Bubble Edge Research Articles

Temperature-dependent interface characteristic of silicon wafer bonding based on an amorphous germanium layer deposited by DC-magnetron sputtering

We report a near-bubble-free low-temperature silicon (Si) wafer bonding with a thin amorphous Ge (a-Ge) intermediate layer. The DC-magnetron-sputtered a-Ge film on Si is demonstrated to be extremely flat (RMS=0.28nm) and hydrophilic (contact angle=3°). The effect of the post-annealing temperature on the surface morphology and crystallinity of a-Ge film at the bonded interface is systematically identified. The relationship among the bubble density, annealing temperature, and crystallinity of a-Ge film is also clearly clarified. The crystallization of a-Ge film firstly appears at the bubble region. More interesting feature is that the crystallization starts from the center of the bubbles and sprawls to the bubble edge gradually. The H2 by-product is finally absorbed by intermediate Ge layer with crystalline phase after post annealing. Moreover, the whole a-Ge film out of the bubble totally crystallizes when the annealing time increases. This Ge integration at the bubble region leads to the decrease of the bubble density, which in turn increases the bonding strength.

Investigation on the effect of geometrical parameters on the performance of a venturi type bubble generator

The venturi type bubble generator adopted in the fission gas removal system for the liquid-fuel thorium molten salt reactor plays a crucial role in strengthening the mass transfer rate. Aiming at clarifying how these three main geometrical parameters, including the injection hole diameter, injection hole number, and divergent angle, influence the bubble size distributions, an experimental study is carried out. The bubble size distribution is extracted from the images captured by the high speed visualization. Post processing algorithm is developed to detect the bubble edges and compute their Sauter mean diameters for each flow condition. By changing the value of each geometrical parameter, the variation of the bubble size distribution is obtained and discussed. It is shown that the injection hole diameter and number basically have little effect on the bubble size distribution while the divergent angle is proved to be sensitive. Finally, the reasons that account for the bubble size variation with three geometrical parameters are explained by the bubble turbulence breakup theory.

Measuring Interlayer Shear Stress in Bilayer Graphene.

Monolayer two-dimensional (2D) crystals exhibit a host of intriguing properties, but the most exciting applications may come from stacking them into multilayer structures. Interlayer and interfacial shear interactions could play a crucial role in the performance and reliability of these applications, but little is known about the key parameters controlling shear deformation across the layers and interfaces between 2D materials. Herein, we report the first measurement of the interlayer shear stress of bilayer graphene based on pressurized microscale bubble loading devices. We demonstrate continuous growth of an interlayer shear zone outside the bubble edge and extract an interlayer shear stress of 40kPa based on a membrane analysis for bilayer graphene bubbles. Meanwhile, a much higher interfacial shear stress of 1.64MPa was determined for monolayer graphene on a silicon oxide substrate. Our results not only provide insights into the interfacial shear responses of the thinnest structures possible, but also establish an experimental method for characterizing the fundamental interlayer shear properties of the emerging 2D materials for potential applications in multilayer systems.

Flotation Bubble Delineation Based on Harris Corner Detection and Local Gray Value Minima

Froth image segmentation is an important and basic part in an online froth monitoring system in mineral processing. The fast and accurate bubble delineation in a froth image is significant for the subsequent froth surface characterization. This paper proposes a froth image segmentation method combining image classification and image segmentation. In the method, an improved Harris corner detection algorithm is applied to classify froth images first. Then, for each class, the images are segmented by automatically choosing the corresponding parameters for identifying bubble edge points through extracting the local gray value minima. Finally, on the basis of the edge points, the bubbles are delineated by using a number of post-processing functions. Compared with the widely used Watershed algorithm and others for a number of lead zinc froth images in a flotation plant, the new method (algorithm) can alleviate the over-segmentation problem effectively. The experimental results show that the new method can produce good bubble delineation results automatically. In addition, its processing speed can also meet the online measurement requirements.

FERMIBUBBLES INFLATED BY WINDS LAUNCHED FROM THE HOT ACCRETION FLOW IN SGR A*

A pair of giant gamma-ray bubbles have been revealed by the {\it Fermi} LAT. In this paper we investigate their formation mechanism. Observations have indicated that the activity of the supermassive black hole located at the Galactic center, Sgr A*, was much stronger than the present time. Specifically, one possibility is that while Sgr A* was also in the hot accretion regime, the accretion rate should be $10^3-10^4$ times higher during the past $\sim 10^7$ yr. On the other hand, recent MHD numerical simulations of hot accretion flows have unambiguously shown the existence of strong winds and obtained their properties. Based on these knowledge, by performing three-dimensional hydrodynamical simulations, we show in this paper that the Fermi bubbles could be inflated by winds launched from the ``past' hot accretion flow in Sgr A*. In our model, the active phase of Sgr A* is required to last for about 10 million years and it was quenched no more than 0.2 million years ago. The Central Molecular Zone (CMZ) is included and it collimates the wind orientation towards the Galactic poles. Viscosity suppresses the Rayleigh-Taylor and Kelvin-Helmholtz instabilities and results in the smoothness of the bubble edge. The main observational features of the bubbles can be well explained. Specifically, the {\it ROSAT} X-ray features are interpreted by the shocked interstellar medium and the interaction region between winds and CMZ gas. The thermal pressure and temperature obtained in our model are in good consistency with the recent {\it Suzaku} observations.

3D reconstruction of single rising bubble in water using digital image processing and characteristic matrix

Reconstructing the shape of a bubble will lay a firm foundation for further description of the dynamic characteristics of bubbly flow, especially for a single rising bubble or separate bubbles whose interaction could be neglected. In this case, the rising bubble is usually simulated as an ellipsoid consisting of two semi-ellipsoids up and down. Thus the projected image of a bubble consists of two semi-ellipses. In this paper, a method for reconstructing the ellipsoid bubble model is described following digital image processing, using the Hough transform in 2D ellipse parameter extraction which could cover most of the bubble edge points in the image. Then a method based on characteristic symmetric matrix is described to detect 3D bubble ellipsoid model parameters from 2D ellipse parameters of projection planes. This method can be applied to bubbles rising with low-velocity in static flow field much in conformity with the projection theory and the shape variation of the rising bubble. This method does not need to solve nonlinear equation sets and provides an easy way to calculate the characteristic matrix of a space ellipsoid model for deformed bubble. For bubble application, two assumed conditions and a calibration factor are proposed to simplify calculation and detection. Errors of ellipsoid center and three axes are minor. Errors of the three rotation angles have no negative effect on further study on bubbly flow.

Exploring the extreme universe with the Fermi Gamma-Ray Space Telescope

The Fermi orbiter, mapping the entire gamma-ray sky every three hours, monitors the cosmos for high-energy phenomena both fleeting and enduring.

THEFERMIBUBBLES. II. THE POTENTIAL ROLES OF VISCOSITY AND COSMIC-RAY DIFFUSION IN JET MODELS

The origin of the Fermi bubbles recently detected by the Fermi Gamma-ray Space Telescope in the inner Galaxy is mysterious. In the companion paper Guo & Mathews (Paper I), we use hydrodynamic simulations to show that they could be produced by a recent powerful AGN jet event. Here we further explore this scenario to study the potential roles of shear viscosity and cosmic ray (CR) diffusion on the morphology and CR distribution of the bubbles. We show that even a relatively low level of viscosity (\mu_{visc} >~ 3 g cm^{-1} s^{-1}, or ~0.1% - 1% of Braginskii viscosity in this context) could effectively suppress the development of Kelvin-Helmholtz instabilities at the bubble surface, resulting in smooth bubble edges as observed. Furthermore, viscosity reduces circulating motions within the bubbles, which would otherwise mix the CR-carrying jet backflow near bubble edges with the bubble interior. Thus viscosity naturally produces an edge-favored CR distribution, an important ingredient to produce the observed flat gamma-ray surface brightness distribution. Generically, such a CR distribution often produces a limb-brightened gamma-ray intensity distribution. However, we show that by incorporating CR diffusion which is strongly suppressed across the bubble surface (as inferred from sharp bubble edges) but is close to canonical values in the bubble interior, we obtain a reasonably flat gamma-ray intensity profile. The similarity of the resulting CR bubble with the observed Fermi bubbles strengthens our previous result in Paper I that the Fermi bubbles were produced by a recent AGN jet event. Studies of the nearby Fermi bubbles may provide a unique opportunity to study the potential roles of plasma viscosity and CR diffusion on the evolution of AGN jets and bubbles.

Dynamics of bubble formation and detachment from an immersed micro-orifice on a plate

Visualization experiments were carried out in the present study to investigate formation and detachment of the bubbles developing from an immersed micro-orifice on a plate in a stagnant and isothermal liquid. A sub-fitting method was proposed to describe the bubble edges and to extrude the bubble characteristics. Taking the microscale effect into consideration, the dynamic behavior of bubbles emerging from various orifices with 0.5, 0.12 and 0.054mm in diameter was analyzed and compared. The experimental results showed that the bubble waiting time, departure time and departure volume decreased with decreasing orifice diameter. Based on the analysis on actual gas flow rate at the orifice, the evolution of bubble formation process was described by three or four stages for different orifices. The bubble formation under the condition of 0.5mm orifice mainly experienced the nucleation stage, while the steady growth stage was the dominator for the micro-orifices with 0.12 and 0.054mm in diameter. The rupture scene and evolution of bubble contact ring at the detachment moment were found to significantly vary with the orifice diameter. The inrush of several trailing bubbles into the detached leading bubble was observed for the orifices with 0.12 and 0.054mm in diameter, resulting in a significant fluctuation in the interface.

MAGNETIC SUBSTRUCTURE IN THE NORTHERN FERMI BUBBLE REVEALED BY POLARIZED MICROWAVE EMISSION

We report a correspondence between giant, polarized microwave structures emerging north from the Galactic plane near the Galactic center and a number of GeV gamma-ray features, including the eastern edge of the recently-discovered northern Fermi Bubble. The polarized microwave features also correspond to structures seen in the all-sky 408 MHz total intensity data, including the Galactic center spur. The magnetic field structure revealed by the polarization data at 23 GHz suggests that neither the emission coincident with the Bubble edge nor the Galactic center spur are likely to be features of the local ISM. On the basis of the observed morphological correspondences, similar inferred spectra, and the similar energetics of all sources, we suggest a direct connection between the Galactic center spur and the northern Fermi Bubble.

The generation of azimuthal magnetic field in laser-induced plasma bubbles

A two-dimensional (2D) analytical model for a laser-induced bubble is rebuilt by taking into account the generation of azimuthal magnetic field. It is shown that the azimuthal magnetic field surrounds the bubble transversely, whose amplitude peaks at the bubble edge and vanishes at laser axis. In a laser wakefield acceleration scheme, the longitudinal electric field accelerates the electrons injected from the rear of the bubble, while the azimuthal magnetic field makes the resulting energetic-electron beam converge. The analytical model is justified by a 2D Particle in Cell (PIC) simulation.

Filamentary Alfvénic structures excited at the edges of equatorial plasma bubbles

Abstract. Recent observations performed by the French DEMETER satellite at altitudes of about 710 km suggest that the generation of equatorial plasma bubbles correlates with the presence of filamentary structures of field aligned currents carried by Alfvén waves. These localized structures are located at the bubble edges. We study the dynamics of the equatorial plasma bubbles, taking into account that their motion is dictated by gravity driven and displacement currents. Ion-polarization currents appear to be crucial for the accurate description of the evolution of plasma bubbles in the high altitude ionosphere. During their eastward/westward motion the bubbles intersect gravity driven currents flowing transversely with respect to the background magnetic field. The circulation of these currents is prohibited by large density depressions located at the bubble edges acting as perfect insulators. As a result, in these localized regions the transverse currents have to be locally closed by field aligned currents. Such a physical process generates kinetic Alfvén waves which appear to be stationary in the plasma bubble reference frame. Using a two-dimensional model and "in situ" wave measurements on board the DEMETER spacecraft, we give estimates for the magnitude of the field aligned currents and the associated Alfvén fields.

Equatorial bubbles as observed with GPS measurements over Pune, India

Ionospheric total electron content (TEC) and scintillations have been recorded continuously since April 2003 using a dual‐frequency GPS receiver at Pune, India (geographic latitude 19.1°N, longitude 74.05°E; 24°N dip), situated in between the magnetic equator and the northern crest of the equatorial anomaly. The TEC often shows bite‐outs when severe amplitude scintillations are observed on the GPS L1 carrier level. The apparent duration of the bite‐outs may be different from the true east‐west duration, as observed with geostationary links, because of the presence of a relative velocity between the irregularity cloud and the satellite. The trajectory of a GPS satellite plays an important role in observing the bubble characteristics. The distributions of amplitude and duration of the bubbles have been obtained during the equinoctial months February through April of 2004. The median values are found to be 9 TEC units (1 TECU = 1016 el/m2) and 3.3 min, respectively. The range error at GPS L1 frequency corresponding to the median TEC depletion is 1.4 m, while that corresponding to the 95th percentile value is 4.5 m. An asymmetry in the east‐west walls of the bubble and sharp edges of the depletions resulting in high range error rates ∼30 cm/min has been noted.

On the Shear Modulus of Two-Dimensional Liquid Foams: A Theoretical Study of the Effect of Geometrical Disorder

The shear modulus of two-dimensional liquid foams in the dry limit of low liquid content has been studied theoretically. The focus is on the effect of geometrical disorder on the shear modulus (besides the influence of surface tension). Various theoretical predictions are formulated that are all based on the assumptions of isotropic geometrical characteristics, incompressible bubbles, and negligible edge curvature. Three of these predictions are based on a transformation of Princen’s theory that is strictly valid only for regular hexagonal bubbles. Another prediction takes into account variations in bubble areas by considering the foam as consisting of approximately regular hexagonal bubbles with varying areas. Two other predictions are solely based on the characteristics of the bubble edges. The first of these is based on the assumption of affine movement of bubble vertices, while the second accounts for nonaffine deformation by considering the interaction with neighboring edges. The theoretical predictions for the shear modulus are compared with the result from a single foam simulation. For the single simulation considered, all predictions, except that based on affine movement of bubble vertices, are close to the value obtained from this simulation.

Patterned regions of porous silicon through trapped hydrogen bubbles

Hydrogen (H2) bubbles trapped around lithographically formed etch pits on silicon single crystal surfaces were used to guide the formation of patterned regions of porous silicon (PSi). Lithography and (electro) chemical etching were utilized to form PSi and guide this growth. It was observed that the pits were surrounded by round regions of PSi suggesting that H2 bubbles defined the areas of PSi formation. PSi porosity seemed to form primarily at the bubble edges, lowering overall etch rate near the bubble center. This is consistent with a variable rate of bubble expansion and an ability of the H2 to mask or slow the reaction in the bubble center. Contrary to our initial expectations, the surface profile suggested that the region away from the bubbles was also etched, but very uniformly.

Prediction of foam growth and its nucleation in free and limited expansion

The influence volume approach (IVA) is often utilized for modeling the mass transfer process dictating bubble growth dynamics in physical foaming. However, the assumed concentration profile in the IVA method is only valid when the changes in dissolved gas concentration are small (less than 5%). In addition, the validity of the IVA method is difficult to justify in chemical foaming applications because of the difficulties involved in defining the dissolved gas concentration profile. In the present work, we define two distinct stages of bubble growth for physical foaming. These two stages are termed as free and limited expansion and are controlled by the bubble nucleation rate. Bubble nucleation is assumed to occur only in the free expansion stage. In this stage, the bubble pressure drops substantially from an initially high pressure in the supersaturated state while the dissolved gas concentration changes very little. The second stage of our two-stage mass transfer model is termed the limited expansion stage and accounts for bubble growth in the late stages of foam evolution, when the pressure changes become small. However, in the limited stage of bubble growth the dissolved gas concentration drops significantly, as the available dissolved gas is depleted. To summarize our two-stage mass transfer model of foam expansion, the pressure difference between the bubble phase and the liquid phase is the primary mechanism for driving mass transfer in the early ( free) stages of foam growth and the concentration difference is the driver for bubble growth in the late ( limited) stages of growth. The first stage can be regarded as the nucleation stage and it is relatively short; while the second stage can be regarded as the bubble growth stage and is much longer. Most of the bubble volume expansion takes place in the second stage. The concentration gradient at the bubble edge, which is often ignored in other models, is analyzed in detail in this paper. The details of our novel mass transfer model are also presented.

Development Of Low-diffusion Flux-splittingMethods For Gas-liquid Flows With InterfacePropagation And Phase Variation

The development of a class of low-diffusion upwinding methods for computing gas-liquid flows with interface propagation and phase variation is presented in this work. An artificial compressibility/low-Mach preconditioning strategy is developed for a hyperbolic two-phase mixture system. The eigenvalues of this system are used to devise extensions of the low-diffusion flux-splitting scheme (LDFSS) method that provide high resolution capturing of interface propagation and phase variation in the gas-liquid system. These schemes are utilized in a polymer injection molding application involving the flow and growth of both chemical and physical foams. In physical foaming processes, a gas is dissolved into the polymer as a physical blowing agent and at a specified temperature and pressure creates foams. Once the dissolution is completed, either the pressure is reduced or the temperature is increased so as to liberate the dissolved gas to create foam. In chemical foams, the blowing agent is created by a chemical reaction as a dissolved gas which vaporizes into a bubble phase creating foam. The foam expansion rate is determined by fundamental physical parameters such as diffusion coefficient, bulk viscosity, bubble radius, bubble number density and dissolved gas concentration at the bubble edge. Foam growth dynamics is modeled using an influence volume approach which is fully coupled into a two-phase (gas-liquid) Navier-Stokes solver. The resulting three-phase foam flow formulation (polymer liquid mixture, foam bubble gas and air phases) provides polymer/air interface tracking as well as quantitative bubble scale and macro-scale flow information. Bubble scale information includes bubble number density and bubble radius; and macro-scale information includes bubble void fraction, mixture mass density and temperature, liquid species mass fraction, and velocity distribution. The method is demonstrated on both a continuous (flow-through) application and a semi-batch (or closed) foam flow system.

On a pair of interacting bubbles in planar Stokes flow

This paper presents a combined numerical and analytical investigation into various problems involving two symmetric interacting constant-pressure bubbles evolving in two-dimensional Stokes flow. The bubbles have constant surface tension on their boundaries and are taken to be in an ambient straining flow. First, a novel numerical method based on conformal mappings is presented to compute the free-surface evolution. Then, a special class of time-evolving exact solutions to the problem is derived and used to check the numerical code. These solutions reveal that, for bubbles with shrinking area, a competition between the imposed strain and surface tension can lead to either a slit or a point as the limiting shape. Numerical solutions of fixed-area bubbles are then computed and reveal that when they are forced together by a straining flow, a thin lubrication layer forms. In the absence of surface tension, large-curvature regions develop at the bubble edges and these are smoothed out by capillary effects. Further, motivated by the viscous sintering application, a study of interaction effects on the pure surface-tension-driven shrinkage of circular bubbles is investigated and compared, in an appropriate limit, to a recently derived ‘elliptical-pore model’.

Hydrogen bubbles and formation of nanoporous silicon during electrochemical etching

Many nanoporous Si structures, including those formed by common electrochemical etching procedures, produce a uniformly etched nanoporous surface. If the electrochemical etch rate is slowed down, details of the etch process can be explored and process parameters may be varied to test hypotheses and obtain controlled nanoporous and defect structures. For example, after electrochemical etching of heavily n-doped (R = 0.05–0.5 Ω·cm) silicon 〈100〉 single crystals at a current density of 10 mA cm−2 in buffer oxide etch (BOE) electrolyte solution, defect craters containing textured nanopores were observed to occur in ring-shaped patterns. The defect craters apparently originate at the hydrogen/BOE bubble interface, which forms during hydrogen evolution in the reaction. The slower hydrogen evolution due to low current density and high BOE viscosity allows sufficient bubble residence time so that a high defect density appears at the bubble edges where local reaction rates are highest. Current-carrying SiOH species are most likely responsible for the widening of the craters. Reducing the defect/doping density in silicon lowers the defect concentration and thereby the density of nanopores. Measurements of photoluminescence lifetime and intensity show a distinct feature when the few nanopores formed at the ring edges are isolated from each other. Overall features observed in the photoluminescence intensity by XPS strongly emphasize the role of surface oxide that influences these properties. Copyright © 2005 John Wiley & Sons, Ltd.

Calibrating optical bubble size by the displaced-mass method

Bubble sizing by optical means is very common, but requires calibration by non-optical means. This is particularly important since apparent bubble size increases with decreasing threshold intensity. A calibration experiment was conducted comparing the displaced water mass from captured bubbles with backlit optical bubble size. The results showed that the appropriate threshold is slightly below the local background intensity. The cause was from off-axis light from the background illumination screen forward reflecting off the bubble edge into the camera. This reflection causes a bright ring which obscures the true bubble edge. An equation describing the forward reflection obscuration was derived and was in agreement with the bias in the optical bubble size. Neglecting this effect, particularly if an inappropriate threshold is used, may cause bubble size underprediction as large as 10% to 15%. The results suggest thresholding at multiple intensities and using a curve fit to extrapolate to the appropriate size will produce the greatest accuracy.