Tiny Bubbles Research Articles

Response surface method for modeling the removal of carbon dioxide from a simulated gas using water absorption enhanced with a liquid-film-forming device

This paper presents the results from using a physical absorption process to absorb gaseous CO2 mixed with N2 using water by producing tiny bubbles via a liquid-film-forming device (LFFD) that improves the solubility of CO2 in water. The influence of various parameters—pressure, initial CO2 concentration, gas-to-liquid ratios, and temperature—on the CO2 removal efficiency and its absorption rate in water were investigated and estimated thoroughly by statistical polynomial models obtained by the utilization of the response surface method (RSM) with a central composite design (CCD). Based on the analysis, a high efficiency of CO2 capture can be reached in conditions such as low pressure, high CO2 concentration at the inlet, low gas/liquid ratio, and low temperature. For instance, the highest removal efficiency in the RSM–CCD experimental matrix of nearly 80% occurred for run number 20, which was conducted at 0.30MPa, CO2 concentration of 35%, gas/liquid ratio of 0.71, and temperature of 15°C. Furthermore, the coefficients of determination, R2, were 0.996 for the removal rate and 0.982 for the absorption rate, implying that the predicted values computed by the constructed models correlate strongly and fit well with the experimental values. The results obtained provide essential information for implementing this method properly and effectively and contribute a promising approach to the problem of CO2 capture in air pollution treatment.

Bond number effects on pore shape in solid

The pore shape affected by Bond number, indicating the ratio between hydrostatic pressure and capillary pressure, of a bubble entrapped by a solidification front is predicted. Bond number is responsible for bubble nucleation, surface area and shape of the bubble or pore cap beyond the solidification front. Surface area determines solute transport across the cap, whereas contact angle of the cap delineates pore shape in solid. The critical velocity for entrapment of a bubble is also related to Bond number. Pore formation in solid influence not only microstructure of materials, but also contemporary issues of biology, engineering, foods, geophysics and climate change, etc. This work extends previous models accounting for mass and momentum transport of solute across a coupled shape of the bubble cap to predict the pore shape in solid. The study also accounts for different directions and magnitudes of solute transport across the cap in different cases. Case 1 is subject to solute transport from the pore across an emerged cap in a concentration boundary layer on the solidification front to surrounding liquid in the early stage. Case 2 indicates solute transport across a submerged cap in a concentration boundary layer to the pore. In contrast to Case 2a, pore value expansion in Case 2b exhibits a more important role in solute concentration at the cap than solute transport across the cap in the late stage. The results show that an increase in Bond number decreases pore radius and time for bubble entrapment in Case 1. An isolated pore cannot be formed in Cases 2a and 2b. The predicted growth and entrapment of a tiny bubble as a pore in solid are found to agree with experimental data. A realistic prediction and control of the pore shape in solid via Bond number has therefore been obtained.

Extraordinarily Rapid Rise of Tiny Bubbles Sliding beneath Superhydrophobic Surfaces.

Tiny bubbles readily stick onto substrates owing to contact angle hysteresis (CAH). Nevertheless, they can slide slowly on a tilted surface with ultralow CAH because capillarity is overcome by buoyancy. It is surprising to observe experimentally that bubbles of 3-15 μL (diameter 1.79-3.06 mm) slide beneath a tilted superhydrophobic surface at a vertical ascent rate faster than that of freely rising ones of high Reynold numbers ≈O(102). As the tilting angle increases, the drag coefficient remains essentially the same as that of a freely rising bubble, but the frontal area of the flat bubble rises monotonically. Nonetheless, the frontal area of the sliding bubble always stays much smaller than that of a freely rising bubble. Consequently, the small drag force associated with the sliding bubbles is attributed to their substantially small frontal areas on superhydrophobic surfaces.

A novel bubble nucleation particle for magnesium composite foam

A new thickening agent (hollow ceramic microsphere, CM) together with a novel bubble nucleation particle (Mg2Si) for magnesium composite foam (MCF) was obtained. Initial morphology and evolution process of Mg2Si during preparation process were described. Meanwhile, initial morphology of Mg2Si which is favorable for absorption of initial tiny bubbles was confirmed. Transition process of Mg2Si phase and the coalescence behavior of bubbles along with the foaming process were analyzed based on the experimental results. According to the results obtained from this work, MCF (using CM as thickening agent) with homogeneous pore structures, superior mechanical properties and excellent corrosion resistance performance could be foreseeable.

A visualized study of the motion of individual bubbles in a venturi-type bubble generator

As an equipment for generating millimeter- or micrometer-sized bubbles, venturi-type bubble generator has a great potential application in online poison gas removal for molten salt reactor, water treatment, mineral flotation, flue gas desulphurization, even in drug delivery, etc. While the process of generating bubbles in a venturi tube is quite different from that in a conventional tube. A visualized study was carried out to illustrate the details of the transportation of individual bubbles in a venturi-type bubble generator. Sizes, velocity, acceleration of the bubbles were obtained by Digital Image Analysis (DIA) method. An extremely rapid deceleration and intense breakup process was observed nearby the entrance of the diverging section of the bubble generator. Bubbles with average diameter larger than 1.5 mm were usually split before collapse, while the smaller one collapsed directly without split. Within several millimeters, the velocity of the bubbles were almost reduced by half, which is believed to play a key role for triggering their collapse. Once a bubble was decelerated to its minimum velocity, it was collapsed promptly into many tiny bubbles around the position of 8–10 mm from the inlet of the diverging section. Increasing the liquid flow rate could intensify the collapse process, while its influence on the position of collapse was weak.

The American Nobel: Oklahoma's Neustadt Prize

I f I had been pressed on what I knew about the Neustadt Prize before going to Oklahoma in November, I probably would’ve said something along the lines that it’s the “American Nobel,” that the prize money is substantial, that it’s held at the University of Oklahoma because of World Literature Today, and that the first author ever published by Open Letter Books—Dubravka Ugrešić—was this year’s recipient. There’s a reason the Neustadt has been labeled as the “greatest literary prize you haven’t heard of,” though—all of which was on display in Norman, Oklahoma, in the week before Halloween. Founded in 1970 as the Books Abroad InternationalPrizeforLiterature(itassumed its much shorter moniker in 1976), the Neustadt is awarded every other year to a poet, novelist, or playwright from anywhere in the world on the basis of “literary merit” alone. On the off years, two things happen: the jury for the next prize releases the names of their finalists and chooses a winner, and the NSK Neustadt Prize for Children’s Literature —funded and administered by three Neustadt sisters, Nancy, Susan, and Kathy— is awarded and celebrated. From the best I can tell, the “American Nobel” nickname comes from the fact that over its forty-five-year history, thirty-two of the winners, jurors, and finalists have gone on to win the Nobel Prize, including Gabriel García Márquez (winner, 1972), Czesław Miłosz (winner, 1978), Tomas Tranströmer (winner, 1990), Mario Vargas Llosa (juror), Derek Walcott (juror), Doris Lessing (finalist ), and Svetlana Alexievich (finalist). That’s a pretty fantastic list of authors, and the actual prize—a $50,000 check and a very weighty feather made of silver, housed in a handcrafted wooden box—is equally impressive. But what most got to me was the care that went into making sure this award has a deep impact on readers and students. One of the reasons behind the two-year cycle for this award—announced one year, with the festival taking place in the ensuing one—is so that students can take a class on the winning author and participate in the festival itself. The idea of taking a “Neustadt Winner” class and being able to meet an internationally renowned author might not seem all that impressive to many of the people reading this, who live in metropolises and are part of the book industry, but for the people of Norman, a tiny little blue bubble in a redder than red state, this is a pretty powerful opportunity. As one would expect, a number of the festival events are pretty standard: an opening -night reception (which I spoke at, as the publisher of five books by Dubravka dating back thirteen years), a keynote address (“A Girl in Litland,” which appears in the current issue of World Literature Today), an insightful roundtable on her works, and a swank closing ceremony at the natural history museum where Dubravka was given her check and silver feather. But along the way, there were some interesting detours. A roundtable discussion about the European refugee crisis. (Although not a refugee per se, Dubravka has been living in exile ever since Yugoslavia split along nationalist lines and she became a “Croatian.”) A slapsticky, raucous staging of her short story “Who Am I?” by students from the University of Oklahoma School of Drama. And a chance for the students in the Neustadt class to apply all their book learning and interview the author in person. • Before going on, I want to take a moment to say that you should absolutely read Dubravka’s books. If this were a different sort of essay, I’d get all personal journal about our fifteen-year friendship, the first time I discovered her (in an interview in Bomb), about how much she’s meant 64 WLT JANUARY–FEBRUARY 2017 cover feature The American Nobel Oklahoma’s Neustadt Prize by Chad W. Post from left Ellen Elias-Bursać, Dubravka Ugrešić, and Chad Post photo : jen rickard blair to Open Letter (our one and only National Book Critics Circle Award finalist!), and the emotional reaction I had when I was referenced in her...

Superhydrophobic Cones for Continuous Collection and Directional Transportation of CO2 Microbubbles in CO2 Supersaturated Solutions

Microbubbles are tiny bubbles with diameters below 50 μm. Because of their minute buoyant force, the microbubbles stagnate in aqueous media for a long time, and they sometimes cause serious damage. Most traditional methods chosen for elimination of gas bubbles utilize buoyancy forces including chemical methods and physical methods, and they only have a minor effect on microbubbles. Several approaches have been developed to collect and transport microbubbles in aqueous media. However, the realization of innovative strategies to directly collect and transport microbubbles in aqueous media remains a big challenge. In nature, both spider silk and cactus spines take advantage of their conical-shaped surface to yield the gradient of Laplace pressure and surface free energy for collecting fog droplets from the environment. Inspired by this, we introduce here the gradient of Laplace pressure and surface free energy to the interface of superhydrophobic copper cones (SCCs), which can continuously collect and directionally transport CO2 microbubbles (from tip side to base side) in CO2-supersaturated solution. A gas layer was formed when the microbubbles encounter the SCCs. This offers a channel for microbubble directional transportation. The efficiency of microbubble transport is significantly affected by the apex angle of SCCs and the carbon dioxide concentration. The former provides different gradients of Laplace pressure as the driving force. The latter represents the capacity, which offers the quantity of CO2 microbubbles for collection and transportation. We believe that this approach provides a simple and valid way to remove microbubbles.

In-Oleo Microgasometry of Nanoliter-Scale Gas Volumes with Image-Based Detection.

Gasometric assays involve measurements of the amounts of gases that are released during physical or chemical processes. The available instrumentation for gasometric analysis is generally difficult to use and requires large sample volumes. In some cases, toxic materials (mercury) are involved in the analysis process. Here, we propose a microscale gasometric assay using silicone oil as matrix. Microliter-volume (∼2.5 μL) aqueous droplets, containing sample and reagent and/or catalyst, are introduced to the oil matrix and merged. Nanoliters of gaseous products are released to the surrounding oil matrix forming tiny spherical bubbles. Due to the huge differences between refractive indices of the released gas and the surrounding liquids (aqueous assay solution, oil), the gas bubbles are clearly visible from the top, when the assay reservoir is illuminated from the bottom with light-emitting diodes. The released gas bubbles are documented by recording videos of the assay reservoir. Individual frames within these videos are then analyzed by a graphical software to obtain diameters of every gas bubble at each time point. Following a fixed period of time (typically, 5-90 s) after the sample/reagent droplet merger, the volume of the released gas scales with the amount of the substrate (analyte) present in the sample droplet. For example, hydrogen peroxide can be decomposed to oxygen by 0.44 U catalase enzyme and semiquantified in the range up to ∼1.0 μmol. Glutathione can be detected in a two-step procedure ((1) oxidation of glutathione by hydrogen peroxide; (2) decomposition of the hydrogen peroxide residue by catalase).

Behavior of liquid in a thin type vessel irradiated with high-intensity aerial ultrasonic waves

A high-intensity aerial ultrasonic waves can remove liquid that has penetrated into the pores instantaneously. Further, tiny particles adhered to a solid surface is removed from the surface of object by radiation of the ultrasonic waves instantaneously. These techniques are implemented to a target by an action aerial ultrasonic waves directly. If these operations can be realized similarly even when the ultrasonic waves pass through the object, the field of application is expected to spread extremely. For example, if it is possible to exert a force to the liquid injected into a small container from the outside of the container by irradiating the aerial ultrasonic waves, various applications can be greatly expected. We have investigated the observations of liquid behavior when the high-intensity aerial ultrasonic waves were irradiated to the very small vessel filled with liquid. As a result, tiny bubbles were generated from the vicinity irradiated with the aerial ultrasonic waves, and the bubbles had a fierce tiny vibration. Further, in the observation of the liquid in the vessel by using ink, the ink is greatly diffused throughout the vessel and the behavior of the bubbles was confirmed to contribute to the diffusion of the ink extensively.

Effect of hydrodynamic cavitation in the tissue erosion by pulsed high-intensity focused ultrasound (pHIFU)

High-intensity focused ultrasound (HIFU) is emerging as an effective therapeutic modality in clinics. Besides the thermal ablation, tissue disintegration is also possible because of the interaction between the distorted HIFU bursts and either bubble cloud or boiling bubble. Hydrodynamic cavitation is another type of cavitation and has been employed widely in industry, but its role in mechanical erosion to tissue is not clearly known. In this study, the bubble dynamics immediately after the termination of HIFU exposure in the transparent gel phantom was captured by high-speed photography, from which the bubble displacement towards the transducer and the changes of bubble size was quantitatively determined. The characteristics of hydrodynamic cavitation due to the release of the acoustic radiation force and relaxation of compressed surrounding medium were found to associate with the number of pulses delivered and HIFU parameters (i.e. pulse duration and pulse repetition frequency). Because of the initial big bubble (~1 mm), large bubble expansion (up to 1.76 folds), and quick bubble motion (up to ~1 m s−1) hydrodynamic cavitation is significant after HIFU exposure and may lead to mechanical erosion. The shielding effect of residual tiny bubbles would reduce the acoustic energy delivered to the pre-existing bubble at the focus and, subsequently, the hydrodynamic cavitation effect. Tadpole shape of mechanical erosion in ex vivo porcine kidney samples was similar to the contour of bubble dynamics in the gel. Liquefied tissue was observed to emit towards the transducer through the punctured tissue after HIFU exposure in the sonography. In summary, the release of HIFU exposure-induced hydrodynamic cavitation produces significant bubble expansion and motion, which may be another important mechanism of tissue erosion. Understanding its mechanism and optimizing the outcome would broaden and enhance HIFU applications.

Microbubbles and Microparticles are Not Faithful Tracers of Turbulent Acceleration.

We report on the Lagrangian statistics of acceleration of small (sub-Kolmogorov) bubbles and tracer particles with Stokes number St≪1 in turbulent flow. At a decreasing Reynolds number, the bubble accelerations show deviations from that of tracer particles; i.e., they deviate from the Heisenberg-Yaglom prediction and show a quicker decorrelation despite their small size and minute St. Using direct numerical simulations, we show that these effects arise due the drift of these particles through the turbulent flow. We theoretically predict this gravity-driven effect for developed isotropic turbulence, with the ratio of Stokes to Froude number or equivalently the particle drift velocity governing the enhancement of acceleration variance and the reductions in correlation time and intermittency. Our predictions are in good agreement with experimental and numerical results. The present findings are relevant to a range of scenarios encompassing tiny bubbles and droplets that drift through the turbulent oceans and the atmosphere. They also question the common usage of microbubbles and microdroplets as tracers in turbulence research.

E&P Notes (July 2016)

E&P Notes Small Bubbles, Big Rewards for Separation Technology Reminiscent of the song made famous by late Hawaiian crooner Don Ho, tiny bubbles are the focal point of a new innovation aimed at transforming produced water from a costly byproduct into a valuable asset. Termed nanobubbles, they are several times smaller than a human red blood cell, which allows them to play with the physics of how dissolved gas interacts with liquids, according to Nano Gas Technologies. The suburban Chicago-based startup says its technology is capable of cheaply producing these nanobubbles to treat produced wastewater that is among the “worst of the worst.” The technology works by pushing gas, either oxygen or nitrogen, through a nozzle head that shoots the tiny bubbles into a treatment tank. The result is what the company’s chief executive officer, Len Bland, calls “fluffy water” that causes suspended solids to fall and oil to float to the top where it is easily skimmed off. “If you want to put it in technical terms,” he said, “we change the specific gravity of the liquid so that it enables the oil that is stuck in the water to rise.” Search for Flight MH 370 Shows Durability of AUVs The unprecedented search effort for Malaysian Airlines flight 370 has yet to achieve its main goal of locating the vanished aircraft and the 227 persons on board. However, it has served as an endurance test of sorts for offshore surveying systems such as the autonomous underwater vehicle (AUV), which just a couple of years ago was considered an emerging technology with a small track record. Edward Saade, president of the ocean surveying firm Fugro Pelagos, the company contracted by the Australian government to carry out much of the search operation, said he knows of no other commercial project where AUVs have been successfully deployed for such an extensive period of time. Speaking at the 2016 Offshore Technology Conference in Houston, Saade said that to cover large swaths of subsea terrain as quickly as possible and capture high-resolution data, Fugro first deployed vessels equipped with conventional deep-tow side scanning sonar. Then starting in January of last year, the AUV vessel was deployed from Perth. “The purpose of the AUV was to try and get into all those areas that turned out to be blocked in the side-scan operation, either by the steep slopes or other types of data anomalies,” Saade said. “We would preprogram the AUV, launch it and it would do its thing for about 24–28 hours and very effectively fill in all the gaps.”

A Milestone: Approval of CEUS for Diagnostic Liver Imaging in Adults and Children in the USA.

A Milestone: Approval of CEUS for Diagnostic Liver Imaging in Adults and Children in the USA.

Transition process leading to microbubble emission boiling on horizontal circular heated surface in subcooled pool

Microbubble emission boiling (MEB) produces a higher heat flux than critical heat flux (CHF) and therefore has been investigated in terms of its heat transfer characteristics as well as the conditions under which MEB occurs. Its physical mechanism, however, is not yet clearly understood. We carried out a series of experiments to examine boiling on horizontal circular heated surfaces of 5mm and of 10mm in diameter, in a subcooled pool, paying close attention to the transition process to MEB. High-speed observation results show that, in the MEB regime, the growth, condensation, and collapse of the vapor bubbles occur within a very short time. In addition, a number of fine bubbles are emitted from the collapse of the vapor bubbles. By tracking these tiny bubbles, we clearly visualize that the collapse of the vapor bubbles drives the liquid near the bubbles towards the heated surface, such that the convection field around the vapor bubbles under MEB significantly differs from that under nucleate boiling. Moreover, the axial temperature gradient in a heated block (quasi-heat flux) indicates a clear difference between nucleate boiling and MEB. A combination of quasi-heat flux and the measurement of the behavior of the vapor bubbles allows us to discuss the transition to MEB.

Aqueous dispersions of nanobubbles: Generation, properties and features

Nanobubbles (NBs) have interesting and peculiar properties such as high stability, longevity and high surface area per volume, leading to important applications in mining-metallurgy and environmental areas. NBs are also of interest in interfacial phenomena studies involving long-range hydrophobic attraction, microfluidics, and adsorption at hydrophobic surfaces. However, little data are available on effective generation of concentrated NBs water dispersions and on their physicochemical and interfacial properties. In this work, air was dissolved into water at pH 7 and different pressures, and a flow was depressurized through a needle valve to generate 150–200nm (mean diameter) NBs and MBs-microbubbles (about 70μm). Microphotographs of the NBs were taken only in the presence of blue methylene dye as the contrast medium. Main results showed that a high concentration of NBs (number per volume) was obtained by decreasing the saturation pressure and surface tension. The number of NBs, at 2.5bar, increased from 1.0×108NBmL−1 at 72.5mNm−1 to 1.6×109NBmL−1 at 49mNm−1 (100mgL−1 α-Terpineol). The NBs mean diameter and concentration only slightly varied within 14days, which demonstrates the high stability of these highly concentrated NBs aqueous dispersions. Finally, after the NBs were attached to the surface of a grain of pyrite (fairly hydrophobic mineral), the NBs dramatically increased the population of MBs, which shows the enhancement of particle hydrophobicity due to NBs adhesion. The results were explained in terms of interfacial phenomena and it is believed that these tiny bubbles, dispersed in water at high concentrations, will lead to cleaner and more sustainable mineral flotation.

Tiny Bubbles; Composite Cocktails for Medical Applications

Tiny bubbles or glass microspheres/microcapsules are used in our society today in many ways. The microspheres can be solid or hollow and represent particles that are generally in the range of 0.1–100 μm. Solid and hollow microspheres have significantly different densities, properties, and capabilities and are commercially available. Special morphologies, formulations, and compositions can also be achieved by a variety of techniques in the laboratory. One of the most interesting morphologies of these tiny bubbles, are porous wall hollow glass microspheres, which are glass micro‐balloons in which through‐wall porosity was induced in the thin outer shells and controlled, on a scale of 10–100 nm. This unique porosity can be used to fill the microspheres with a variety of cargos of various contents that can later be released on demand. For medical applications, an interdisciplinary team investigated these tiny bubbles, as part of a new series of composites or medical cocktails. The microspheres are key components in the composites and can be used alone or mixed in various amounts and ratios, with a variety of contents, outer coatings, and/or biocompatible matrices, resulting in a wide range of medical cocktails designed and tailored for various applications.

Ethanol based foamed asphalt as potential alternative for low emission asphalt technology

Abstract Foamed asphalt typically relies on water as a foaming agent because water becomes gaseous at elevated temperatures, generating numerous tiny bubbles in the asphalt and causing spontaneous foaming. In this study, ethanol was used as a potential alternative to water as a foaming agent. Ethanol is expected to be a physical blowing agent in the same manner as water, except it requires less energy to foam due to its 78 °C boiling point. This study compares the performance of water and ethanol as foaming agents through the measurements of rotational viscosity, the reduction in temperature during foaming, and volatile loss. The ethanol-foamed asphalt binders were prepared at 80 °C and 100 °C, while the water-foamed asphalt binders were prepared at 100 °C and 120 °C. Additionally, the rolling thin film oven (RTFO) was used to generate short-term aging of the foamed asphalt binders. A rotational viscometer was used to determine the viscosity of the asphalt binders at 80 °C, 100 °C, 120 °C, 140 °C, and 160 °C. Overall, ethanol can function in the same manner as water but requires less energy to foam. It is proven based on the smaller drop in temperature of the asphalt binder foamed using ethanol compared with that prepared with water. This is due to the lower latent heat capacity of ethanol, which requires less energy to vaporize compared with water. Through the rotational viscometer test, ethanol performs better in lowering the viscosity of asphalt binders, which is essential in allowing production processes at low temperatures, as well as a better workability and aggregate coating. Ethanol can be expelled from the foamed asphalt binders at a higher rate due to its lower boiling point and latent heat.

Acoustic response of bubbles inside a cylindrical cavitationbubble cluster generated by low-frequency ultrasound

An ultrasonic horn can radiate a strong ultrasonic wave into viscous liquid contained in a tank or cylindrical cup, and bubble clusters could be generated by the high-intensity ultrasound in the liquid. In the bubble clusters, interaction of bubbles exists because of the secondary radiation of bubbles. Therefore, the oscillations of bubbles are coupled. On the other hand, the surrounding liquid pressure of the bubbles in the cluster is influenced by the oscillations of the bubbles, which induces a pressure gradient on the boundary of the cluster. Therefore, the oscillation of a bubble inside a cluster is contracted by the formation of the cluster and its structure evolution. In this paper, a cylindrical cavitation bubble cluster is considered as a mixture drop of bubbles and liquids, and the motion of the cluster boundary is proposed with a second two-dimensional (2D) Rayleigh equation related to the difference between the inner mixture pressure and the outside liquid pressure on the boundary. Based on the bubble cluster boundary dynamical equation, a new mathematical model is developed to describe the motion of cavitation bubbles inside a cylindrical cluster when the effects of coupled oscillation are included. Comparing the new model equation with the Rayleigh-Plesset equation of single bubble in unbounded liquid, it is easy to draw the conclusion that the contraction of oscillating bubbles is strengthened by the coupled oscillation of bubbles and the boundary motion. In the cylindrical cluster, the oscillation of bubbles is suppressed, and the natural frequency of bubbles is reduced. The proposed model is used as a basis for the numerical investigation of the nonlinear acoustic response of bubbles. The suppression of the bubble oscillation is strengthened by increasing the number density of bubbles. Comparing numerical curves of the maximum radius of the oscillating bubble, it is shown that there are local peaks which are related to the resonance response of bubbles. In some unstable parameter regions, the maximum radius of the oscillating bubble varies sensitively with the tiny change of the parameters. The parameter space distribution of the unstable regions is related to the initial bubble radius and driving frequency of ultrasound. According to the numerical results related to the parameters, such as bubble number density, initial radius, driving frequency and pressure amplitude of ultrasound, it is found that the unstable acoustic response could be amplified for bubbles of smaller initial radius driven by a low-frequency ultrasound. For cavitation bubbles of initial radii ranging from 1 m to 10 m in low-frequency ultrasonic field, the unstable regions of parameter spaces related to the evolution of maximum radius become broader with the decrease of bubble initial radius and driving frequency of ultrasound. Therefore, the tiny bubbles inside cylindrical clusters have stronger nonlinear properties and the change of the parameters in the dynamical model equation has greater influence on the tiny bubbles.

圧力振動を用いた壁面近傍に生じる剪断波の実験的検討

We observed 2D flow birefringence profile near a vertical flat walls of an upside-down air tight quartz cell filled with CTAB/NaSal aqueous solution. When a tiny bubble is placed inside the top wall of the cell, an oscillating flow can be generated by pressing a rubber film at the bottom of the cell by a sinusoidal vibrating piston whose vibrating amplitude, frequency and center positon of the cell can be tuned precisely. As the bubble is getting contracting and expanding alternately, shear waves were observed clearly, which was propagated from the vertical wall using the 2D high speed polarization camera. In this study, a correlation of the propagated waves and propagation speed of the shear waves were studied and discussed experimentally.

気泡の崩壊時における気液界面構造の分子動力学解析

In our previous study, the phenomenon that a tiny vapor bubble re-appeared after its first collapse in a molecular scale was observed in a Molecular Dynamics (MD) simulation. In this study, we tried to elucidate the mechanism of the phenomenon in order to further examine whether we can express this phenomenon in continuum mechanics, and we reinvestigated a bubble collapse by molecular dynamics simulation. Here, we compared the liquid-vapor interfacial structure in the non-equilibrium state at a bubble collapse with the liquid-vapor interfacial structure in an equilibrium or a near equilibrium state. As a result, non-equilibrium characteristics were strong at the time of a bubble collapse, and the liquid-vapor interfacial structure was greatly different from that at equilibrium or near equilibrium condition. In addition, we found that the rebound of a bubble in the non-equilibrium state is the cause of becoming strong repulsive force between molecules in the liquid-vapor interfacial region.