Function Of Bubble Size Research Articles

Energy-efficient bubble aeration guided by bubble dynamics model: From bubble formation at submerged orifice to oxygen utilization during uprising

Bubble aeration is a significant consumer of energy in wastewater treatment processes. Optimizing oxygen transfer, from an energy usage perspective, is crucial in the development of clean production techniques and for the stimulation of the circular economy. Although large efforts have been made in enhancing this process, particularly in ameliorating bubble diffusers, a main problem lies in energy wastage due to over-pursuing ultra-fine bubble generation. In this study, a dynamic model, that integrated both bubble expansion and detachment at submerged orifices, and consequential oxygen utilization during the uprising, was developed in order to better facilitate energy savings during the control of bubble generation. We demonstrate through calibrated models that bubble formation is governed by many factors including gas flow rate, orifice radius, surface tension, liquid viscosity, and surface wettability. Small bubbles are more prone to form when subjected to conditions of high wettability, small orifice size and low flow rate, since the bubbles are easily detached by the increasing upward forces in these scenarios. The oxygen transfer during the rising process was calculated as a function of bubble size, gas type and water depth. Bubbles with a radius smaller than a threshold value can shrink and collapse while below the water surface and thus provide 100% oxygen utilization. The threshold values of bubble radius are determined to be 150 μm and 250 μm at 5 m and 15 m initial water depth, respectively. The parameter set for bubble generation in order to obtain the desired bubble size may then be inversely determined by our bubble formation model, and it's considered as optimal aeration strategy to realize precise aeration. With these results, our study highlights the viability of the approach employing the proposed model in order to direct energy-efficient bubble generation.

Laser induced spherical bubble dynamics in partially confined geometry with acoustic feedback from container walls

We investigated laser-induced cavitation dynamics in a small container with elastic thin walls and free or partially confined surface both experimentally and by numerical investigations. The cuvette was only 8–25 times larger than the bubble in its center. The liquid surface was either free, or two thirds were confined by a piston-shaped pressure transducer. Different degrees of confinement were realized by filling the liquid up to the transducer surface or to the top of the cuvette. For reference, some experiments were performed in free liquid. We recorded the bubble dynamics simultaneously by high-speed photography, acoustic measurements, and detection of probe beam scattering. Simultaneous single-shot recording of radius-time curves and oscillation times enabled to perform detailed investigations of the bubble dynamics as a function of bubble size, acoustic feedback from the elastic walls, and degree of surface confinement. The bubble dynamics was numerically simulated using a Rayleigh-Plesset model extended by terms describing the acoustically mediated feedback from the bubble’s environment.Bubble oscillations were approximately spherical as long as no secondary cavitation by tensile stress occurred. Bubble expansion was always similar to the dynamics in free liquid, and the environment influenced mainly the collapse phase and subsequent oscillations. For large bubbles, strong confinement led to a slight reduction of maximum bubble size and to a pronounced reduction of the oscillation time, and both effects increased with bubble size. The joint action of breakdown-induced shock wave and bubble expansion excites cuvette wall vibrations, which produce alternating pressure waves that are focused onto the bubble. This results in a prolongation of the collapse phase and an enlargement of the second oscillation, or in time-delayed re-oscillations. The details of the bubble dynamics depend in a complex manner on the degree of surface confinement and on bubble size. Numerical simulations of the first bubble oscillation agreed well with experimental data. They suggest that the alternating rarefaction/compression waves from breakdown-induced wall vibrations cause a prolongation of the first oscillation. By contrast, liquid mass movement in the cuvette corners result in wall vibrations causing late re-oscillations. The strong and rich interaction between the bubble and its surroundings may be relevant for a variety of applications such as intraluminal laser surgery and laser-induced cavitation in microfluidics.

A phase-resolved force analysis of bubble trapping behind cylinders in liquid-gas flows

A liquid-gas flow across a bluff body can result in non-uniform distribution of void fraction due to the interaction of the bubbles with the bluff-body wake. This experimental investigation was conducted to characterize a bubble-trapping region and the clustering dynamics of two distinct bubble sizes in an upward water channel with a rectangular cross-section. Bubble trapping in the near wake of a cylinder is shown as a function of bubble size, liquid flow rate, and gas flow rate through patterns of local void fraction. Particle tracking velocimetry and particle image velocimetry were used to calculate time-averaged trajectories of 3 mm diameter and 0.5 mm diameter bubbles as they flowed around a cylinder. The liquid Reynolds number (Re), based on the cylinder diameter of 9.5 mm, was varied from Re = 100 to Re = 3,000. In addition, the injection of liquid flow tracers and application of particle shadow image velocimetry allowed measurements of the time-averaged liquid velocities in the wake. The phase-resolved tracking results were evaluated to determine the effects of the added mass, pressure gradient, lift, drag, and buoyancy forces acting on the air bubbles. Trapping of both bubble sizes was observed across a range of operating conditions; however, this behavior was not explained solely by the Reynolds number of the flow. The force balance analysis revealed that inertial forces and lift forces acting on the bubbles both contributed to the clustering, which occurred when either the inertial forces or the lift forces acting on the bubble were sufficiently high compared to the bubble drag forces. Both the inertia- and lift-to-drag ratios were necessary to predict the bubble clustering dynamics of the distinct bubble sizes investigated.

Dynamic behavior of helium bubbles in Fe9Cr1.5W0.4Si F/M steel at elevated temperature

The degradation of mechanical properties caused by transmutation/deuterium-tritium reaction produced helium (He) is directly related to the behavior of He bubbles in nuclear materials. Here, the dynamic behavior of He bubbles was in-situ studied in an annealed Fe9Cr1.5W0.4Si ferrite-martensite (F/M) steel using transmission electron microscopy. The critical temperature at which He bubbles started to appear obvious Brownian motion was ∼1013 K. The mobile spherical bubbles gradually evolved into immobile polyhedral bubbles through migration and coalescence, and the critical size between mobile spherical and immobile polyhedral was ∼4 nm. The macroscopic diffusion coefficients of bubbles at 1023 K were obtained as a function of bubble size by quantifying the bubble motion in the two-dimensional plane. Meanwhile, the motion characteristics of bubbles away from or close to dislocation were compared and analyzed in detail. Due to the attraction of dislocations, the migration path of He bubbles exhibited a similar 1D Brownian motion. This was attributed to the short-range elastic interaction between He bubbles and dislocations. These results provide direct evidence and potential insights for understanding the dynamic behavior of He bubbles in metals.

Tip-Enhanced Photoluminescence of Freestanding Lateral Heterobubbles.

Two-dimensional (2D) semiconducting materials have promising applications in flexible optoelectronics, nanophotonics, and sensing based on the broad tunability of their optical and electronic properties. 2D nanobubbles form exciton funnels due to localized strain that can be used as local emitters for information processing. Their nanoscale optical characterization requires the use of near-field scanning probe microscopy (SPM). However, previous near-field studies of 2D materials were performed on SiO2/Si and metallic substrates using the plasmonic gap mode to increase the signal-to-noise ratio. Another challenge is the deterministic control of bubble size and location. We addressed these challenges by investigating the photoluminescence (PL) signals of freestanding monolayer lateral WSe2-MoSe2 heterostructures under the influence of strain exerted by a plasmonic SPM tip. For first time, we performed tip-enhanced PL imaging of freestanding 2D materials and studied the competition between the PL enhancement mechanisms by nanoindentation as a function of the tip-sample distance. We observed the tunability of PL as a function of bubble size, which opens new possibilities to design optoelectronic nanodevices.

Volume and pressure of helium bubbles inside liquid Pb16Li. A molecular dynamics study

The behaviour of helium impurities inside metals has been well studied in the last 30 years, however, little attention has been devoted to helium atoms inside liquid metals. Here we have investigated the nucleation and coalescence processes of helium atoms inside liquid eutectic lithium–lead alloys using atomistic simulations. Several key findings regarding He bubbles inside liquid PbLi eutectic are presented. The radius versus the number of atoms has been calculated in the temperature range 600–1000 K. The trend can be fitted and likely extrapolated to larger bubbles (micrometer size). The value of thermal expansion of He bubbles is given as well and compared to the thermal expansion of bulk He. The pressure inside He bubbles has been calculated as a function of bubble size. Finally, the importance of accurate interatomic potentials for the He–metal interaction is discussed.

Translational dynamics of individual microbubbles with millisecond scale ultrasound pulses.

It is established that radiation forces can be used to transport ultrasound contrast agents, particularly for molecular imaging applications. However, the ability to model and control this process in the context of therapeutic ultrasound is limited by a paucity of data on the translational dynamics of encapsulated microbubbles under the influence of longer pulses. In this work, the translation of individual microbubbles, isolated with optical tweezers, was experimentally investigated over a range of diameters (1.8-8.8 μm, n = 187) and pressures (25, 50, 100, 150, and 200 kPa) with millisecond pulses. Data were compared with theoretical predictions of the translational dynamics, assessing the role of shell and history force effects. A pronounced feature of the displacement curves was an effective threshold size, below which there was only minimal translation. At higher pressures (≥150 kPa) a noticeable structure emerged where multiple local maxima occurred as a function of bubble size. The ability to accurately capture these salient features depended on the encapsulation model employed. In low Reynolds number conditions (i.e., low pressures, or high pressures, off-resonance) the inclusion of history force more accurately fit the data. After pulse cessation, bubbles exhibited substantial displacements consistent with the influence of history effects.

Interfacial mass transfer limitations of the Fischer-Tropsch synthesis operated in a slurry bubble column reactor at industrial conditions

At high catalyst volume fractions the Fischer-Tropsch synthesis (FTS) operated in a slurry bubble column (SBC) is driven into the mass transfer limited regime. This study utilized literature models for the gas-liquid mass transfer coefficients in a multifluid-population balance model in which the gas-phase composition was a function of bubble size. The results confirmed that mass transfer limitations occur and that the choice of mass transfer coefficient model is crucial, yielding final conversion results ranging from 45% to 92% depending on the choice of kL models. At smaller kL values the composition is highly dependent on bubble size, whilst for the largest kL values the composition is not a function of bubble size at all. The population balance modeling (PBM) allowed for explicitly keeping track of the bubble size distribution. Varying the inlet Sauter-mean diameter (SMD) resulted in a linear decrease in conversion as the inlet SMD was increased from 5 mm to 20 mm. Illustrative models for the bubble size dependency of kL were implemented, which provided additional information compared to traditional models which use (bubble size) averaged values for the liquid-phase mass transfer coefficient kL and/or the gas-liquid interfacial area a and composition.

Size Control of Sessile Microbubbles for Reproducibly Driven Acoustic Streaming

This article presents technology to control the size of sessile gas bubbles in porous microchannels, which is of particular interest for microfluidic lab-on-a-chip applications using silicone-based materials. It shows how such bubbles can be removed, enlarged, dwindled, or maintained at constant size in an automated way, using a proportional-integral-derivative (PID) algorithm for pressure control. The technique is used to show how in a microchannel the streaming flow close to an acoustically actuated bubble changes in topology and strength, as a function of bubble size. This approach could be useful in potentially any experiment involving bubbles in porous microchannels.

A numerical study of the bubble induced pressure fluctuation in gas-fluidized beds

Pressure fluctuation analysis has been widely accepted as an efficient way for bubble size estimation in fluidized beds since the local bubble induced pressure fluctuation, which is believed to be a function of bubble size, can be separated away from the global pressure waves. The spectral data decomposition method developed by van der Schaaf et al. (2002) Van der Schaaf et al. (2002) has been widely used in this regard. However, it has been found in various experimental studies that the proportionality constant between the reference data (obtained via measurements by various techniques or predicted by well-established correlations) and the estimated bubble size differs significantly in different applications. In this work we try to understand the scattered proportionality constants via a numerical study based on the Euler-Euler two-fluid model. The simulation results indicate that the local bubble induced pressure fluctuation is affected not only by bubble size, but also by the lateral distance between the rising bubble and detecting point, bubble shape, bed diameter, and bubble coalescence. Without consideration of these factors, the spectral data decomposition method is subject to large deviation for bubble size estimation.

Drainage of particle stabilized foam film

Abstract A model for drainage of aqueous foam film with a partial coverage of particles accounts for an inner clean air-liquid film surrounded by a film stabilized by close packed particles at the outer periphery. The drainage of the inner film occurs due to capillary pressure gradient. The film thickness is found to decrease fastest at the center with the inner film being thinner than the outer film eventually leading to its rupture as a result of film thickness becoming zero at the center. The velocity of film drainage is found to be higher for lower particle concentration and higher surface tension. The lifetime (time of rupture) of the film is found to be larger for larger bubble size and liquid holdups (higher initial film thickness). For a film stabilized by close packed monolayer or multilayer of particles, the disjoining pressure is larger for larger particle number concentration and particle size, increases with surface tension only for film stabilized by single or two layers of particles and is a function of bubble size only for film stabilized by multilayers of particles. The maximum disjoining pressure decreases with contact angle approaching zero for a critical contact angle of 129° and increases linearly with surface tension. At mechanical equilibrium, the particle stabilized foam gets drier for larger bubble sizes, contact angles and particle number concentrations.

Blast wave attenuation in liquid foams: role of gas and evidence of an optimal bubble size.

Liquid foams are excellent systems to mitigate pressure waves such as acoustic or blast waves. The understanding of the underlying dissipation mechanisms however still remains an active matter of debate. In this paper, we investigate the attenuation of a weak blast wave by a liquid foam. The wave is produced with a shock tube and impacts a foam, with a cylindrical geometry. We measure the wave attenuation and velocity in the foam as a function of bubble size, liquid fraction, and the nature of the gas. We show that the attenuation depends on the nature of the gas and we experimentally evidence a maximum of dissipation for a given bubble size. All features are qualitatively captured by a model based on thermal dissipation in the gas.

Interactions between Individual Ultrasound-Stimulated Microbubbles and Fibrin Clots

The use of ultrasound-stimulated microbubbles (USMBs) to promote thrombolysis is well established, but there remains considerable uncertainty about the mechanisms of this process. Here we examine the microscale interactions between individual USMBs and fibrin clots as a function of bubble size, exposure conditions and clot type. Microbubbles (n = 185) were placed adjacent to clot boundaries (“coarse” or “fine”) using optical tweezers and exposed to 1-MHz ultrasound as a function of pressure (0.1–0.39 MPa). High-speed (10 kfps) imaging was employed, and clots were subsequently assessed with 2-photon microscopy. For fine clots, 46% of bubbles “embedded” within 10 μm of the clot boundary at pressures of 0.1 and 0.2 MPa, whereas at 0.39 MPa, 53% of bubbles penetrated and transited into the clots with an incidence inversely related to their diameter. A substantial fraction of penetrating bubbles induced fibrin network damage and promoted the uptake of nanobeads. In coarse clots, penetration occurred more readily and at lower pressures than in fine clots. The results therefore provide direct evidence of therapeutically relevant effects of USMBs and indicate their dependence on size, exposure conditions and clot properties.

Mixing and segregation of solid mixtures in bubbling fluidized beds under conditions pertinent to the fuel reactor of a chemical looping system

Abstract Performance of chemical looping combustion processes can be improved drastically by enhancing the overall redox characteristics of the system through the use of binary mixtures of oxygen carriers. However, binary mixtures of oxygen carrier particles are often found to differ in both size and density and therefore have the tendency to segregate under certain operating conditions. In this work, a numerical study was conducted to investigate the mixing and segregation behaviour of binary mixtures of particles with different sizes and densities in a bubbling fluidized bed under conditions pertinent to the fuel reactor of a cold flow model (i.e. a non-reacting replica) of a 10 kW th chemical looping combustor. The motion of particles was tracked individually by discrete element model (DEM), whilst the gas flow was modelled by computational fluid dynamics (CFD). Gas–particle interactions were considered by a two-way coupling method. Further, a modified version of Lacey's method was developed to calculate the mixing index, taking into account both the heterogeneity of solids spatial distribution and particle size differences. Results showed that the modified Lacey's method provided very consistent and stable mixing indexes, proving to be effective for an in-situ quantitative description of mixing. It was also found that as the size ratio of the binary mixture of particles reduced, the mixing index increased indicating better mixing conditions. The agreement between the DEM/CFD model predictions and the experimental data was found to be satisfactory. The optimum conditions for mixing of binary mixtures appeared to be a function of bubble size, bubble rising rate and bubbling dynamics (e.g., splitting and coalescence). Application of the DEM/CFD model for prediction of layer inversion phenomenon in gas–solid fluidized beds was also demonstrated.

Acoustic estimates of size distribution for bubbles released from the seabed.

Bubbles rising from the seabed evolve as they rise through the water column due to gas transfer and changes in hydrostatic pressure. Consequently, the bubble’s acoustic response can change quite dramatically as it rises. For depth integrated measurements of target strength from these bubble clouds, depth dependent changes can confound acoustic inversions for bubble size distribution, void fraction, and the amount of bubbles (and hence gas) reaching the ocean surface. One approach to solving this problem is to implement a model for the evolution of the bubble as it rises through the surface and to then compute an effective, depth integrated scattering response for the bubble plume as a function of bubble size at the source. This effective scattering strength can then be used for inversions of frequency dependent, depth integrated target strength measurements to find the source distribution of bubbles. Such an inversion procedure has been implemented using an incoherent forward model, and tested for a steady stream of bubbles pumped through an orifice at a depth of 11 m. Broadband (1–30 kHz) target strength estimates were made at a distance of approximately 10 m. Inversions for the source distributions of bubbles are compared with estimates made directly with underwater video. Results of this acoustic inversion, and possible coherent bubble cloud effects, will be discussed.

Resolution of d- and l-glucoses by chiral N-octyl-β- d-glycoside–Cu(II) complex adsorbed at the gas/liquid interface of small bubbles

A new technique of the jet drop method (JDM) was applied to a chiral molecular discrimination of optically active d- or l-glucose (guest) by chiral N-octyl-β- d-glycoside (OβDG)–Cu(II) complex (host) at the gas/liquid interface of small bubbles. The discrimination of glucoses as the guests is possible using OβDG adsorbed at the gas/liquid interface of bubbles where it acts as the host, either in the presence or the absence of Cu(II) ions. In order to make clear the host–guest interaction at the gas/liquid interface, the composition of 5000 top jet drops periodically collected onto a slide glass receiver was analyzed. The relative concentration ( η i − 1) and the surface excess amount, Γ i 0 of species i such as d- and l-glucoses, and OβDG were determined as a function of bubble size, d b and bulk concentration C b. The partition coefficient, Π i = Γ i 0 / C b was also evaluated for each component. The adsorption of these materials either in the presence or absence of Cu(II) ions, was assigned to the Freundlich type, and the discrimination of d- and l-glucoses with OβDG was evaluated in terms of the Freundlich constant, k i and 1/ n. The discrimination ability of OβDG was also evaluated by determining the equilibrium constants, K c of complex formations for the respective glucoses in the presence and absence of Cu(II) ions. It was found that l-glucose can form a more stable complex with OβDG–Cu(II).

Atomistic simulation of He bubble in Fe as obstacle to dislocation

Degradation of mechanical properties due to nanometric irradiation induced defects is one of the challenging issues in designing materials for future fusion reactors. Various types of defects such as voids and He bubbles may be produced due to high dose of neutron irradiation due to fusion reaction. We study the influence of He bubble on the mobility of an edge dislocation in pure bcc-Fe using molecular dynamics simulation as a function of bubble size, He density and temperature. It appears that low contents He bubbles are penetrable defects, which size and temperature rise make them harder and softer, respectively. At high He contents a size dependent loop punching is observed, which at larger bubble sizes leads to a multistep dislocation-defect interaction. It also appears that the bubble surface curvature and temperature are the main parameters in the screw segments annihilation needed for the release of the dislocation from the bubble.

A mechanism for internal reflux in foam fractionation

Multiple equilibrium stages can be engendered in foam fractionation, a process used for the enrichment of streams of proteins, by returning some of the foamate stream to the top of the column as external reflux liquor. However, it was recognised, 40 years ago that reflux could be autogenously created through the coalescence of bubbles in fractionation columns. By invoking the hydrodynamic theory of rising foam, we suggest a mechanism for the creation of internal reflux in foam fractionation. This method can give internal reflux rate as a function of bubble size. However, since the bubble size profile in a rising foam cannot be estimated, we cannot yet estimate how internal reflux varies with position in the column.

Effect of nanoscale bubbles on viscous flow and rupture in thin liquid films

Nanoscale bubbles between a thin liquid film and a hydrophobic solid substrate affect rupture by relaxing the no-slip boundary condition for the viscous flow in the liquid. They also cause appreciable changes in the shape of the flow domain and the value of disjoining pressure when the film thickness is comparable to the bubble size. We investigate all three effects systematically for a single gas bubble on the substrate under the liquid film in the framework of a lubrication-type model. A scaling law for film thickness versus time is found, but the scaling exponent is different from the case when no bubble is present. Rupture time is studied as a function of bubble size and disjoining pressure parameters. Presence of bubbles is shown to significantly decrease rupture time and can also be the cause of instability and rupture in otherwise stable films.

Gas Transfer During Bubbler Destratification of Reservoirs

In this paper, dimensional analysis has been carried out to derive general equations that predict: the total gas transferred to the ambient reservoir water from an air bubbler, total volume entrained, and total energy consumed for a known or equivalent linear stratification. The equations are tested by comparison with a one-dimensional bubbler model developed by the authors. It is shown that the oxygen transfer to the water column can be significant if small bubbles are used. The mechanical destratification efficiency ηmech (%), destratification time per unit surface area Γ (s∕ m2 ) , oxygen dissolution efficiency Ω (%), and oxygen transferred per unit input energy are examined as functions of bubble size. It is concluded that an average bubble radius of 1 mm should be considered for design purposes. However, if oxygen transfer from the bubbler is not considered important, then a bubble size of up to 4 mm is acceptable for destratification purposes.