Bubble Center Research Articles

Processing and decay of 6S-1 and 6S-2 RNAs in Bacillus subtilis.

Noncoding 6S RNAs regulate transcription by binding to the active site of bacterial RNA polymerase holoenzymes. Processing and decay of 6S-1 and 6S-2 RNA were investigated in Bacillus subtilis by northern blot and RNA-seq analyses using different RNase knockout strains, as well as by in vitro processing assays. For both 6S RNA paralogs, we identified a key-but mechanistically different-role of RNase J1. RNase J1 catalyzes 5'-end maturation of 6S-1 RNA, yet relatively inefficient and possibly via the enzyme's "sliding endonuclease" activity. 5'-end maturation has no detectable effect on 6S-1 RNA function, but rather regulates its decay: The generated 5'-monophosphate on matured 6S-1 RNA propels endonucleolytic cleavage in its apical loop region. The major 6S-2 RNA degradation pathway is initiated by endonucleolytic cleavage in the 5'-central bubble to trigger 5'-to-3'-exoribonucleolytic degradation of the downstream fragment by RNase J1. The four 3'-exonucleases of B. subtilis-RNase R, PNPase, YhaM, and particularly RNase PH-are involved in 3'-end trimming of both 6S RNAs, degradation of 6S-1 RNA fragments, and decay of abortive transcripts (so-called product RNAs, ∼14 nt in length) synthesized on 6S-1 RNA during outgrowth from stationary phase. In the case of the growth-retarded RNase Y deletion strain, we were unable to infer a specific role of RNase Y in 6S RNA decay. Yet, a participation of RNase Y in 6S RNA decay still remains possible, as evidence for such a function may have been obscured by overlapping substrate specificities of RNase Y, RNase J1, and RNase J2.

Effects of pressure and particle size on bubble behaviors in a pseudo 2D pressured fluidized bed with Geldart A/B, B and D particles

Bubble behaviors of three types of particles from Geldart A/B, B and D groups were comprehensively investigated in a pseudo 2D pressurized fluidized bed via digital image analysis. Based on the analysis of a large quantity of experimental data, the effects of pressure and particle size on the spatial distribution of bubbles, bubble diameter, rise velocity, bubble fraction and flow regimes were obtained. Results show that bubbles are smaller and uniformly dispersed throughout the bed of Geldart A/B particles, while they become densely populated and grow into much larger sizes in the bed of Geldart B particles. A single peak of large bubbles in the center and emptying of bubble near the walls is formed in the bed of Geldart D particles due to the preferable rising path of bubbles towards the center and the significant coalescence of bubbles. Bubble diameter decreases with increasing pressure, and the effect of pressure is more pronounced on Geldart D particles. Bubble rise velocity increases with the increasing particle size and decreases as pressure rises. Large bubbles/slugs hardly appear in the bed of Geldart A/B particles, while they are clearly visible in the bed of Geldart B and D particles. The flow regime transition from slugging to fast fluidization shifts to the lower gas velocity for Geldart D particles compared with Geldart B particles.

壁面近傍での気泡崩壊時の二方向同時撮影画像と衝撃圧分布との関係

The pressure measurement using a fiber optic prove hydrophone and simultaneous two-directional observation with a high-speed video camera have been conducted to investigate the collapse of a laser-induced bubble near a rigid wall and the corresponding shockwave generation. Pressure measurements at multiple locations near the rigid wall and observations of frontal and overhead views relate to the locations of pitting damage due to bubble collapse observed by Philipp and Lauterborn (J. Fluid Mech., 1998). In γ=s/Rmax≅1.5 where s is the distance between the bubble center and the wall surface and Rmax is the equivalent bubble radius at the maximum growth, the second bubble collapse occurs on the rigid wall: the overhead view reveals that the bubble collapses asymmetrically with a toroidal (ring) shape and bubble blobs remain at the bubble center and on both sides of the ring in the direction of laser injection. It is shown that the area of the bubble blobs on the ring correlates with the high pressure on the wall. This suggests that the collapse of these blobs leads to the high pressures and asymmetric pitting damages on the wall.

Interactions between a central bubble and a surrounding bubble cluster

The interaction of multiple bubbles is a complex physical problem. A simplified case of multiple bubbles is studied theoretically with a bubble located at the center of a circular bubble cluster. All bubbles in the cluster are equally spaced and own the same initial conditions as the central bubble. The unified theory for bubble dynamics [35] is applied to model the interaction between the central bubble and the circular bubble cluster. To account for the effect of the propagation time of pressure waves, the emission source of the wave is obtained by interpolating the physical information on the time axis. An underwater explosion experiment with two bubbles of different scales is used to validate the theoretical model. The effect of the bubble cluster with a variation in scale on the pulsation characteristics of the central bubble is studied.

Discovery of a New Molecular Bubble–Outflow Structure in the Taurus B18 Cloud

Star formation can produce bubbles and outflows, as a result of stellar feedback. Outflows and bubbles inject momentum and energy into the surrounding interstellar medium, and so are related to the overall energy balance of the molecular cloud. Molecular bubbles can be resolved by higher-resolution radio telescopes to quantify the effect of star formation on molecular clouds. We report here the identification of a new molecular bubble with an outflow, and a Herbig–Haro object, HH 319, located at the bubble center. Multiwavelength data have been utilized to study its spatial structure, energy injection, and dynamical timescale. This bubble has a kinetic energy of 5.8 × 1043 erg within the smallest radius of a bubble in Taurus, 0.077 pc. The bubble formed ∼70,000 yr ago. According to the proper-motion velocities of protostars from Gaia EDR3, the T Tauri binary stars (FY Tau and FZ Tau) at the southwest edge of the bubble may have produced the outflow–bubble structure. This is an unusual new structure found in low- and intermediate-mass star formation regions. Only a bubble in Orion A, driven by V380 Ori, has a similar structure. The bubble–outflow structure provides additional observational evidence for the theory of stellar wind from T Tauri stars. It enhances our understanding of how stellar feedback acts on molecular clouds.

The messy death of a multiple star system and the resulting planetary nebula as observed by JWST

Planetary nebulae—the ejected envelopes of red giant stars—provide us with a history of the last, mass-losing phases of 90% of stars initially more massive than the Sun. Here we analyse images of the planetary nebula NGC 3132 from the James Webb Space Telescope (JWST) Early Release Observations. A structured, extended hydrogen halo surrounding an ionized central bubble is imprinted with spiral structures, probably shaped by a low-mass companion orbiting the central star at about 40–60 au. The images also reveal a mid-infrared excess at the central star, interpreted as a dusty disk, which is indicative of an interaction with another closer companion. Including the previously known A-type visual companion, the progenitor of the NGC 3132 planetary nebula must have been at least a stellar quartet. The JWST images allow us to generate a model of the illumination, ionization and hydrodynamics of the molecular halo, demonstrating the power of JWST to investigate complex stellar outflows. Furthermore, new measurements of the A-type visual companion allow us to derive the value for the mass of the progenitor of a central star with excellent precision: 2.86 ± 0.06 M⊙. These results serve as pathfinders for future JWST observations of planetary nebulae, providing unique insight into fundamental astrophysical processes including colliding winds and binary star interactions, with implications for supernovae and gravitational-wave systems. JWST observations of the planetary nebula NGC 3132 reveal a rich molecular environment containing evidence of multiple stellar interactions on a variety of scales.

Numerical Investigation of Vortex Shedding from a 5:1 Rectangular Cylinder at Different Angles of Attack

Although flow around a 5:1 rectangular cylinder at small angles of attack (AoA) has been extensively studied, when the AoA becomes larger, the research is rare. Therefore, this study performs Unsteady Reynolds-Averaged Navier-Stokes simulations (URANS) using the k-ω SST turbulence model for unsteady flow around a two-dimensional 5:1 rectangular cylinder at different AoAs up to 45°. A strong dependence of the flow characteristics on AoA is observed through the analysis of the time-averaged lift coefficient, drag coefficient, and Strouhal number. The peak of lift and drag coefficient is observed to be correlated, respectively, to the leading- and trailing-edge vortex based on the analysis of the flow. The x′-directional length of the main recirculation bubble on the top side and the distance from the bubble center to the leading edge of the cylinder both reach the maximum when α = 15°. In addition, the standard deviation σx′ of the time-averaged velocity U¯x′ along the cylinder shows a trend of increasing at first and then decreasing, and that σy′ also shows the same trend at α = 20°~45°; it fluctuates within a range of 0.05~0.2 at α = 0°~20°. Finally, two principal modes of vortex shedding are observed with α = 15° being their turning point, (i) “1 + 1” mode: in one vortex shedding period, two major vortices shed off from the top and bottom sides of the cylinder at α ≤ 15°; (ii) “2 + 2” mode, four vortices shed off from the top and bottom sides of the cylinder at α > 15° in one vortex shedding period.

Determining collision efficiency in multi-bubble-particle systems in presence of turbulence

Bubble-particle collision efficiency in the existing flotation literature is determined in a single bubble-particle system assuming an ideal flow field in absence of any turbulence which does not fully represent reality of the actual flotation systems. To address this knowledge gap, in this study, we numerically computed collision efficiency in a multi-bubble-particle system (particle diameter dP = 30 µm, bubble diameter dB = 1 mm) using a two-way coupled Eulerian-Lagrangian 3D computational fluid dynamics model with different solids concentrations (0.385 %–3.080 %) at turbulence intensities (Ti = 0 %–20 %). Collision efficiency of multiple particles with a single central bubble was determined in presence of different configurations of neighbouring bubbles. Two simplified multi-bubble configurations – monolayer and multilayer comprising different number of bubbles/layer, were investigated. It was noted that side-by-side collisions in monolayer configuration did not occur because particles did not have sufficient time and kinetic energy to migrate to a surrounding bubble. On contrary, in multilayer configurations, particles traversing around a leading bubble had a chance to get entrained in the rear wake and collide with a trailing bubble which resulted in higher collision efficiency compared to monolayer configurations. Modelling showed that bulk energy dissipation rate increased with number of bubbles per layer, number of bubble layers, turbulence intensity and solids concentration. For the lowest and highest solids concentration, the optimal collision efficiency occurred at Ti = 20 %, while for the intermediate solid concentration, it occurred at Ti between 4 % and 11 %. In all cases, collision efficiency increased with the energy dissipation rate.

Dynamic Response of Nanoscale He Bubbles in Single Crystal Al during Release from a High-Pressure State

Previous researches have presented some knowledge about the shock loading and unloading of the nanoscale He bubble. However, the He bubble will undergo a long high-pressure adiabatic relaxation process after being shocked. This work focuses on the release path of the nanoscale He bubble in single crystal Al from a stable high-pressure state by molecular dynamics method. Firstly, we consider the case that two opposite release waves meet at the center of the He bubble. Combined with the analysis of deformation mechanism and stress waveform around the He bubble, the difference of evolution law of He bubble under different pressure conditions is revealed. The evolution of the number and distribution of voids with or without the He bubble is compared. And the nucleation region gradually extends to both sides of the He bubble with the increase of initial Hugoniot pressure. Moreover, when a single unloading wave is considered to sweep through the He bubble, the microjet formation in the He bubble is found due to the pressure gradient effect. The shape and velocity distribution of internal and surface jets are discussed.

The left-right symmetrical and asymmetrical deformations in a three-bubble system

This paper studies the simplest system that can possess left-right symmetrical and asymmetrical surroundings, three bubbles in a line. Assuming that the deformations are small, the surfaces of bubbles are described by a combination of the first three Legendre polynomials, that is, spherical symmetrical mode P0, L-R antisymmetrical mode P1, and symmetrical mode P2. A dynamical model is built to describe aspherical oscillations of central and two side bubbles. It is found that when three identical bubbles are separated uniformly, the central bubble only has a P2 component and P1 component tends to zero, while two side bubbles have both P1 and P2 components. When three identical bubbles are separated by different distances, they can be degenerated into a two-bubble system and a free bubble. The bubble deformations contain both P1 and P2 components in the two-bubble system, while both aspherical components P1 and P2 of the free bubble tend to zero. If side bubbles are different in ambient radii but located symmetrically on the left and right of the central bubble, the side bubble pulsated more strongly plays an important role on the deformation of the central one.

Cavitation bubble collapse and rebound in water: Influence of phase transitions

The influence of the mass transfer across the bubble surface due to evaporation and condensation on the cavitation (vapor) bubble collapse and rebound in water under room conditions has been studied. The bubble is spherical with an initial radius of 1.92mm. The dynamics of the vapor in the bubble and the surrounding liquid is governed by the gas dynamics equations. The effects of the liquid viscosity and heat conductivity of both fluids are taken into account. The liquid and vapor states are mainly described by the known wide-range equations by Nigmatulin and Bolotnova. The mass transfer is governed by the accommodation coefficient αac in the Hertz–Knudsen–Langmuir formula. It has been found that at αac in the range 0.001−0.075 the vapor in the bubble is homobaric at collapse. As αac increases in the range αac>0.075, an isentropic compression wave convergent to the bubble center is formed in the bubble. At focusing of this wave at the bubble center, a divergent (reflected from the center) isentropic compression wave occurs. With rising αac the intensities of these convergent and divergent waves increase. The divergent wave partially penetrates to the liquid in the form of an isentropic divergent wave. With growing αac the penetrated wave becomes steeper. At αac≈0.25 the divergent isentropic wave in the bubble transforms into a shock wave during its propagation to the bubble surface. This shock wave partially penetrates to the liquid in the form of a divergent shock wave. At small αac the outgoing pulse in liquid is shockless. Starting with αac≈0.03, it becomes discontinuous. In the range 0.03≤αac≤0.12 the shock pulse results from only large pressure gradients in the liquid in the vicinity of the bubble at the beginning of rebound. In the range 0.12<αac<0.25 the discontinuous outgoing pulse is formed from the divergent isentropic compression wave arisen in liquid as a result of partial escape of the isentropic wave out of the bubble. At αac>0.25 the shock pulse is formed directly on the bubbles surface as a result of the partial penetration of the divergent shock wave from the bubble to liquid. The obtained numerical results are in good agreement with available numerical and experimental data.

Collapse of a bubble close to a solid boundary

A bubble equation of motion close to the solid plane is obtained by using the perturbation method. This bubble equation can explain the phenomena of the reentrant microjet penetrating the bubble produced by the external disturbances if the distance between the bubble center to the solid boundary is small enough as well as the external pressure is large enough. Furthermore, two critical points are found. One critical point is the distance between the bubble center to the solid boundary. The other is external pressure. The critical boundary at which the reentrant microjet can just be produced is given. The critical boundary depends on both the distance between the bubble center to the solid boundary and the ratio of external pressure to the initial pressure of the liquid.

Deformation of Impacting Interface of Electrically Charged Drops on Solid Surfaces

Predicting the dynamics of an electrically charged drop impacting a solid surface is of great importance in various industrial applications such as spray coating and painting. An impacting drop decelerates immediately before the touch-down by the lubrication pressure of the intervening air. The pressure build-up forms a dimple resulting in the entrapment of bubbles under the drop. We measured and visualized the dimple and bubble formations using high-speed interferometry and back-light imaging techniques by focusing on the effect of the electrostatic charge on the surface deformation. We clarified that the dimple size decreased, and the kink of the dimple became more flattened with increasing the charge, which resulted in forming a micro-bubble band around a central bubble. By comparing the dimple profile obtained with interferometry and bubble images, we revealed that an air layer, thinner than 266 nm, raptures into small pieces forming a microbubble band while a thicker air layer collapses into a bubble. Replacing a substrate from conductive to dielectric hinders the decrease in dimple volume, confirming Maxwell stress acting between the drop bottom surface and the substrate plays a crucial role in determining the bubble entrapment.

Reaching the bubble may not be enough: news media role in online political polarization

Politics in different countries show diverse degrees of polarization, which tends to be stronger on social media, given how easy it became to connect and engage with like-minded individuals on the web. A way of reducing polarization would be by distributing cross-partisan news among individuals with distinct political orientations, i.e., “reaching the bubbles”. This study investigates whether this holds in the context of nationwide elections in Brazil and Canada. We collected politics-related tweets shared during the 2018 Brazilian presidential election and the 2019 Canadian federal election. Next, we proposed an updated centrality metric that enables identifying highly central bubble reachers, nodes that can distribute content among users with diverging political opinions—a fundamental metric for the proposed study. After that, we analyzed how users engage with news content shared by bubble reachers, its source, and its topics, considering its political orientation. Among other results, we found that, even though news media disseminate content that interests different sides of the political spectrum, users tend to engage considerably more with content that aligns with their political orientation, regardless of the topic.

Bubble growth in water vortex unit and its effect on suction force

The vortex unit is a non-contact negative pressure generating mechanism. It blows high-pressure fluid tangentially into a cylindrical chamber through several nozzles to form a swirling flow and relies on the centrifugal inertial effect of the swirling flow to generate negative pressure and suction force. To date, all related studies and applications have used compressed air as the working fluid. Because the density of water is much greater than that of air, in this study, water was used as the working fluid to greatly improve the centrifugal inertial effect of the swirling flow. Based on this innovative idea, the swirling flow of the vortex unit with water as the working fluid was investigated through experiments. A convergence-growth-stabilization phenomenon of bubbles was discovered, and the effect of the converged central bubble on the central pressure and suction force was elucidated. The mechanism of the gap height and flow rate on the converged central bubble and suction force was also studied. The reason for the formation of the converged central bubble is that the negative pressure causes the gas to evolve out of the water, and the dispersed tiny bubbles move to the center of the vortex chamber under the influence of centrifugal inertia to form a large bubble. This study provides new research directions and usages of the vortex unit and provides important basic knowledge for further research.

Analysis of doublet bubble dynamics near a rigid wall in ferroparticle nanofluids

This study aims to characterize the interaction of doublet bubbles growing near a solid wall in ferroparticle nanofluids. Accordingly, we analyzed the behavior of spherical bubbles near a rigid wall considering liquid compressibility of suspended ferro-nanoparticles. In addition to the interaction effect between two bubbles, we considered the rigid wall forces and thermophysical configurations of the particles in nanofluids to understand the bubble growth. The equation of motion of the doublet bubbles was formulated based on the continuity equation, Euler equation, wave equation, and thermophysical configurations of the particles in nanofluids. Subsequently, the developed model was analytically solved by modifying the Plesset–Zwick technique. Throughout the bubble growth, we examined the thermal effects on the cavitation bubble dynamics, such as the distance between the interacting bubbles, vertical distance between the bubble center and rigid wall, and ferro-nanoparticle volume concentration. The analysis results revealed that the bubble growth was directly proportional to the Jacob number and thermal diffusivity, and inversely proportional to the distance between the boundary rigid wall and bubble center. Furthermore, it can be deduced from the results that the presence of the wall significantly influenced the bubble growth and made a significant deviation between the moving velocities of the interfaces both near and far from the wall. Ultimately, the phenomenon was physically interpreted based on the theory of bubble dynamics, and the water pressure induced by the bubble growth was estimated. The results were well aligned with the theoretical and experimental results from previous studies.

The initial impact of drops cushioned by an air or vapour layer with applications to the dynamic Leidenfrost regime

This work is devoted to the study of the conditions under which a drop directed normally towards a superheated or isothermal smooth substrate prevents the initial contact with the solid by skating over a micrometre-sized vapour or air layer. The results have been obtained analysing the gas flow at the spatio-temporal region where the maximum liquid pressure is attained, which is also where and when the minimum values of the film thickness are reached. For the common case in which $We St^{-1/6}\gtrsim 1$ , where $We=\rho _l U^{2} R/\gamma$ and $St=\rho _l U R/\eta _a$ denote, respectively, the Weber and Stokes numbers, we find that capillary effects are negligible and the ratio between the minimum film thickness and the local drop radius of curvature is $h_m/R\propto St^{-7/6}$ , with $\rho _l$ , $\gamma$ , $\eta _a$ , $U$ and $R$ indicating the liquid density, interfacial tension coefficient, gas viscosity, impact velocity and drop radius, respectively. In contrast, when $We St^{-1/6}\lesssim 1$ , capillary effects can no longer be neglected and $h_m/R\propto We^{-1/3} St^{-10/9}$ . The predicted values of the minimum film thickness are compared with published experimental data, finding good agreement between predictions and measurements for the cases of both isothermal and superheated substrates. In addition, using mass conservation, we have also deduced an equation providing the minimum value of the substrate temperature for which a cylindrical central vapour bubble of constant height $h_d/R\propto St^{-2/3}$ , with $h_d\gg h_m$ , grows radially at the wetting velocity deduced in Riboux &amp; Gordillo (Phys. Rev. Lett., vol. 113, 2014, 024507). The predicted values are in good agreement with the dynamic Leidenfrost temperatures reported by Shirota et al. (Phys. Rev. Lett., vol. 116, 2016, 064501).

Characterization of jet parameters related to cavitation bubble dynamics in a vicinity of a flat liquid-liquid interface

Presented study analyses different jet parameters that appear during and after bubble collapse, namely bubble center displacement, length of a jet-pierced bubble and size of the rebounded bubble and their relation to anisotropy parameter in a vicinity of a liquid–liquid interface. In experiments cavitation bubble was produced with a focused pulsed laser and its dynamics was recorded by a high-speed camera. Two different types of liquid-liquid interfaces consisting of distilled water and two types of silicone oil were used for the study in order to be able to compare the influence of anisotropy parameter on the magnitude of considered jet parameters. The elongation of the cavitation bubble during jet protrusion was analysed and compared for both interface types. The study exhibits systemic approach to jet parameter consideration and presents an important step in understanding the behaviour of cavitation bubble near liquid-liquid interfaces.

Pulsating bubbles dynamics near a concave surface

Bubble dynamics near different boundaries have always been a hot topic in ocean engineering and many other fields. In the present study, we investigate the dynamics of high-pressure pulsating bubbles near a concave surface via experimental and numerical methods. Pulsating cavitation bubbles are generated by the electric discharge technique, and the violent expansion, contraction, and jet formation are observed through high-speed photography. Additionally, a multi-vortex ring model implemented in the boundary integral method is established to simulate the bubble motion near a curved surface. The precise non-spherical bubble shapes during bubble pulsation and detailed information on the jet velocity and wall pressure are obtained. The numerical results are found to be in good agreement with the experimental results, indicating the validity of the numerical model. Finally, a quantitative study is provided on the influence of two parameters: the distance parameter γ=L/Rm (L is the distance between the initial bubble center and the lowest point of the concave surface and Rm is the maximum bubble radius) and the concave curvature ξ=Rm/Rs (Rs is the radius of concave boundary).

CFD–DEM simulations of wet particles fluidization with a new evolution model for liquid bridge

AbstractA new model for liquid‐bridge evolution with consideration of particle dynamics, is proposed to improve Computational Fluid Dynamics‐Discrete Element Method (CFD–DEM) simulations of wet particles fluidization under high liquid loading and viscosity. A liquid bridge is allowed to form and remains stable only when the normal relative velocity of two particles is lower than a critical value vnc. A large vnc leads to an increase of liquid‐bridge or cohesive force. The model can be reduced to the conventional liquid‐bridge model in literature when vnc = 0 or ∞. With the new model, the prediction of bubble properties including bubble center, aspect ratio, and volume agrees well with the experimental data in literature. In particular, under high liquid loading, bubble disintegration due to particle agglomerating is reasonably captured. The simulations demonstrate the advantage of the new model that can extend the liquid‐bridge models and CFD–DEM for high liquid loading and viscosity.