Bubble Growth Research Articles

Towards Controlled Transfer of (001) β-Ga2O3 to (0001) 4H-SiC Substrates

We demonstrate successful surface blistering of He-implanted (001) β-Ga2O3 substrates, bonding to (0001) 4H-SiC, and initial results towards large-area transfer of (001) β-Ga2O3 to 4H-SiC. Surface blistering of unbonded, implanted substrates is an important indication of successful exfoliation and transfer of films, which is achieved by initiating He bubble nucleation during a low temperature anneal followed by bubble growth at a high temperature anneal. Prior to annealing, implanted substrates were bonded to (0001) 4H-SiC at room temperature using a thin ~5 nm Ti interlayer. However, the β-Ga2O3 substrate did not wafer split from the bonded structure after annealing. Instead, small area transfers up to ~200 μm were achieved (~7% of the total bonded area transferred while the entire structure remained bonded). Further optimization of implant parameters is underway. These are promising results towards achieving large wafer-scale (001) β-Ga2O3 composite wafers suitable for β-Ga2O3 devices with efficient thermal management characteristics.

Pool-boiling enhancement on periodic micro/nano ripple-structured surfaces fabricated by femtosecond laser

A study was conducted on whether periodic micro/nano ripple structures generated on metal surfaces using femtosecond laser processing could improve pool boiling heat transfer. Depending on the laser irradiation conditions on the metal surface, a surface structure with different geometrical characteristics is created. Changes in sample surface morphology caused by the laser irradiation intensity are examined using scanning electron microscopy and confocal laser microscopy. Pool boiling tests are conducted to compare three laser irradiation conditions fabricated samples and the bare copper surface. The working fluid is deionized water. The pool boiling curves of each sample surface were compared with each other. Visualization of bubble nucleation is performed using a high-speed camera. The superheat, bubble departure diameter, and bubble growth period of each sample are measured and compared at the onset of nucleate boiling (ONB). The most efficient periodic micro/nano ripple-structured surface reduces the superheat at the ONB by 6.13 °C in comparison with that at the bare Cu surface. The enhanced surface structure reduced the bubble departure diameter and renewal period by 60.5% and 55.6% in comparison with that of the smooth surface. Further, the structured surface showed 178.5% enhancement in heat transfer coefficient and 39.1% in critical heat flux, respectively. Thus, the combination of increased nucleate site density, high nucleation site activation, and a wicking effect in the periodic micro/nano ripple structure significantly improves the pool boiling heat transfer.

Synergistic effect of helium and hydrogen for vacancy-like defects in pure Fe and Fe9Cr alloy

A synergistic effect between the helium and hydrogen elements exists in fusion reactors materials. To explore its microscopic mechanism, this paper meticulously investigates the H and He action for vacancy-like defects in pure Fe and Fe9Cr alloys by using first-principles calculations. The He and H vacancy-like defects were performed within atomistic simulations in body-centered cubic Fe and Fe9Cr alloys. Via analyzing the structures of the clusters, we find that He atoms in the VacHenHm clusters are more energetically favorable to occupy the center of the vacancy and the H atoms will attach to the free surface. Based on the comprehensive analysis of the vacancy trapping energy, vacancy formation energy, formation volumes, and the electron density maps, He elements play a more dominant role within the H/He synergistic effect, while H elements aggravate the bubble growth. There is no evident chemical bonding between He and H atoms. Concerning the Cr atoms, the Cr-He possesses the strongest repulsive energy within the considered diatomic molecules. The Cr element could weaken the aggregation and reduce the diffusion barriers of the vacancies, thus inhibiting the growth of vacancy-like defects to some extent. These results would help to provide insights into the synergistic effect of He and H for vacancy-like defects in Fe9Cr alloy.

A Mathematical Model of Financial Bubbles: A Behavioral Approach

In this work, we propose a mathematical model to describe the price trends of unsustainable growth, abrupt collapse, and eventual stabilization characteristic of financial bubbles. The proposed model uses a set of ordinary differential equations to depict the role played by social contagion and herd behavior in the formation of financial bubbles from a behavioral standpoint, in which the market population is divided into neutral, bull (optimistic), bear (pessimistic), and quitter subgroups. The market demand is taken to be a function of both price and bull population, and the market supply is taken to be a function of both price and bear population. In such a manner, the spread of optimism and pessimism controls the supply and demand dynamics of the market and offers a dynamical characterization of the asset price behavior of a financial bubble.

Modeling inception and evolution of near-wall vapor thermo-cavitation bubbles via a lattice Boltzmann method

The proposed method simulates the growth and collapse mechanisms of lasers or sparks cavitation bubbles based on a double-distributed pseudo-potential thermal lattice Boltzmann method, which gives a high-temperature spot equal to or larger than the critical temperature of the fluid. This technique offers valuable insights into both the hydro- and the thermodynamics evolution of near-wall bulk bubbles. Our findings indicate that the dimensionless bubble-wall distance and the maximum radius share a correlative relationship, expressible via a piecewise function. The critical radius required for bubble inception exhibits a direct correlation with surface tension. We also observe a power-law relationship between surface tension and the maximum radius: lower surface tension results in increased deformation and a larger jet volume. The simulation results reveal the significant influence of initial input temperature on the pressure dynamics within the gas nucleus, leading to varied surface expansion velocities. However, this factor only marginally affects the surface wall expansion velocities. An increased input temperature results in intensified collapse mechanisms. Nevertheless, the elevated temperature during the initial collapse may cause an increase in pressure within the remaining bubble components, potentially leading to oscillations in the bubble radius.

Computational modelling and investigation of nucleate boiling with periodic exponential heat flux-based power-transients

The present work proposes a numerical methodology to perform single bubble nucleate boiling simulation subjected to power transients with exponential heating. Cyclic variation in transient heat flux is utilized which enables—proper characterization of the power transient parameters, parametric investigation of power transients on single bubble dynamics, and comparison between steady and transient heating condition. For constant time averaged heat flux in power cycle, it was observed that heating parameters are not having any influence on bubble departure diameter and bubble growth period whereas the bubble departure frequency is enhanced until vertical coalescence is observed. Parametric investigations based on time period of periodic heat flux and time averaged heat flux reveal the existence of a range in excursion time period which cause increase in bubble departure frequency, with applicability for controlling bubble dynamics with power transients and improving efficiency of thermal management devices using nucleate boiling.

Depressurization-induced drop breakup through bubble growth

Depressurization-induced drop breakup through bubble growth

Temperature dependence of surface microstructure of 316 L stainless steel exposed to low-energy hydrogen ion irradiation

The austenitic 316 L stainless steel (316LSS) is an important metal for nuclear installations due to its high strength, good plastic toughness and excellent corrosion resistance, and is commonly used as the plasma-facing and structural material. In this study, 316LSS samples are irradiated with low-energy hydrogen ions (35 eV) at the temperature of 400–1300 K to investigate the surface microstructure evolution. The experimental results show that the surface of 316LSS samples remains relatively smooth at 400 K and 600 K after irradiation. Significant surface roughening including spherical protrusions is formed on the surface at 800 K and 1100 K, and step-like streaks are observed at 1300 K. However, no drastic change in the surface morphology is observed in the separate annealing experiments at 800–1300 K. It has been shown that hydrogen ion irradiation mainly causes the surface microstructure evolution of 316LSS samples. After hydrogen plasma exposure, a large number of adatoms are first formed on the surface of 316LSS because of bubble growth. The adatoms then preferentially reach the defects by diffusion at certain temperatures, and aggregate into spherical protrusions with low surface energy. The qualitative simulation results show that the irradiation temperature affects the diffusion and aggregation of adatoms on the surface, and the radius of the spherical protrusions gradually increases due to the decrease of the surface binding energy and the increase of the irradiation fluence.

The role of surface wettability on the growth of vapour bubbles

The formation of heterogeneous vapour bubbles is widely studied due to its importance to two-phase thermal management systems, ultrasonic cleaning, and turbomachinery performance. However, the role that the surface plays in determining the growth of a bubble is still poorly understood. Currently, theoretical understanding of heterogeneous vapour bubble growth is limited to hemispherical bubbles or completely spherical bubbles next to a surface. We have previously developed an inertio-thermal model to accurately predict how heat transfer from the surrounding fluid affects homogeneous vapour bubble growth from the nanoscale to the macroscale (Sullivan et al. J. Fluid Mech. 948(A55):1-15, 2022). By accounting for the presence of the surface and its wettability on both the geometry of the bubble and on the available thermal energy in the surrounding fluid, we extend our model to capture heterogeneous vapour bubble growth. Using molecular simulations, we show not only how the strength of fluid-solid interaction affects the growth rate, but also how the formation of an adsorbed fluid layer under the bubble on lyophilic surfaces plays a vital role in determining the bubble shape and subsequent dynamics. These insights have potential to improve the performance of systems involving a change of phase from liquid to vapour by better understanding the role of surface wettability on this process.

Effect of nucleation frequency and compressible buffer volume on the thermofluidic characteristics for flow boiling through a microchannel heat sink

A 1-D transient model of flow boiling in a rectangular microchannel is developed to assess the pressure drop and heat transfer characteristics, particularly emphasizing the impacts of nucleation frequency and compressible buffer volume. The combined influence of nucleation frequency with channel height, inlet velocity and heat flux is also studied. The increased nucleation frequency reduces the pressure drop amplitude and enhances the heat transfer coefficient. As the buffer volume is increased, the pressure drop and heat transfer coefficient are reduced due to the predominant occurrence of reverse flow in the channel. The amplitude and duration of reverse flow are identified as two important key parameters. As the reverse flow strongly affects the thermal-hydraulics of the channel, a sensitivity analysis is performed to assess the variation of these two key parameters over a range of operating conditions. The variation of amplitude and duration of reverse flow strongly depends on the trends of channel pressure drop, which is the consequence of the competing effects of bubble growth rate and reverse flow. Lastly, the effect of variable vapor properties on pressure drop and heat transfer characteristics of the channel has been discussed.

Competitive methane bubble growth in aquatic muds

Methane (CH4) bubbles residing in shallow aquatic muds pose a significant threat to the environment. Impeded by the muddy sediment opacity and insufficient resolution for their characterization, past studies overlooked bubble interactions as they grow. Here, the competitive fracture-driven growth of CH4 bubble pair with different initial sizes in the aquatic muds is simulated, using a mechanical/reaction-transport numerical model. Mechanical and solute transport interactions were found to dominate at the different stages of the bubble growth, both retarding the smaller competitive bubble growth. The stress from large competitive bubble affects the inner pressure and diffusive CH4 flux to the smaller bubble, causing its slower initial growth (at t < 40 s). The large competitive bubble also diverts CH4 from the smaller one later on, thus inhibiting its growth even more. Bubble stress interactions may produce more laterally oriented smaller bubble and significant deformations of the larger one, both amplified under the decreasing distance between the growing bubble pair. Competitive bubble growth may shape a bubble size distribution pattern observed in the lab experiments and in situ, which may promote muddy sediment CH4 gas retention and produce gas domes. The latter acts as pockmark precursors whose formation induces a violent gas release to the water column and potentially to the atmosphere. Our study presents a basis for a proper upscaling to various effective gassy muddy sediment characteristics and gas retention evolution transient models, while preserving the single bubble growth descriptors. It contributes to evaluation and even reduction of a long-persisting uncertainty related to CH4 flux from the shallow aquatic sediments.

A coupled level-set and volume-of-fluid (CLSVOF) method for prediction of microgravity flow boiling with low inlet subcooling on the international space station

The present study is part of a series of NASA-supported investigations of flow boiling of n-perfluorohexane (n-PFH, C6F14) under microgravity on the International Space Station (ISS). The fluid physics and heat transfer characteristics of flow boiling with low inlet subcooling are explored using the Flow Boiling and Condensation Experiment (FBCE) (which is actual name of the test facility), the largest endeavor in microgravity two-phase research conducted to date. Experimental data are collected from the FBCE's Flow Boiling Module (FBM), which features a rectangular channel with 5.0-mm x 2.5-mm cross-section and 114.6-mm heated length. Results for three flow rates are used to explore the effects of mass velocity on flow structure and heat transfer characteristics in the absence of the body force. A Computational Fluid Dynamics (CFD) model is constructed to predict the measured FBM results. The CFD model employs the Coupled Level-Set and Volume-of-Fluid (CLSVOF) method, which is modified with improved interface capture and additional force terms in the momentum equation to determine the bubble dynamics more accurately. Validity of the CFD model is assessed in two ways, by comparing predictions against video-captured interfacial behavior and heat transfer data. The CFD model shows good ability to capture detailed evolution of the interfacial structure both across and along the flow channel, including bubble nucleation, growth, departure, and coalescence, as well as axial development of dominant flow patterns. Details of the flow structure are examined via axial development of both flow velocity and void fraction profiles. Similarly, heat transfer results are presented in terms of streamwise variations of both wall temperature and fluid temperature, which are also predicted accurately with the CFD model. Overall, this study proves the CFD model is an effective method for both design and performance assessment of flow boiling subsystems in space vehicles.

Droplet boiling on micro-pillar array surface – Nucleate boiling regime

Despite spray cooling in the form of droplet boiling on a textured surface being a very promising phase-change heat dissipating method, the understanding of droplet/bubble two-phase dynamics in the nucleate boiling is extraordinarily limited. In this study, we report sessile droplet boiling on micro-pillar array surface in the nucleate boiling regime using a three-dimensional lattice Boltzmann model comprehensively. Effects of micro-pillar size on bubble behaviors inside droplet are discussed in detail, covering bubble nucleation, growth, coalescence, and rupture. For the micro-pillars with large side length or small spacing, nucleation sites are activated around micro-pillar top surface. The preferential activation location of nucleation sites is determined by temperature, confined space and fluid flow. In bubble growth stage, the variation of bubble radius with time follows the square root law, being consistent with previous experiments. Bubbles merge into a large central bubble beneath droplet for the short micro-pillars while into a vapor layer for the long micro-pillars. Emergence of large central bubble prolongs droplet lifetime but deteriorates heat transfer. In addition, increasing micro-pillar side length or decreasing micro-pillar height can delay activation of nucleation sites.

The dissolution behavior of He atom in AlNbTiZr high entropy alloy by first principles

The dissolution behavior of He atom in AlNbTiZr high entropy alloy (HEA) was studied from first principles and a new mechanism for the enhanced irradiation resistance with the unusual interaction between AlNbTiZr and He is found. It is interesting to note that the dissolution energy is inversely proportional to Al atom and proportional to Nb atom, i.e., the helium tends to occupy the sites where there exists a local chemical ordering characterized by an atomic environment with high content of Al or low content of Nb atoms, and thus Al atoms are conducive to dissolving He atoms. And interesting enough, differing from traditional metal, the site-to-site interaction is different in AlNbTiZr HEA, which leads to a special sub-nanoscale structure, Al cage. What's more, the Al cage can not only promote the ability of vacancy trapping for He, but also greatly suppress the growth of He bubbles by pinning the He atoms, further improving the irradiation resistance. Consequently, it is feasible to produce some Al cage in dispersion state to improve irradiation resistance of AlNbTiZr. Due to the infinite possibilities of the composition of HEA, this finding can guide future design of novel HEA with improved irradiation resistance performances.

Limiting and Binary Schemes of Vapor Bubble Growth in an Infinite Volume of Liquid

The problem of spherically symmetric growth of a vapor bubble in an infinite volume of a uniformly superheated liquid is considered. The concept of limiting growth schemes, each of which is determined by the influence of a single physical factor, is put forward. An analytical approximation of the Scriven integral is proposed, its asymptotics are analyzed. Analytical solutions for binary growth schemes with due account of the influence of two physical factors on bubble growth are obtained.

A numerical study on bubble dynamics and heat transfer of flow boiling in an inclined microchannel

Flow boiling, as an important heat transfer method, is widely applied in many devices. In this study, a gas-liquid thermal lattice Boltzmann model is adopted to study the flow boiling in an inclined microchannel. Specifically, the bubble growth processes and heat transfer efficiency are studied for different inclined angles. The numerical results show that the liquids occur phase change almost at the same time while the inclined angles are different. As the inclined angle increases, the bubble departure time decreases. Also, the heater temperatures change periodically for the studied cases. The maximum heater temperature is not affected by the inclined angle while the minimum heater temperature decreases when the inclined angle of the microchannel increases. Therefore, the larger inclined angle increases the flow boiling heat transfer efficiency.

A lattice Boltzmann investigation of liquid viscosity effects on the evolution of a cavitation bubble attached to chemically patterned walls

The thermal lattice Boltzmann model is applied to explore liquid viscosity effects on a single cavitation bubble attached to chemically patterned walls. A conversion method based on the surface tension and the non-ideal equation of state parameters is proposed. According to the force analysis, it is found that the local pressure difference and the unbalanced Young's force are two main controlling factors for contactpoint dynamics. The dynamic contact angle is larger than the equilibrium contact angle throughout the evolution process for a hydrophilic wall, which results in a hysteresis effect in the bubble growth process due to the unbalanced Young's force and accelerates the contact point retraction velocity in the collapse stage. For hydrophobic walls, the unbalanced Young's force accelerates the contact radius expanding, resulting in a larger maximum contact radius than for a bubble attached to a hydrophilic wall. The hysteresis effects caused by the unbalanced Young's force slow down the contact points retraction in the early collapse stage and accelerate the retraction later because of dramatic interface deformation. The bubble is punctured over a larger volume with a hydrophilic wall than with a hydrophilic wall, resulting in a smaller collapse intensity. An exponential relationship between the micro-jet volume and the cosine function of the equilibrium contact angle at the collapse point is found. Furthermore, the jet volume before bubble collapse decreases, and the collapse time delays with the increase in viscosity.

Pre‐Eruptive Outgassing and Pressurization, and Post‐Fragmentation Bubble Nucleation, Recorded by Vesicles in Breadcrust Bombs From Vulcanian Activity at Guagua Pichincha Volcano, Ecuador

AbstractBreadcrust bombs formed during Vulcanian eruptions are assumed to originate from the shallow plug or dome. Their rim to core texture reflects the competition between cooling and degassing timescales, which results in a dense crust with isolated vesicles contrasting with a highly vesicular vesicle network in the interior. Due to relatively fast quenching, the crust can shed light on pre‐ and syn‐eruptive conditions prior to or during fragmentation, whereas the interior allows us to explore post‐fragmentation vesiculation. Investigation of pre‐ to post‐fragmentation processes in breadcrust bombs from the 1999 Vulcanian activity at Guagua Pichincha, Ecuador, via 2D and 3D textural analysis reveals a complex vesiculation history, with multiple, spatially localized nucleation and growth events. Large vesicles (Type 1), present in low number density in the crust, are interpreted as pre‐eruptive bubbles formed by outgassing and collapse of a permeable bubble network during ascent or stalling in the plug. Haloes of small, syn‐fragmentation vesicles (Type 2), distributed about large vesicles, are formed by pressurization and enrichment of volatiles in these haloes. The nature of the pressurization process in the plug is discussed in light of seismicity and ground deformation signals, and previous textural and chemical studies. A third population (Type 3) of post‐fragmentation small vesicles appears in the interior of the bomb, and growth and coalescence of Type 2 and 3 vesicles causes the transition from isolated to interconnected bubble network in the interior. We model the evolution of viscosity, bubble growth rate, diffusion timescales, bubble radius and porosity during fragmentation and cooling. These models reveal that thermal quenching dominates in the crust whereas the interior undergoes a viscosity quench caused by degassing, and that the transition from crust to interior corresponds to the onset of percolation and development of permeability in the bubble network.

Preparation of high-purity nickel by floating zone refining under a hydrogen atmosphere

This study focuses on preparing high-purity nickel using floating zone refining under hydrogen atmosphere to overcome challenges related to impurity removal. The utilized high-vacuum floating zone refining device minimizes contamination and external impurities. The purification process's effectiveness is particularly notable for gaseous impurities, such as carbon and oxygen, with removal rates reaching 94.5% and 96.3%, respectively. The purification mechanism of the melt is considered as involving the nucleation and growth of hydrogen bubbles in the molten zone, and the adsorption of impurities on the bubble surface, followed by the ascent and overflow of bubbles, which effectively carry impurities out of the molten zone.

The effect of screw dislocation on Helium Bubble growth in Tungsten: molecular dynamic simulation study

The effect of a 1/2[111] screw dislocation on the growth of helium bubble in tungsten is investigated by molecular dynamics simulation method. When helium bubble grows at sites less than 3 nm away from the dislocation, tungsten self-interstitial atoms (SIAs) pushed out by the growing bubble are absorbed by and then migrate rapidly along the dislocation line, inducing the screw dislocation to evolve into a helical configuration. The existence of screw dislocation can facilitate the growth of helium bubble. The interaction energies between the screw dislocation and point defects (helium, SIA and vacancy) are calculated. The interaction energies are all negative, indicating that the screw dislocation can attract and serve as a sink for these point defects. Further investigation shows that the interactions between the screw dislocation and these point defects are local, and stem from the special stress distribution pattern of the screw dislocation.