising zone and the bubble breaking coalescence zone in the rising tube. After the bubbles were blown in, they changed from regular spherical shapes to flat shapes in the single bubble rising zone, and then broke apart into small bubbles in the bubble breaking coalescence zone. Variations in the gas flow rate and vacuum degree had significant effects on the regional distribution of bubble motion and bubble residence time. The critical height of the single bubble rising zone and the bubble breaking coalescence zone were stable when the bubble travel distance was greater than 280 mm.

The effect of interstitial carbon atoms on defect evolution in high entropy alloys under helium irradiation

In this work, a design strategy based on a typical multi-principal element alloy (Fe50Mn30Co10Cr10, at. %) was proposed to tune the irradiation behavior by adding interstitial carbon atoms (Fe49.5Mn30Co10Cr10C0.5, at. %). Advanced scanning transmission electron microscopy was employed to study the evolution of dislocation loops and helium bubbles in alloys after 400 keV He+ ion irradiation at three different temperatures ranging from 350 °C to 450 °C. The results showed that the interstitial carbon atoms inhibited the growth of He bubbles and irradiation hardening. The high-resolution energy dispersive spectroscopy mapping revealed that the segregation at the dislocation loops in Fe49.5Mn30Co10Cr10C0.5 was efficiently lessened compared to that in Fe50Mn30Co10Cr10, which shows an even superior performance than the equiatomic FeMnCoCrNi Cantor alloy. Radiation-induced “W-shaped” elemental oscillation patterns with a width of about 1 nm around dislocation loops, were observed for the first time in high-entropy alloys. Such nanoscale local elemental fluctuations are suggested to modify the lattice friction and thus have an impact on the mechanical properties of the material. First-principles calculations suggest that the interstitial carbon atoms can effectively enhance the site-to-site local lattice distortion and modify vacancy migration energy near carbon atoms. Our systematic experiments demonstrate that adding interstitial carbon atoms into multi-principal element or high-entropy alloy is beneficial for the reduction in irradiation hardening and volumetric swelling, as well as radiation-induced segregation.

Comparison of Intergroup Mass Transfer Coefficient Correlations in Two-Group IATE for Subcooled Boiling Flow

The interfacial area concentration (IAC) is of vital importance in determining the interfacial transfer terms of mass, momentum, and energy between phases in the two-fluid model. The two-group (2-G) interfacial area transport equation (IATE) dynamically models the IAC evolution of large and small bubbles, and the intergroup transfer bookkeeps the mass transferring between two groups. Different intergroup mass transfer coefficient correlations are employed in the 2-G IATE model, which include one old correlation and three new correlations previously developed by the authors. The two-group IATE is benchmarked with subcooled boiling experimental dataset. The group-1 interfacial area concentration results show that the three modified correlations improve the physically incorrect prediction by the old correlation. With the modified correlations, the 2-G IATE is foundationally capable of predicting the group-1 interfacial area concentration in subcooled boiling flow.