Microbubbles In Liquids Research Articles

Mass transfer of microbubble in liquid under multifrequency acoustic excitation - A theoretical study

Microbubble’s mass transfer under external acoustic excitation holds immense potential across various technological fields. However, the current state of acoustic technology faces limitations due to inadequate control over bubble size in liquids under external excitation. Here, we conducted numerical investigations of the mass transfer behavior of microbubbles in liquids under multifrequency acoustic excitations with different frequencies (in the MHz range), pressure amplitudes (in the range of several atmospheric pressures), and amplitude ratios. We identified various pressure threshold regions for the growth of gas bubbles (radii range from a few microns to tens of microns) and observed common intersections between single and multifrequency excitations that enable effective control of the growth intervals and final size of bubbles by adjusting the ratio of pressure amplitude and frequency value. Allocating power to the lower frequency component of multifrequency acoustic excitation is recommended to facilitate mass transfer or diffusion, as small-frequency acoustic excitation has a more significant effect than the higher frequency in the growth region. Our study provides a better understanding of the dynamics of bubbles under complex excitations and has practical implications for developing methods to control and promote bubble-related processes.

Transparent, Flexible, and Highly Sensitive Piezocomposite Capable of Harvesting and Monitoring Kinetic Movements of Microbubbles in Liquid

AbstractIn this study, a zinc oxide (ZnO)‐decorated nickel microfiber (ZNMF)‐based piezoelectric nanogenerator (ZNMF‐PENG) using electrospinning, metal electroplating, electrospraying, and ceramic growing techniques is fabricated. The combination of these techniques enables the ZNMF‐PENG to possess high transparency and flexibility that are difficult to achieve through the existing piezoceramic‐based PENGs. In particular, the presence of innumerable piezoceramic ZnO nanowires inside the ZNMF‐PENG allows for detecting microbubble movements having an extremely low buoyancy force of 0.009 N, which is beneficial for detecting cavitation. Moreover, in comparison to previously reported PENGs fabricated using an electrospinning technique, the ZNMF‐PENG demonstrates the highest energy‐harvesting efficiency of 8750 V N−1 m−2. The novel approach in materials and methods proposed in this study is expected to contribute to the further advancement in developing transparent, flexible, and performance‐improved PENGs applicable to various industrial applications.

Sonochemistry of actinides: from ions to nanoparticles and beyond

AbstractSonochemistry studies chemical and physical effects in liquids submitted to power ultrasound. These effects arise not from a direct interaction of molecules with sound waves, but rather from the acoustic cavitation: the nucleation, growth, and implosive collapse of microbubbles in liquids submitted to power ultrasound. The violent implosion of bubbles leads to the formation of chemically reactive species. In principle, each cavitation bubble can be considered as a microreactor initiating chemical reactions at mild conditions. In addition, microjets and shock waves accompanied bubble collapse produce fragmentation, dispersion and erosion of solid surfaces or particles. Microbubbles oscillating in liquids also enable nucleation and precipitation of nanosized actinide compounds with specific morphology. This review focuses on the versatile sonochemical processes with actinide ions and particles in homogenous solutions and heterogenous systems. The redox reactions in aqueous solutions, dissolution or precipitation of refractory solids, synthesis of actinide nanoparticles, and ultrasonically driving decontamination are considered. The guideline for further research is also discussed.

Conic optical fiber probe for generation and characterization of microbubbles in liquids

A novel optical fiber probe has been developed to provide mechanical stability to microbubbles generated in fluids, the tip of the fiber is etched with hydrofluoric acid to pierce a truncated horn that fastens the microbubbles to the fiber tip and prevents misalignment or detachment caused by convection currents, vibrations or shocks in the liquid. Microbubbles are photo-thermally generated on the etched fiber and used as Fabry-Perot cavity sensor. Two methods were used to interrogate the probe: the first one, in the wavelength domain, is suitable for calibration in static or quasi static situations; the second one, in the time domain, can be used in dynamic environments. Experimental results in the wavelength domain show that the microbubble size rises linearly with temperature and decreases with the inverse of pressure; the average slopes are 27.1 μm/°C and 88.3 μm/bar respectively. Dynamic variations of temperature have been measured in the time domain, temperature changes down to 0.007 °C have been detected at a readout rate of 10 s-−1. Bubbles have been subjected to pressure shocks of 2.4 bar at a speed of 25 bar/s, pressure changes of 3.4 mbar have been resolved in the time domain at a readout rate of 20,000 s−1.

Mesoscopic modelling of microbubble in liquid with finite density ratio of gas to liquid

A microbubble model is developed with the mesoscopic simulation tool, dissipative particle dynamics (DPD) and many-body dissipative particle dynamics (MDPD). Standard DPD particles are employed to represent bubble phase at low density, and MDPD particles are for the liquid phase. The microbubble can be stable in liquid, in contrast to the vacuum bubble model. Gas-liquid interface is well presented with density and pressure jumps. The density ratio of gas to liquid can be lower than 0.1 by increasing the cut-off radius of bubble particles. Oscillating behavior of the microbubble model is investigated and validated by comparing with the Rayleigh-Plesset equation. The current model shows correct dynamic response, and the fluctuating behavior is captured as well. The lower the density ratio of the microbubble model, the closer the oscillating frequency to that of continuum theory.

Activating molecules, ions, and solid particles with acoustic cavitation.

The chemical and physical effects of ultrasound arise not from a direct interaction of molecules with sound waves, but rather from the acoustic cavitation: the nucleation, growth, and implosive collapse of microbubbles in liquids submitted to power ultrasound. The violent implosion of bubbles leads to the formation of chemically reactive species and to the emission of light, named sonoluminescence. In this manuscript, we describe the techniques allowing study of extreme intrabubble conditions and chemical reactivity of acoustic cavitation in solutions. The analysis of sonoluminescence spectra of water sparged with noble gases provides evidence for nonequilibrium plasma formation. The photons and the "hot" particles generated by cavitation bubbles enable to excite the non-volatile species in solutions increasing their chemical reactivity. For example the mechanism of ultrabright sonoluminescence of uranyl ions in acidic solutions varies with uranium concentration: sonophotoluminescence dominates in diluted solutions, and collisional excitation contributes at higher uranium concentration. Secondary sonochemical products may arise from chemically active species that are formed inside the bubble, but then diffuse into the liquid phase and react with solution precursors to form a variety of products. For instance, the sonochemical reduction of Pt(IV) in pure water provides an innovative synthetic route for monodispersed nanoparticles of metallic platinum without any templates or capping agents. Many studies reveal the advantages of ultrasound to activate the divided solids. In general, the mechanical effects of ultrasound strongly contribute in heterogeneous systems in addition to chemical effects. In particular, the sonolysis of PuO2 powder in pure water yields stable colloids of plutonium due to both effects.

Nature of the Supercritical Mesophase

It has been reported that at temperatures above the critical there is no “continuity of liquid and gas”, as originally hypothesized by van der Waals [1]. Rather, both gas and liquid phases, with characteristic properties as such, extend to supercritical temperatures [2]-[4]. Each phase is bounded by the locus of a percolation transition, i.e. a higher-order thermodynamic phase change associated with percolation of gas clusters in a large void, or liquid interstitial vacancies in a large cluster. Between these two-phase bounds, it is reported there exists a mesophase that resembles an otherwise homogeneous dispersion of gas micro-bubbles in liquid (foam) and a dispersion of liquid micro-droplets in gas (mist). Such a colloidal-like state of a pure one-component fluid represents a hitherto unchartered equilibrium state of matter besides pure solid, liquid or gas. Here we provide compelling evidence, from molecular dynamics (MD) simulations, for the existence of this supercritical mesophase and its colloidal nature. We report preliminary results of computer simulations for a model fluid using a simplistic representation of atoms or molecules, i.e. a hard-core repulsion with an attraction so short that the atoms are referred to as “adhesive spheres”. Molecular clusters, and hence percolation transitions, are unambiguously defined. Graphics of color-coded clusters show colloidal characteristics of the supercritical mesophase. We append this Letter to Natural Science with a debate on the scientific merits of its content courtesy of correspondence with Nature (Appendix).

Breakdown processes in gas micro-bubbles in liquids under electric stress

The present work is concerned with a theoretical analysis of the breakdown characteristics of gas-filled micro-bubbles formed in insulating liquids stressed with electric field. It is assumed that the gas inside these bubbles is air which allows the use of experimental Paschen curve data for air in this analysis. Two main discharge mechanisms have been considered, the Townsend discharge and impulse breakdown. The combination of bubble diameter, D, gas pressure, p, and duration, τ, of the field stress determines the type of breakdown. Parameters which are required for the Townsend mechanism of breakdown and impulse breakdown to occur inside gas bubbles have been obtained and these conditions have been represented as boundary lines in the (Dp, τp) coordinate system. It is shown that there are such combinations of these parameters which satisfy neither Townsend nor impulse breakdown conditions. Experimental data on breakdown in air for these intermediate values of (Dp, tp) between the Townsend and the impulse discharges are not available in the literature and the breakdown behavior under such conditions is not well defined.

Flow Properties of Microbubble/Polyethylene Glycol Mixtures Passing through Orifices and Slits

Recently, a considerable amount of research has been conducted on microbubbles, and accordingly many papers have reported on the research results. For a microbubble/water mixture passed through a capillary tube to which lard oil had been applied, a mass removal rate of 0.3 was found, whereas a rate of 0.9 for only the microbubble/water mixture. Gotoh et al. separated oils in an emulsion by using microbubbles, and Tachibana reported on the medical application of a microbubble/liquid mixture as an ultrasonic contrast agent. Suzuki et al. showed that by suspending microbubbles in liquids, the turbulent structure was changed and the Reynolds stress was lowered. In addition, the drag coefficient was found to be reduced for microbubble/liquid mixtures in rectangular channels and capillary tubes by Murai et al. and Guin et al.. Thus, as seen above, a number of theoretical and applied studies have been previously conducted. However, only microbubble/water mixtures have been typically investigated. Studies on microbubble mixtures in complex liquids, for example, surfactant solutions and polymer solutions, are limited. In general, anomalous and interesting results have been reported from investigations on complex liquids. Okawara et al. measured the pressure drop in slit flows of a wormlike micelle surfactant and found that flow properties changed as the surfactant structure was changed. Ouchi et al. conducted an experiment using an aqueous solution of cetyltrimethylammonium bromide and sodium salicylate, which form wormlike micelles, and reported on the shear induced structure and instabilities. Ushida et al. examined the flow properties of several types of surfactants that form spherical micelles, and found a drag reduction in elongational flows. Hasegawa et al. measured the flow properties and the rheological properties of dilute aqueous solutions of polyethylene oxide. Furthermore, Ushida et al. investigated the pressure drops of surfactant/nanobubble mixtures. However, the study of nanoand micro-bubble/dilute polymer mixtures was rare case. In the present research, a dilute polymer solution in which microbubbles were suspended was passed through different sizes of orifices and slits. The Flow Properties of Microbubble/Polyethylene Glycol Mixtures Passing through Orifices and Slits

Optical and Acoustic Observations of Bubble Adhered to Piezoelectric Transducer under Ultrasound Field: Pressure Signal Caused by Bubble Behavior

In order to clarify the mechanical effects caused by micro-bubbles in liquids, the detailed behavior of a single bubble adhered to a solid surface was observed optically using an ultra high-speed video camera (maximum recording rate: 1,000,000 frames/s). Making use of a thin piezoelectric transducer on the solid surface, the pressure signal caused by the bubble behavior was simultaneously observed. In addition, the influence of the initial bubble radius R0 on mechanical effects was investigated. It was confirmed that a micro-jet was generated from the center of the bubble, which was followed by a counter-jet, in the range of R0/Rres≈0.65–1.10 (Rres: the resonance radius of the free bubble). An impulsive positive pressure to the surface was also observed synchronously with the momentary increase of bubble volume acceleration. Moreover, it was found that the strength of the impulsive pressure was connected to the peak value of volume acceleration. The pressure amplitude showed a maximum value when R0/Rres was approximately 0.9.

Generation of Microplasma Using Pulsed Discharge of Ethanol Droplet in Air

An ethanol droplet of 300 µm in diameter is used to create a dense microplasma by pulsed discharge. The pulsed discharge took place when the droplet came into the space between miniature electrodes across which a dc high voltage was applied. The vaporization and dissociation of the ethanol droplet were confirmed in the discharge because the atomic line of hydrogen was observed by optical emission spectroscopy. Molecular hydrogen was detected as a product after the discharge of the ethanol droplet. The discharge of a droplet in air and that of a micro bubble in liquid are in opposite configurations, which is a duality in nature. A single microbubble discharge in pure water was conducted. The gas breakdown of air took place in the bubble and bright emission was observed locally at the interface between the bubble and the electrode.

Influence of dissolved gas on optical breakdown and small-angle scattering of light in liquids

It is demonstrated that the small-angle laser beam scattering pattern in liquid depends greatly on the dissolved gas content: the intensity of the scattering steeply drops after degassing doubly distilled water. It was experimentally found that the parameters or small-angle scattering of light correlate strongly with those of optical breakdown in liquid. Namely, the fractal dimension and the gyration radius of scatterers in aqueous KCl solutions behave similar to the optical breakdown probability in those liquids versus salt concentration, while the optical breakdown cannot be induced after degassing. The effect might be associated with the presence of stable gas microbubbles in liquid.

Evidence for a relation between inspired gas mixture and the left ventricular contrast achieved with albunex® in a Canine Model

In a previous experiment, a marked reduction in the right- and left-sided contrast effect of Albunex was noted in an intubated animal spontaneously breathing isoflurane in 100% oxygen. The theory suggests that the time course of echogenicity of microbubbles in liquid is dependent on the pressure and the gradients of dissolved gases. The present set of experiments tested whether the loss of contrast occurs at commonly used therapeutic concentrations of inspired oxygen. This research tested the hypothesis that the left ventricular (LV) contrast effect achieved with intravenous injection of the ultrasound contrast agent Albunex is related to the inspired oxygen content. Intubated dogs were maintained in a spontaneously respiring anesthetic state on isoflurane and mixtures of oxygen (12-50%) in nitrogen. FIO2 was held steady for 15 min prior to injection of 0.08 ml/kg of Albunex. The contrast effects were recorded from a transthoracic short-axis view. Left and right ventricular brightness curves were generated from digitized sequences of end-diastolic frames. The minimum and maximum brightness and area under the time-brightness curves were determined. The LV maximum brightness and area under the curve showed significant negative correlations (p = < 0.004) with the FIO2, while the minimum brightness showed a significant positive correlation (p = < 0.002). No significant correlations were found for the right ventricular brightness parameters. These findings show an important relationship between the FIO2 and loss of the contrast effect of Albunex. This loss occurs at oxygen concentrations in the therapeutic range, but could be overcome by increasing the dose of Albunex. The mechanism is likely related to an outward nitrogen gradient causing a loss of echogenicity. The clinical implication is that patients on supplemental oxygen may require higher doses of Albunex to achieve optimal opacification.

Nucleation and stabilization of microbubbles in liquids

An important aspect of the processes of cavitation and boiling is the concept of a nucleus that acts as a preferential site for the inception of these events. It is commonly thought that except for rare instances or specially controlled experiments, all cavitation and boiling sites originate at the location of such a nucleus. In order to study these important phenomena, then, it is imperative that as much as possible be known about nucleation in cavitation and boiling. It is generally accepted that free air bubbles normally do not act as nucleation sites because they are inherently unstable to dissolution due to surface tension. Thus, the study of nucleation is necessarily associated with mechanisms for stabilizing microbubbles or pockets of gas within the liquid. In this paper, various stabilization models that have been proposed are reviewed as well as the experimental evidence that supports the specific models. One particular model, the crevice model, is examined in some detail, and its predictions are used to explain several different measurements of boiling and cavitation inception. Finally, some evidence that has recently become available concerning the damaging aspects of high intensity ultrasound is examined. Many aspects of this evidence point to the existence of cavitation as the damage mechanism. Also given in this paper are preliminary explanations of these effects due to the growth of microbubbles or cavitation nuclei by rectified diffusion.

Thermodynamic Theory of Bubble Nucleation Induced in Liquids by High-Energy Particles

A theory is proposed on the creation of microbubbles in liquids by ionizing particles. A thermodynamic point of view allows one to specify the conditions required for the growth of the gaseous embryos statistically present in liquid. In the case of aerated water, the process shows the existence of an energy threshold for the production of 1-μ-radius bubbles as required for ultrasonic cavitation; the theoretical values of particle energy agree with experimental results. Also proposed is a mechanism for one qualitative determination of the stable existence of microbubbles in water.