Conventional Bubbles Research Articles

Self-Propelled Tubular Micromotors Powered by Hydrogen Bubbles under Mild Conditions: A Major Step toward Biological Applications with Live Cells.

Polymer-based tubular micromotors, featuring an inner layer of Pt nanoparticles (PtNPs), exhibit vigorous propulsion by emitting H2 bubbles in an aqueous ammonia borane (NH3BH3) solution. The hydrolysis of NH3BH3 on the PtNPs facilitates the continuous release of H2 gas from the open-end terminus, driving its forward movement. Unlike conventional O2 bubbles' systems that rely on hydrogen peroxide (H2O2) as fuel, these micromotors can operate in the presence of live cells within the NH3BH3 medium. Consequently, micromotors functionalized with the lectin concanavalin A demonstrate the capability to capture and release Escherichia coli (E. coli) without inducing cell death. Remaining bacteria can be detected by using standard culture techniques. Conversely, micromotors coated with TiO2 nanoparticles enable photosterilization of E. coli without fuel-induced damage. The self-stirring motion of the tubes enhances both bacterial capture and sterilization efficiency. These advancements obviate the necessity for H2O2 as a fuel source, and pave the way for the applications of micromotors in biological contexts.

A novel approach to the separation of macerals in inertinite-rich coal: Charged micro-nanobubble flotation

The classification and utilization of coal macerals have become a vital focus for the advancement of the modern coal chemical industry and mineral processing. Flotation is a well-established technique for large-scale mineral separation, but as for maceral separation, it is often limited by a low recovery rate and poor selectivity. This study investigates the utilization of charged micro-nanobubble (CMNB) flotation to separate the vitrinite from a typical low-ash and inertinite-rich coal in Xinjiang. Results show that CMNB flotation demonstrated notable index advantages over conventional flotation. Optimal flotation was achieved by the CMNBs produced from a 0.75 mM sodium dodecyl sulfate (SDS) solution, with a yield, concentrate vitrinite content, and vitrinite recovery of 49.58 %, 74.31 %, and 77.28 %, respectively. It was revealed that the CMNBs not only promoted the aggregation of vitrinite particles but also increased the adhesion of vitrinite particles and their aggregates to conventional flotation bubbles, resulting in enhanced flotation performance.

The role of microbubble dose in combined microflotation of fine particles

Flotation of small particles is one of the global challenges facing the mineral raw materials processing industry. Large amounts of non-ferrous and rare metals are lost in the flotation tailings in the form of mineral particles below 15 µm in size as a result of the low effectiveness of their capture by coarse bubbles generated in conventional flotation machines. The method of combined microflotation, developed in recent years, uses conventional coarse bubbles (CB) and microbubbles (MB) produced in the stand-alone generator of air-in-water microdispersion, which serves as the flotation carriers. Depending on the MB dose, the effect of their application may be positive or negative. The theoretical analysis of various mechanisms of particle transfer onto the surface of coarse bubbles and further into the froth layer allowed to obtain the formula for the optimal MB dose f=d<sub>d</sub>/2d<sub>p</sub>r<sub>p</sub>, where d<sub>d</sub> is MB size; d<sub>p</sub> and r<sub>p</sub> respectively are the size and the density of particles. Experiments performed on the copper ore flotation tailings at the Atalaya Mining (Spain) and Chaarat Kapan (Armenia) concentrators showed that, besides the optimal MB dose in the range of 1-3 ml/g, there is another optimal MB dose in the range of 10-20 ml/g, where the copper recovery increases by several percent compared to the reference test (f = 0). The deep minimum in copper recovery is observed in the area between the optimal MB doses, which is by several percent lower than the value in the reference test.

Thickness-Tunable Zoology of Magnetic Spin Textures Observed in Fe5GeTe2.

The family of two-dimensional (2D) van der Waals (vdW) materials provides a playground for tuning structural and magnetic interactions to create a wide variety of spin textures. Of particular interest is the ferromagnetic compound Fe5GeTe2 that we show displays a range of complex spin textures as well as complex crystal structures. Here, using a high-brailliance laboratory X-ray source, we show that the majority (1 × 1) Fe5GeTe2 (FGT5) phase exhibits a structure that was previously considered as being centrosymmetric but rather lacks inversion symmetry. In addition, FGT5 exhibits a minority phase that exhibits a long-range ordered (√3 × √3)-R30° superstructure. This superstructure is highly interesting in that it is innately 2D without any lattice periodicity perpendicular to the vdW layers, and furthermore, the superstructure is a result of ordered Te vacancies in one of the topmost layers of the FGT5 sheets rather than being a result of vertical Fe ordering as earlier suggested. We show, from direct real-space magnetic imaging, evidence for three distinct magnetic ground states in lamellae of FGT5 that are stabilized with increasing lamella thickness, namely, a multidomain state, a stripe phase, and an unusual fractal state. In the stripe phase we also observe unconventional type-I and type-II bubbles where the spin texture in the central region of the bubbles is nonuniform, unlike conventional bubbles. In addition, we find a bobber or a cocoon-like spin texture in thick (∼170 μm) FGT5 that emerges from the fractal state in the presence of a magnetic field. Among all the 2D vdW magnets we have thus demonstrated that FGT5 hosts perhaps the richest variety of magnetic phases that, thereby, make it a highly interesting platform for the subtle tuning of magnetic interactions.

Current status of research on nanobubbles in particle flotation

Froth flotation, as one of the most widely used separation approaches in mineral processing, is commonly used to recover valuable components from minerals. However, maintaining high flotation efficiencies is a serious challenge for conventional froth flotation in the face of decreasing particle size of the minerals to be sorted. To date, there have been plenty of reports on the software of nano-bubbles (NBS) in flotation, and the experimental consequences show that nano-bubbles' introduction has given rise to improvement's different grades in the recovery of varieties of minerals, which highlights the great potential of nano-bubbles for mineral flotation. Nanobubbles have smaller bubble radii and unusually high stability compared to conventional flotation bubbles, and their related behavior in flotation has been a hot research topic. This paper reviews some of the methods of preparing nanobubbles, equipment techniques for characterizing nanobubbles, factors affecting their stability, and some of the popular doctrines. In particular, the reinforcing mechanism of nanobubbles in the particle flotation process is discussed, first, the nanobubbles improve the electrostatic attractiveness with the particles by achieving the charge inversion while the nanobubbles that was adsorbed on the particles' surface will cover a share of the charge, which decreases the electrostatic repulsive force between the particles; and second, the nanobubbles can act as a bridge between the surfaces of the two particles, which advances the agglomeration between the particles. This review aims to be able to further advance the research related to the industrialization of nanobubbles.

Enhancement of ozonation using microbubbles – Micropollutant removal, mass transfer and bromate formation

Microbubble technology is a promising development in the optimisation of gas–liquid contacting processes. When applied to ozonation, microbubbles have demonstrated significant enhancements to mass transfer, dissolved ozone residual and the speed and extent of compound removal. However, the mechanism by which microbubbles enhance performance over conventional bubbles is not well understood and numerous explanations exist within the literature. To elucidate the critical components that drive such enhancements the performance of microbubbles (Sauter mean diameter 37 µm) and conventional bubbles (5.4 mm) were compared under identical conditions in terms volumetric mass transfer coefficient, steady state dissolved ozone concentration, rate constant for ozone self-decomposition and the rate constant for degradation of two pesticides: mecoprop and metaldehyde. Overall, the improvement observed in performance can be attributed to the increase in the volumetric mass transfer coefficient through the combination of an increase in specific interfacial area and a decrease in the mass transfer coefficient. The increase in area outweighed the decrease in mass transfer coefficient such that an overall improvement factor of 1.6 was observed for microbubbles over conventional bubbles. All other differences were an artefact of the enhanced mass transfer leading to higher dissolved ozone concentrations when operating at a fixed input dose. For the first time it has been shown that when normalised to the amount of ozone transferred to the water, no enhancement in hydroxyl radical production, bromate formation or impact from the background constituents could be observed.

Advancing aerobic digestion efficiency using ultrafine bubbles in wastewater treatment

The application of ultrafine/nanobubbles (NBs) was tested as an aeration mechanism in a laboratory scale, semi-batch, biological wastewater treatment unit and compared with a similar scale, conventional bubbling supported aerobic digestion unit. The results showed that the oxygen utilization rate (OUR) and volumetric oxygen mass transfer coefficient (kla) were doubled from 0.075 to 0.159 mg O2/min and 0.07 to 0.13 respectively, through the application of NBs compared to the use of conventional bubbles (CBs). This was achieved by exploiting the sufficient residence time NBs can have in the aqueous system along with its capacity of high mass transfer surface area to volume ratio. The twofold increment in the OUR and kla due to the application NBs improved the biodegradation rate of the organic matter from 5.83 to 17.5 mg/l.hr compared to the application of CBs. Thus, 60 % of the hydraulic residence time reduction was achieved by the application NBs compared to CBs to achieve a similar amount of organic waste degradation. Moreover, the membrane filtration results also showed a reduction in the sludge load and fouling rate for aerobic membrane bioreactors supported by NB systems. A possible mechanism for sludge reduction was also proposed based on the results achieved.

The catalytic oxidation process of atrazine by ozone microbubbles: Bubble formation, ozone mass transfer and hydroxyl radical generation

Ozone microbubbles have received increasing attention since they can produce hydroxyl radical (•OH) to decompose ozone-resistant pollutants. Besides, compared with conventional bubbles, microbubbles have a larger specific surface area and higher mass transfer efficiency. However, the research on the micro-interface reaction mechanism of ozone microbubbles is still relatively scarce. Herein, we systematically studied the stability of microbubbles, ozone mass transfer and atrazine (ATZ) degradation through multifactor analysis. The results revealed that bubble size was dominant in the stability of microbubbles, and gas flow rate played a major role in ozone mass transfer and degradation effects. Besides, the bubble stability accounted for the different effects of pH on ozone mass transfer in two aeration systems. Finally, kinetic models were built and employed to simulate the kinetics of ATZ degradation by •OH. The results revealed that conventional bubbles could produce •OH faster compared with microbubbles under alkaline conditions. These findings shed light on the interfacial reaction mechanisms of ozone microbubbles.

A modelling approach to explore the optimum bubble size for micro-nanobubble aeration

Bubble aeration has been widely applied in water/wastewater treatment, however its low gas utilization rate results in high energy consumption. Application of micro-nanobubbles (MNB) has emerged as a process with the potential to significantly increase gas utilisation due to their high relative surface area and high gas-liquid mass transfer efficiency. In this study, we demonstrate through calibrated models that MNB of an optimum bubble size can shrink and burst at or below the water surface enabling (1) all encapsulated gas to thoroughly dissolve in water, and (2) the bursting of nanobubbles to potentially generate free radicals. Through the understanding of MNB dimensional characteristics and bubble behaviour in water, a dynamic model that integrated force balance (i.e. buoyancy force, gravity, drag force, Basset force and virtual mass force), and mass transfer was developed to describe the rising velocity and radius variation of MNB along its upward trajectory. Unlike for conventional millimetre-sized bubbles, intensive gas dissolution of MNBs led to radius reduction for small bubbles, while a large initial radius triggers bubble swelling. The initial water depth was also crucial, where greater depth could drive the potential for bubble shrinkage so that they were more liable to contract. For example, the optimum bubble size of air (42–194 μm) and oxygen (127–470 μm) MNB that could achieve complete gas transfer (100% gas utilisation) for a range of specific water depths (0.5–10 m) were calculated. The modelling results for microbubbles (10–530 μm) were well validated by the experimental data (R2>0.85). However, the validation of the modelling results for nanobubble (<1 μm) aeration requires further study due to a lack of available empirical data. In this study, the proposed model and analysis provided new insights into understanding bubble dynamics in water and offered fundamental guidance for practitioners looking to upgrade bubble aeration system.

Numerical simulation of a confined cavitating gas bubble driven by ultrasound

This work investigates the flow disturbance generated by an ultrasonically driven gas bubble confined in a narrow gap over one acoustic cycle. Here, we provide a more accurate representation of ultrasonic cleaning by implementing a volume-of-fluid model in OpenFOAM that simulates the ultrasound as a sinusoidally time-varying pressure boundary condition. A modified Rayleigh–Plesset equation is solved to select an acoustic forcing that instigates bubble collapse. Simulations reveal the interaction between the inflow from the acoustic forcing and the flow deflected by the confining walls intensifies the strength of the self-piercing micro-jet(s), and consequently of the unsteady boundary layer flow, compared to the traditional collapse near a single rigid wall. Depending on the gap height and the position of bubble inception inside the gap, three distinct collapse regimes involving dual-jets or directed-jets are identified, each resulting in a different shear-stress footprint on the confining boundaries. Plots of the spatiotemporal evolution of the shear flow (that is difficult to measure experimentally) reveal peak shear-stress magnitudes at collapse that are double those reported for an undriven laser-induced bubble in similar geometric confinement. This twofold increase is attributed to the ultrasonic signal driving the collapse. Surprisingly, in our simulations we have not encountered a transferred-jet regime previously observed for an unforced bubble collapsing in a similar configuration. This unexpected finding highlights the different physics involved in modeling acoustically driven bubbles compared with the conventional laser-induced bubbles used in experiments.

Effect of the ultrasonic standing wave frequency on the attractive mineralization for fine coal particle flotation

Froth flotation for mineral beneficiation is one of the most important separation techniques; however, it has several challenges for processing fine and ultrafine particles. Attractive mineralization between particles and bubbles by ultrasonic standing wave (USW) is a novel and high-efficiency method that could assist fine particle flotation. Frequency is an important ultrasound parameter, whose effectiveness mechanisms on the attractive mineralization did not compressively address. This study explored the effect of the USW field with various frequencies on the fine coal flotation for filling this gap. Herein, a high-speed camera and a focused beam reflectance measurement (FBRM) were used to analyze three sub-processes of the attractive mineralization, including the microbubbles’ formation, the conventional flotation bubbles (CFBs)’ dispersion, and the particles’ movement. It was found that the maximum flotation metallurgical responses were obtained under the highest examined USW frequency (600 kHz). However, the flotation outcomes by a low USW frequency (50 kHz) were even lower than the conventional flotation tests. Observation and theoretical calculation results revealed these results were originated from the influence of frequency on the carrier bubbles’ formation and the action of the secondary acoustic force during USW-assisted flotation. These outcomes demonstrated that frequency is a key factor determining the success of attractive mineralization for fine particles’ flotation.

Effect of the Ultrasonic Standing Wave Frequency on the Attractive Mineralization for Fine Particle Flotation

Froth flotation for mineral beneficiation is the most important separation technique; however, it has several challenges for processing fine and ultrafine particles. Attractive mineralization between particles and bubbles by ultrasonic standing wave (USW) is a novel and high-efficiency method that could assist fine particle flotation. Frequency is an important ultrasound parameter, which its effectiveness mechanisms on the attractive mineralization did not compressively address. This study explored the effect of the USW field with various frequencies on the fine coal flotation for filling this gap. Herein, a high-speed camera and a focused beam reflectance measurement (FBRM) were used to analyze three sub-processes of the attractive mineralization, including the microbubbles' formation, the conventional flotation bubbles (CFBs)' dispersion, and the particles' movement. It was found that the maximum flotation metallurgical responses were obtained under the highest examined USW frequency (600 kHz). However, the flotation outcomes by a low USW frequency (50 kHz) were even lower than the conventional flotation tests. Observation and theoretical calculation results revealed these results were originated from the influence of frequency on the carrier bubbles' formation and the action of the secondary acoustic force during USW-assisted flotation. These outcomes demonstrated that frequency is a key factor determining the success of attractive mineralization for fine particles' flotation.

The breakup dynamics of bubbles stabilized by nanoparticles in a microfluidic Y-junction

The breakup flow patterns, the neck dynamics and the tip dynamics of bubbles stabilized by adsorptive nanoparticles in microfluidic Y-junction were investigated. Three flow patterns were observed in the Y-junction: non-breakup, breakup with partial obstruction, and breakup with permanent obstruction. Compared with bubbles stabilized only by surfactants (conventional bubbles), bubbles stabilized by nanoparticles (hardening bubbles) are more difficult to breakup. The breakup process with partial obstruction could be divided into three stages: squeezing, transition and pinch-off. Nanoparticles have little effect on the squeezing stage, but could greatly prolong the transition stage while accelerate the pinch-off stage. The difference between hardening and conventional bubbles is weakened, even disappears as the dispersion flow rate increases. Besides, the “rigid” surface of hardening bubbles could cause the decrease of the curvature of daughter bubble tips and the tunnels between the daughter bubbles and the channel wall in comparison with conventional bubbles.

A Hybrid Device for Enhancing Flotation of Fine Particles by Combining Micro-Bubbles with Conventional Bubbles

Flotation in the mining industry is a very significant separation technique. It is known that fine and ultra-fine particles are difficult to float, leading to losses of valuable minerals, mainly due to their low collision efficiency with bubbles. Flotation of fine particles can be enhanced either by increasing the apparent particle size or by decreasing the bubble size. Literature review reveals that electroflotation resulted in higher recoveries of ultrafine particles as compared with dispersed-air flotation, because electrolytic bubbles are smaller in size. To this end, the best practical approach is to combine conventional air bubbles and micro-bubbles from water electrolysis. Therefore, the design, fabrication, and operation of a bench-scale micro-bubble generator through water electrolysis is proposed. Moreover, this electrolysis unit is adapted in a mechanical Denver-type flotation cell. The resulting hybrid flotation device is capable of producing bubbles within a wide range of diameters. The significance of this process is that micro-bubbles, attached tothe surface of fine particles, facilitate the attachment of conventional-sized bubbles and subsequently increase the flotation recovery of particles. Experimental flotation results so far on the hybrid device indicate the enhancement of fine particle recovery by approximately 10% with the addition of micro-bubbles.

Numerical study of the interaction between a pulsating coated microbubble and a rigid wall. I. Translational motion

Coated microbubbles exhibit a complex surface rheology. When accelerated towards a wall subject to an acoustic disturbance they behave like a deformable solid and assume a prolate rather than oblate shape, as is the case with conventional bubbles, due to the dominance of friction over pressure drag. This results from the balance between viscoelastic stresses that develop on the protective shell and pressure and viscous stresses from the surrounding fluid which generate a progressively more prolate shape as the translational Reynolds number increases. Proper coating design can optimally control particle shape and motion in applications, notably in biomedical ones.

Reverse Combined Microflotation of Fine Magnetite from a Mixture with Glass Beads

Magnetite is an essential iron-bearing mineral. The primary method of magnetite ore beneficiation involves successive steps of crushing, grinding, and magnetic separation. Reverse cationic flotation is used at the final stage to remove silicate and aluminosilicate impurities from the magnetite concentrate and reduce silica content to 1–3%, depending on metallurgical processing route (electrometallurgy, direct iron reduction). In view of the stringent demands of the magnetite concentrate grade, before flotation, the ore is currently routinely ground down to a particle size below 35 µm, and magnetite particles are ground to a size below 10 µm. This significantly reduces the efficiency of flotation and increases iron loss in the tailings due to the hydraulic report in froth being up to 15–25%. Combined microflotation (CMF) looks to be a promising method of increasing fine-particle flotation efficiency, as it uses relatively small amounts of microbubbles alongside conventional coarse bubbles. Microbubbles act as flotation carriers, collecting gangue particles on their surface, which then coarse bubbles float. The purpose of this study is to explore the effectiveness of CMF for processing a model mixture that contained magnetite particles smaller than 10 µm and glass beads (Ballotini) below 37 µm in size when the initial iron content in the mixture was 63.76%. Commercial reagent Lilaflot 821M was used as both collector and frother. The flotation procedure, which included the introduction of 15 g/t of the collector before the start of flotation, and the addition of 5 g/t of the collector in combination with a microbubble dose of 0.018 m3/t 6 min after starting flotation, ensured an increase in the concentrate grade to 67.63% Fe and iron recovery of 91.16%.

Effect of Nanobubbles on the Flotation Performance of Oxidized Coal.

In this study, the effects of air bubbles and nanobubbles on flotation performance and kinetics of oxidized coal were investigated. The surface properties of the coal sample before and after oxidation were characterized by a scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The nanobubbles on highly oriented pyrolytic graphite (HOPG) were observed by an atomic force microscope (AFM). The interaction between coal and conventional bubbles in the absence and presence of nanobubbles was explained by induction time. Flotation results showed that oxidized coal flotation in the presence of nanobubbles resulted in 10% higher combustible matter recovery than conventional air bubble flotation. Moreover, it was found that the flotation of oxidized coal in the absence and presence of nanobubbles can be best described using the first-order model with the rectangular model. AFM images analysis showed that a large number of nanobubbles were produced and attached to the oxidized coal surface. The induction times of the oxidized coal in the absence and presence of nanobubbles were 1000 and 39 ms, respectively, indicating that the existence of nanobubbles effectively promotes the interaction between oxidized coal and macroair bubbles. In addition, the agglomeration between oxidized coal particles also occurred spontaneously in the presence of nanobubbles, which was helpful in improving the combustible matter recovery and flotation rate of oxidized coal.

Flotation of fine particles in the presence of combined microbubbles and conventional bubbles

It is known that fine and ultrafine particles are difficult to float mainly due to their low collision efficiency with bubbles. In this paper, the effect of microbubble addition, on the recovery of fine quartz particles floated with conventional bubbles is discussed. Microbubbles stabilised by collectors were produced using an external air–in–water micro-dispersion generator and added to the pulp in a flotation cell before the introduction of conventional bubbles. It was found that when microbubbles were used, much less collector (about half) was needed to achieve the same or even slightly higher recovery. At the same time, the quartz flotation rate was also higher when microbubbles were used. The advantage of microbubbles may be attributed to a higher attachment efficiency of the conventional bubbles with the fine particles covered with microbubbles. The interaction between microbubble–collector–quartz and the involved mechanism will be identified.

The role of interface in microbubble ozonation of aromatic compounds

Due to its high mass transfer efficiency, microbubble ozonation has been widely used in water treatment to degrade refractory compounds. Compared to conventional bubbles (diameters larger than 1 mm), microbubbles (diameter less than 50 μm) have special interfacial characteristics that are probably advantageous in ozonation. However, the mechanisms involved are still unclear and therefore our primary aim here was to explore the interfacial effect of microbubbles during ozonation process. Phenol and nitrobenzene degradation by ozone microbubbles and conventional bubbles were carried out across a broad pH range. We found that microbubble decomposition of pollutants was markedly more efficient than conventional bubbles in terms of ozone consumption. Hydroxyl radical scavenger experiments of phenol revealed that the enhancement of microbubble might result from the increase of ozone concentration in interfacial region and a mathematical simulation further proved this synthesis by showing that ozone concentration is not homogenous throughout the reaction medium and forms a steep gradient in the liquid film of microbubbles. As for nitrobenzene, the acceleration of hydroxyl radicals was supposed to be the dominate factor which might be the consequence of high gas concentration in liquid film. These findings shed light on the mechanism of interfacial reaction in microbubble ozonation.

Study relationships between flotation variables and recovery of coarse particles in the absence and presence of nanobubble

Recent investigations indicated that using nanobubbles (NBs) in flotation separation of fine particles (<25 μm) has several advantages; however, a detail study on performances of various flotation parameters (such as hydrodynamic variables and particle properties) and their impacts on recovery of coarse particles (>100 μm) in the presence of NBs have not been fully understood. This work was explored how NBs can change impacts of Reynolds number, conventional flotation bubbles (CBs), air flow rate and particle sizes on flotation recovery of coarse particles (−425 + 106 μm). Several flotation experiments were carried out by using pure quartz in the presence and absence of NBs. Kendall’s tau (τ) as an accurate statistical method was introduced and applied through the provided dataset from the experiments to assess the impacts of NBs. In the absence of NBs; τ assessments demonstrated that there are negative correlations between particle size, air flow rate, Reynold number and recovery (τ ∼ −0.81, −0.18 and −0.12, respectively), and a positive relationship between CBs and recovery (τ ∼ 0.08). In general, results indicated that recovery of coarse particles was increased by ∼14% (on average) in the presence of NBs; however, the magnitude of relationships was not changed (just correlations between Reynolds and CBs vs. recovery were changed: τ ∼ −0.17 and 0.13, respectively). Assessing the simultaneous impacts of Re and CBs on recovery showed that in the presence of NBs; the highest recovery (on average) may receive in lower Re values and higher CB sizes in comparison with in the absence of NBs. Taking all these interactions into consideration, it can be concluded that increasing the rate of bubble-particle attachment and decreasing the impact of Re can be the main reasons for the enhancement of coarse particle recovery in the presence of NBs.