Bubble State Research Articles

The role of irradiation-enhanced interstitial diffusion in over-pressurizing fission gas bubbles in UO2

Fission gas bubbles in UO2 nuclear fuel have been observed to exhibit pressures in excess of the equilibrium bubble pressure; however, the cause of bubble over-pressurization has not yet been demonstrated. The mechanical interaction between a bubble and the surrounding matrix or grain boundary depends on the internal pressure of the bubble and local stress state, such that over-pressurized bubbles are thought to be responsible for fragmentation and pulverization, when exposed to a temperature ramp. Here, we investigate the role of U interstitials, produced through irradiation, in over-pressurizing bubbles by using a combined molecular dynamics (MD) and cluster dynamics approach. Firstly, the energies for the capture of interstitials and vacancies by bubbles have been determined from MD as a function of the ratio of gas atoms to vacancies that make up the bubble. Secondly, these reaction energies have been implemented in the cluster dynamics code Centipede to predict bubble over-pressurization as a function of temperature for typical fission rates. It was found that there is a transition from low pressure bubbles (at high temperatures) to high pressure bubbles (at lower temperatures). The cause of this behavior was shown to be the creation of irradiation-induced interstitials that are highly mobile relative to vacancies at low temperature; whereas, vacancies are sufficiently mobile at high temperatures to limit bubble pressures. This result supports the hypothesis that over-pressurized bubbles form during steady-state operation and that this behavior is highly sensitive to the local pellet temperature.

A generalized kinetic theory of Ostwald ripening in porous media

Partially miscible bubbles (e.g., CO2) trapped inside a porous medium and surrounded by a wetting phase (e.g., water) occur in a number of applications including underground hydrogen storage, geologic carbon sequestration, and the operation of electrochemcial devices such as fuel cells and electrolyzers. Such bubbles evolve due to a process called Ostwald ripening that is driven by differences in their interfacial curvature. For spherical bubbles, small bubbles shrink and vanish while feeding into larger ones, resulting in one large bubble at equilibrium. Within the confinement of a porous medium, however, bubbles can attain a distribution of sizes at equilibrium that have identical curvature. This work concerns itself with the formulation of a kinetic theory that predicts the statistical evolution of bubble states, defined as the sizes of the pores within which bubbles are trapped and the extent to which those pores are saturated with bubbles. The theory consists of a population balance equation and appropriate closure approximations. Systematic comparisons against a previously published pore network model (PNM) are conducted to validate the theory. Our theory generalizes existing variants in the literature limited to spherical bubbles trapped in homogeneous media to non-spherical (deformed) bubbles inside microstructures with arbitrary heterogeneity and spatial correlation in pore/throat sizes. We discuss the applicability, limitations, and implications of the theory towards future extensions.

Behind the Non-Uniform Breakup of Bubble Slug in Y-Shaped Microchannel: Dynamics and Mechanisms.

Bubble flow in confined geometries is a problem of fundamental and technological significance. Among all the forms, bubble breakup in bifurcated microchannels is one of the most commonly encountered scenarios, where an in-depth understanding is necessary for better leveraging the process. This study numerically investigates the non-uniform breakup of a bubble slug in Y-shaped microchannels under different flow ratios, Reynolds numbers, and initial bubble volumes. Overall, the bubble can either breakup or non-breakup when passing through the bifurcation and shows different forms depending on flow regimes. The flow ratio-Reynolds number phase diagrams indicate a power-law transition line of breakup and non-breakup. The bubble takes longer to break up with rising flow ratios yet breaks earlier with higher Reynolds numbers and volumes. Non-breakup takes less time than the breakup patterns. Flow ratio is the origin of non-uniform breakup. Both the Reynolds number and initial volume influence the bubble states when reaching the bifurcation and thus affect subsequent processes. Bubble neck dynamics are analyzed to describe the breakup further. The volume distribution after breaking up is found to have a quadratic relation with the flow ratio. Our study is hoped to provide insights for practical applications related to non-uniform bubble breakups.

Peak effect and dynamics of stripe- and pattern-forming systems on a periodic one-dimensional substrate.

We examine the ordering, pinning, and dynamics of two-dimensional pattern-forming systems interacting with a periodic one-dimensional substrate. In the absence of the substrate, particles with competing long-range repulsion and short-range attraction form anisotropic crystal, stripe, and bubble states. When the system is tuned across the stripe transition in the presence of a substrate, we find that there is a peak effect in the critical depinning force when the stripes align and become commensurate with the substrate. Under an applied drive, the anisotropic crystal and stripe states can exhibit soliton depinning and plastic flow. When the stripes depin plastically, they dynamically reorder into a moving stripe state that is perpendicular to the substrate trough direction. We also find that when the substrate spacing is smaller than the widths of the bubbles or stripes, the system forms pinned stripe states that are perpendicular to the substrate trough direction. The system exhibits multiple reentrant pinning effects as a function of increasing attraction, with the anisotropic crystal and large bubble states experiencing weak pinning but the stripe and smaller bubble states showing stronger pinning. We map out the different dynamic phases as a function of filling, the strength of the attractive interaction term, the substrate strength, and the drive, and demonstrate that the different phases produce identifiable features in the transport curves and particle orderings.

Liquid–Solid Triboelectric Probes for Bubbles Status Monitoring

AbstractBubble status monitoring is essential for designing material nanostructures and optimizing industrial processes. However, tracking the fluid dynamics of bubbles is challenging due to their random distribution and irregular dynamic changes. In this study, a tube liquid–solid triboelectric nanogenerator serves as a probe to investigate bubbles at the liquid–solid interface. The study explores the impact of bubble fluid dynamics on liquid–solid contact electrification, examining variables such as flow rates, volumes, and release intervals of bubbles. The study demonstrates that the liquid–solid triboelectric probe can work as a non‐intrusive and self‐powered sensor for monitoring bubble states in chemical fluid control. The sensor exhibits a sensitivity of 13.2 V·cm−3, a signal‐bubble volume correlation coefficient of 0.9964, and a rapid response time of 0.15 s. The sensor, integrated with a wireless module, enables continuous remote detection. This work provides a valuable tool for investigating tri‐phase contact electrification dynamics and presents promising technology for bubble state monitoring.

Air entrainment by a hydrofoil in a high-speed water flow for design of bubbly drag reduction ships

We have investigated the flow behavior around a hydrofoil bubble generator in a high-speed channel flow up to 10 m/s for the first time to understand the air entrainment characteristics of this hydrofoil device caused purely by the inertia effect of water flow. This study is motivated by our previous success in achieving ship drag reduction for full-scale vessels (Kumagai et al., 2015). Depending on the water flow velocity U and the angle of attack of the hydrofoil α, three different major patterns were identified: continuous generation of dispersed bubbles, intermittent formation of an air cavity, and steady formation of a super air cavity that stretched behind the hydrofoil. The transition from the dispersed bubble state to the air cavity formation regime is explained theoretically using the equation of bubble motion coupled with a potential flow analysis. The air volume flow rate increased linearly with U, and its slope was steeper in the dispersed bubble state than in the air cavity formation regime. The highest air flow rate was realized at U = 9 m/s, which corresponds to the nominal air layer thickness at ta = 4.1 mm, and reaches the feasible range for realization of bubbly drag reduction for a long ship hull.

An Experimental Database for the Analysis of Bursting of a Laminar Separation Bubble

The bursting phenomenon consists in the switch of a laminar separation bubble from a short to a long configuration. In the former case, reduced effects on profile pressure distribution are typically observed with respect to the attached condition. On the contrary, long bubbles provoke significant variations in the loading coefficient upstream of the separation position, with increased risk of stall of the lifting surfaces. The present work presents an experimental database describing separated boundary layers evolving under different Reynolds numbers, adverse pressure gradients and free-stream turbulence levels. Overall, more than 80 flow conditions were tested concerning short and long bubbles for the characterization of separated flows under turbine-like conditions. Measurements were performed on a flat plate geometry using a fast-response Particle Image Velocimetry (PIV) system. For each flow case, two sets of 6000 flow records were acquired with an acquisition frequency equal to 300 and 1000 Hz. Based on existing criteria for the identification of the bursting phenomenon, the flow cases were clustered in terms of short and long bubble states. Additionally, the kind of instability (i.e., convective or absolute) developing into the separated boundary layer was identified based on flow statistics. The present data captures the existing link between the bursting of a laminar separation bubble and the onset of the absolute instability of the separated shear layer, with stationary vortices forming in the dead air region.

Vertical diffusion of bubbles injected beneath a flat-bottomed ship for frictional drag reduction

There has been concern about the short effective distance of frictional drag reduction achieved by air lubrication, but recent model ship experiments have shown over 30 m longer than our expectation. Also, the reduction ratio can be improved through the artificial generation of void waves appearing beneath the hull. These topics relate to the spatiotemporal bubble distribution and an understanding of them requires clarification of the distribution. In this study, optical probes were used to measure the distributions of bubble states beneath a 20-m long flat-bottomed ship towed with 6–8 m/s. The measurements explained the long effective distance of drag reduction and the process of void wave generation and development. The drag reduction ratio was about 7–25% and mainly depended on the void fraction within 10 mm of the hull. The vertical distribution of the void fraction remained unchanged over long downstream distances. The development of void waves was affected by the streamwise velocity difference between bubble clusters near the hull. The velocity was determined by the number density of the bubbles, decreasing to the half of ship speed until reaching a certain number density, 100 per meter in our experiments, and then becoming constant at higher density.

A numerical study on oscillatory characteristics of unstable steam jet in water flow within a vertical pipe

In this study, a numerical investigation was conducted to examine the condensation and pressure oscillation characteristics of low mass flux steam jets in water flow within a vertical pipe under different operating conditions. Three bubble states were observed during the injection of subsonic steam in an upward direction into a subcooled water flow: bubble growth, bubble necking, and bubble detachment. Through a synchronously analysis of the bubble morphology and interface pressure, the oscillation mechanism of the unstable steam jet was revealed. The results showed that the pressure oscillation characteristics were consistent with the behavior of the vapor bubble. The frequency of pressure oscillation increased with the increase of steam inlet pressure and water Reynolds number, while decreased with the decrease of subcooled water temperature. The intensity of pressure oscillation exhibited a distinct peak at the position corresponding to the detachment of the primary and secondary bubbles. Moreover, a new predictive correlation for pressure oscillations in flowing water was presented by the frequency of steam injection, the condensation driving potential, the steam mass flux, the Weber number, and the water Reynolds number. The predicted values showed good agreement with the experimental values, with a maximum deviation of 24 %

Mixing enhancement in a straight microchannel with ultrasonically activated attached bubbles

Liquids and reagents mixing at the microscales are of great significance to various lab-on-chip applications such as drug delivery and microreactor. This work demonstrated mixing enhancement based on microstreaming induced by ultrasonically activated attached bubbles in a straight microchannel. The bubbles were experimentally trapped in a microchannel fabricated with hydrophobic surfaces, and they demonstrated good stability to flow rate due to the lower surface energy of the system. The effect of the applied voltage and flow rate on the mixing performance was numerically and experimentally studied. Both results show that reducing the flow rate or increasing the voltage would improve the mixing efficiency. Furthermore, the proof-of-concept was successfully tested by utilizing it as a microreactor for sodium hydroxide and phenolphthalein at a throughput of 4 μL/min. The proposed chip can achieve diverse mixing processes based on the trapped bubble state than other microfluidic mixers. We believe the proposed design will be valuable for developing bubble-based biochemical and biological applications.

Observation of hybrid magnetic skyrmion bubbles in Fe3Sn2 nanodisks

It has been widely established that there exist two types of magnetic bubbles. Here, the authors present the theory and experiment of a novel three-dimensional bubble state, referred to as the type-III bubble, which is composed of N\'eel-twisted skyrmions with topological charge $Q$ = -1 in the near-surface layers and type-II bubbles with $Q$ = 0 in the interior layers. Furthermore, the authors demonstrate a method for creating the type-III bubble and the topological magnetic transformation among the three types of bubbles.

Creation and erasure of polar bubble domains in PbTiO3 films by mechanical stress and light illuminations

The controllable manipulation of polar topological structures (e.g. skyrmion bubble) in ferroelectric materials have been considered as a cornerstone for future programmable nano-electronics. Here, we present the effective creation and erasure of polar bubble states PbTiO3 (PTO) multilayers trigged by mechanical stress and light illumination, respectively. It was found that applying atomic force microscope (AFM) tip force can induced formation of nanoscale bubble domains from the initial monodomain state. Moreover, the created bubble domain can be eliminated by exposure to ultraviolet or infrared light illumination. The above results can be understood by modulation of depolarization screening charges and bias fields, as reflected by scanning Kelvin potential microscopic (SKPM) observations, whereby the flexoelectric effect from the tip force tends to remove the screening charges on top surface and modulate the bias field that favors the formation of bubble state while light illumination tends to recover the screen charges and favor the monodomain state. The results provide a good example for multi-field manipulation of polar topologies, which might create a new avenue towards the immerging new concept electronic devices.

Underwater bubble plumes multi-scale morphological feature extraction and state recognition method

Underwater bubble plumes multi-scale morphological feature extraction and state recognition method

Synergistic effect of coating copper foam with carbon nanotubes on pool boiling heat transfer performance

Pool boiling heat transfer is a commonly used method for efficient phase change heat transfer. This paper studied the synergistic enhancement of heat transfer performance of pool boiling in deionized water by introducing an MWCNT (multi-walled nanotube) coating on a foam copper frame, and the influence of MWCNT content was explored. The results showed that the MWCNT coating strengthened the critical heat flux (CHF) and heat transfer coefficient (HTC) compared to a polished Cu surface. When 2.4 mg of MWCNTs were deposited, the maximum CHF was reached (785.2 kW·m−2), 3.41 times that of the polished Cu surface; and when 2.8 mg were deposited, the maximum HTC was reached (33.41 kW·m−2· k−1), 2.50 times that of the polished Cu surface. The results show that this heat transfer surface improves the performance of pool boiling heat transfer. A high-speed camera was used to record the bubble state during the boiling process of the pool. A simulation of the physical model in ANSYS Workbench returned the same results as the experiments.

Mixed bubbles in a one-dimensional Bose-Bose mixture

We investigate a Bose-Bose mixture across the miscible-immiscible phase transition governed by quantum fluctuations in one dimension. We find the recently predicted so-called mixed bubbles as ground states close to the mean-field miscible-immiscible threshold. These bubbles form a pocket of miscibility, separated by one of the components. The collective excitations reflect the symmetry breaking resulting from the bubble formation. The partial miscibility of the system allows for persistent currents in an annular confinement. Intriguingly, the mixed bubble acts like an intrinsic weak link, connecting the rotational behavior of the mixed bubble state to current efforts in atomtronic applications.

The unpredictable nature of bubble evolution

Unpredictable dynamics arising from a sensitivity to initial conditions is commonly associated with chaos. We demonstrate how similar unpredictability manifests in a nonlinear system that possesses a large number of long-term outcomes, namely the propagation of an air bubble within a viscous fluid-filled channel. The system under investigation supports various stable states of single-bubble propagation. In addition, bubbles can readily break up during their propagation. Upon subjecting steadily-propagating bubbles to finite-amplitude perturbations in the form of localised channel constrictions, we identify localised regions of the driving flow rate for which the resulting evolutions are unpredictable. Visibly-indistinguishable bubbles are observed to evolve towards a multitude of long-term outcomes, including each of the stable states available to the initial bubble and various states of permanently-changed bubble topology. By combining high-precision experimental results with simulations of a depth-averaged lubrication model of the system, we determine that this behaviour is driven by a sensitive dependence on initial conditions within the vicinity of an unstable periodic orbit.

Ultrafast Activation and Tuning of Topological Textures in Ferroelectric Nanostructures

AbstractPolar topological textures are recently discovered in ultra‐thin ferroelectrics, including stripe, labyrinth, or bubble states. Enabling to shape on‐demand, these topologies can provide an original platform for potential applications. Here, using first‐principle‐based calculations, it is shown that THz electric field pulses are an efficient means for generating a plethora of topological states in ultrathin films made by a prototypical ferroelectric system, depending on the characteristics of these pulses and films. Consequently, these ultrafast electrical pulses in ultra‐thin ferroelectrics offer key ingredients for mimicking neuronal and synaptic behaviors, including all‐optical devices, and open a new avenue for topological objects to neuromorphic systems.

Bubble Behaviors in a Dense Gas‐Solids Fluidized Bed: Analysis of Pressure Fluctuation

AbstractThe pressure signals were analyzed by time‐domain and frequency‐domain methods, including standard deviation, autocorrelation function, and power spectral density function, to investigate the bubble behavior and determine the regime transition. Results indicated that the autocorrelation function of pressure signals derived from bubbles was periodic fluctuation. The periodic nature of the autocorrelation function disappeared and the dominant frequency increased when the transition from bubbling to the turbulent regime occurred. The decrease of static bed height facilitated the explosion of bubbles, which accelerated the transition from the bubbling to the turbulent regime. The fluidization index was used as an indicator for the bubble state in the dense fluidized bed.

Control and tunability of magnetic bubble states in multilayers with strong perpendicular magnetic anisotropy at ambient conditions

The reversal of magnetic bubble helicity through topologically trivial transient states provides an additional degree of freedom that promises the development of multidimensional magnetic memories. A key requirement for this concept is the stabilization of bubble states at ambient conditions on application-compatible substrates. In the present work, we demonstrate a stabilization routine for remanent bubble states in high perpendicular magnetic anisotropy ${\text{[Co(0.44 nm)/Pt(0.7 nm)]}}_{X},$ $X=48$, 100, and 150 multilayers on $\mathrm{Si}/{\mathrm{SiO}}_{2}$ substrates by exploring the effect of external magnetic fields $({H}_{\mathrm{m}})$ of different strength and angles $(\ensuremath{\theta})$ with respect to the film surface normal. By systematic variation of these two parameters, we demonstrate that remanent bubble density and mean bubble diameter can be carefully tuned and optimized for each sample. Our protocol based on magnetometry only reveals the densest remanent bubble states at ${H}_{\mathrm{m}}=0.87{H}_{\mathrm{s}}$ (${H}_{\mathrm{s}}$ is the magnetic saturation field) and $\ensuremath{\theta}={60}^{\ensuremath{\circ}}$--${75}^{\ensuremath{\circ}}$ for all $X$ with a maximum of 3700 domains/100 $\ensuremath{\mu}{\mathrm{m}}^{2}$ for the $X=48$ sample. The experimental observations are supported by micromagnetic simulations, taking into account the nanoscale lateral grain structure of multilayers synthesized by magnetron sputter deposition, and thus helping to understand the different densities of the bubble states found in these systems.

Strain manipulation of ferroelectric skyrmion bubbles in a freestanding PbTiO3 film: A phase field simulation

So far nanoscale topological polar structures have been mainly observed in ferroelectric superlattices, nanodots, and complex heterostructures, originating from the intricate interplay of built-in electric field, polarization, strain, and their gradient-related energies. However, solid substrates hinder the continuous strain manipulation of the ferroelectric topological textures. Recently, a breakthrough in fabricating freestanding films has demonstrated the possibility of continuous strain manipulation, but there has been little experimental or theoretical investigation of whether polar topological structures exist in freestanding films. Herein, by performing phase field simulation on ${(\mathrm{PbTi}{\mathrm{O}}_{3})}_{20}/{(\mathrm{SrTi}{\mathrm{O}}_{3})}_{10}$ freestanding bilayers, we observed the stabilized ferroelectric skyrmion bubbles in a ferroelectric layer. A thickness-dependent phase diagram indicates that the skyrmion bubbles exist when the ferroelectric layer is between eight and 30 unit cells. Meanwhile, we demonstrated that the uniaxial strain is an effective way to manipulate the skyrmion bubble in freestanding films, which not only induces consolidation and rearrangement of skyrmion bubbles, but also induces a phase transition from the topological skyrmion bubble state to a standard ${a}_{1}/{a}_{2}$ domain state. These results provide us guidance with a mechanical approach to control topological polar structures in freestanding films.