Stable Gas Bubbles Research Articles

Generating Stable Nitrogen Bubble Layers on Poly(methyl methacrylate) Films by Photolysis of 2-Azidoanthracene.

2-Azidoanthracene (2N3-AN) can act as a photochemical source of N2 gas when dissolved in an optically transparent polymer such as poly(methyl methacrylate) (PMMA). Irradiation at 365 or 405 nm of a 150 μm-thick polymer film submerged in water causes the rapid appearance of a surface layer of bubbles. The rapid appearance of surface bubbles cannot be explained by normal diffusion of N2 through the polymer and likely results from internal gas pressure buildup during the reaction. For an azide concentration of 0.1 M and a light intensity of 140 mW/cm2, the yield of gas bubbles is calculated to be approximately 40%. The dynamics of bubble growth depend on the surface morphology, light intensity, and 2N3-AN concentration. A combination of nanoscale surface roughness, high azide concentration, and high light intensity is required to attain the threshold N2 gas density necessary for rapid, high-yield bubble formation. The N2 bubbles adhered to the PMMA surface and survived for days under water. The ability to generate stable gas bubbles "on demand" using light permits the demonstration of photoinduced flotation and patterned bubble arrays.

The role of cavitation and gas bubbles in the non-photochemical laser-induced nucleation of sodium acetate

Laser pulse nucleates anhydrous sodium acetate and stable gas bubbles from solution.

Thermodynamic Stability Analysis of Microbubbles Confined in a Liquid Droplet.

A thermodynamic stability analysis of microbubbles in an enclosed volume of a volatile liquid-noncondensable gas solution was performed. The stability conditions for the formation of the bubbles and the relationship between the equilibrium bubble radius and the equilibrium bubble number were investigated. The calculation results show that even if the saturation vapor pressure of a volatile liquid is higher than the pressure of the liquid phase, stable noncondensable gas bubbles appear in a closed system. Under such a condition, if the initial pressure of the noncondensable gas, which is proportional to the total number of moles of a noncondensable gas in the system, increases, stable bubbles appear at a lower temperature. At a fixed parameter of h0, which is defined as the ratio of the saturation vapor pressure of the volatile liquid minus the pressure in the liquid phase to the initial pressure of the noncondensable gas, the equilibrium bubble radius increases with a decrease of the equilibrium bubble number.

Stable Air Nanobubbles in Water: the Importance of Organic Contaminants.

Nanobubbles, surprisingly stable submicrometer gas bubbles in water, appear to be common in water and biological fluids and are of great interest in technical applications ranging from ultrasound contrast agents to flotation in the mining industry. Nanobubbles on surfaces have been more researched than freely floating bulk nanobubbles, and the reason for their stability appears to be better explained. The stability of bulk nanobubbles is less well explained, several theories exist, and even their existence is sometimes questioned. In the present study, an attempt was made to generate nanobubbles through hydrodynamic cavitation as well as through vigorous shaking in test tubes, and it was found that none of these methods generated a detectable concentration of possible bulk nanobubbles if pure water was used, with or without a small addition of NaCl, the equipment was cleaned properly, and certain plastic materials were avoided. These results indicate that trace organic contaminants are necessary for nanobubble stabilization. Experiments were also made with the dissolution of a high concentration of inorganic salts, which generated bubbles by creating air supersaturation. Light scattering submicron particles were found in all solutions and appeared to be actual gas bubbles in at least one case. However, in many cases, these light scattering particles were unaffected by vacuum and pressure and appear, therefore, to be something else other than air bubbles. It is concluded that, in future research on nanobubble stability, it is very important to avoid contamination, as well as to ascertain that light scattering objects really are bubbles and not oil droplets or particles.

Localization and characterization of a gas bubble stream at a Congo deep water seep site using a 3D gridding approach on single-beam echosounder data

Abstract This study investigates the geometry and temporal variation of a prominent gas bubble stream from a seafloor pockmark in the Lower Congo Basin. We present results of a 3D gridding approach of singlebeam echosounder water column data (Parasound 18 kHz) that increases the lateral resolution and improves the localization of anomalies beyond the inherent depth-dependent footprint size of the sonar system. This allows a detailed reconstruction of gas bubble trajectories through the water column, revealing a stable gas bubble stream for the deepest 1000 m. The bubbles are shifted laterally by ∼200 m in the transition zone between North Atlantic Deep Water and Antarctic Intermediate Water with a reconstructed current velocity of ∼4 cm/s. The direction of lateral shift varies throughout the ∼15 h survey possibly because of internal waves and small-scale eddies at the water mass boundary. Furthermore, a stacking of SBES water column data acquired during sampling stations showed a periodic variation in seepage intensity with periods of 6 and 8 min, which are likely related to temporally modulated gas supply to the sea floor. Singlebeam echosounder are commonly limited by their small coverage of the survey area during individual passes and their footprint size in detailed imaging of individual seep sites, especially in the deep sea. The presented gridding method for singlebeam echosounder data may facilitate the use of such data in deep water gas bubble stream identification and localization.

Optically and acoustically triggerable sub-micron phase-change contrast agents for enhanced photoacoustic and ultrasound imaging

We demonstrate a versatile phase-change sub-micron contrast agent providing three modes of contrast enhancement: 1) photoacoustic imaging contrast, 2) ultrasound contrast with optical activation, and 3) ultrasound contrast with acoustic activation. This agent, which we name ‘Cy-droplet’, has the following novel features. It comprises a highly volatile perfluorocarbon for easy versatile activation, and a near-infrared optically absorbing dye chosen to absorb light at a wavelength with good tissue penetration. It is manufactured via a ‘microbubble condensation’ method. The phase-transition of Cy-droplets can be optically triggered by pulsed-laser illumination, inducing photoacoustic signal and forming stable gas bubbles that are visible with echo-ultrasound in situ. Alternatively, Cy-droplets can be converted to microbubble contrast agents upon acoustic activation with clinical ultrasound. Potentially all modes offer extravascular contrast enhancement because of the sub-micron initial size. Such versatility of acoustic and optical ‘triggerability’ can potentially improve multi-modality imaging, molecularly targeted imaging and controlled drug release.

Hydrophobic forces as a key factor in crystalline biofilm formation on ureteral stents.

Current discussions about biofilm formation focus on the solid/liquid interface between a medical device and body fluids. Yet it has been shown that gas bubbles (GB) can stably form on ureteral stents in artificial urine and that their fate depends on the stent's surface properties. The liquid/gas interface constitutes an adhesion site for precipitating salts as well as hydrophobic organic molecules. The surface wettability of polyurethane stents is varied by coating with amorphous hydrogenated carbon (a-C:H). GB and crystalline biofilm formation on the stents are investigated in a novel encrustation device which avoids gravitation- or sample-position-related influences on the results. Bigger and more stable GB form on hydrophobic stents than on hydrophilic, coated stents. Appearance and amount of crystalline deposits differ significantly between the surfaces. With decreasing wettability the number of hollow crystalline spheres and the mass of precipitate increase. On hydrophobic surfaces, stable GB increase precipitation of salts and become incorporated in the growing encrustation layer in vitro. In contrast, GB quickly lift off from hydrophilic surfaces taking part of the precipitate with them. This self-cleaning mechanism slows down the encrustation process. A similar effect may explain the prolonged complication-free indwelling time of amorphous-carbon coated stents in vivo.

Electrochemistry of single nanobubbles. Estimating the critical size of bubble-forming nuclei for gas-evolving electrode reactions.

In this article, we address the fundamental question: "What is the critical size of a single cluster of gas molecules that grows and becomes a stable (or continuously growing) gas bubble during gas evolving reactions?" Electrochemical reactions that produce dissolved gas molecules are ubiquitous in electrochemical technologies, e.g., water electrolysis, photoelectrochemistry, chlorine production, corrosion, and often lead to the formation of gaseous bubbles. Herein, we demonstrate that electrochemical measurements of the dissolved gas concentration, at the instant prior to nucleation of an individual nanobubble of H2, N2, or O2 at a Pt nanodisk electrode, can be analyzed using classical thermodynamic relationships (Henry's law and the Young-Laplace equation - including non-ideal corrections) to provide an estimate of the size of the gas bubble nucleus that grows into a stable bubble. We further demonstrate that this critical nucleus size is independent of the radius of the Pt nanodisk employed (<100 nm radius), and weakly dependent on the nature of the gas. For example, the measured critical surface concentration of H2 of ∼0.23 M at the instant of bubble formation corresponds to a critical H2 nucleus that has a radius of ∼3.6 nm, an internal pressure of ∼350 atm, and contains ∼1700 H2 molecules. The data are consistent with stochastic fluctuations in the density of dissolved gas, at or near the Pt/solution interface, controlling the rate of bubble nucleation. We discuss the growth of the nucleus as a diffusion-limited process and how that process is affected by proximity to an electrode producing ∼1011 gas molecules per second. Our study demonstrates the advantages of studying a single-entity, i.e., an individual nanobubble, in understanding and quantifying complex physicochemical phenomena.

Electrochemical Nucleation of Stable N2 Nanobubbles at Pt Nanoelectrodes.

Exploring the nucleation of gas bubbles at interfaces is of fundamental interest. Herein, we report the nucleation of individual N2 nanobubbles at Pt nanodisk electrodes (6–90 nm) via the irreversible electrooxidation of hydrazine (N2H4 → N2 + 4H(+) + 4e(–)). The nucleation and growth of a stable N2 nanobubble at the Pt electrode is indicated by a sudden drop in voltammetric current, a consequence of restricted mass transport of N2H4 to the electrode surface following the liquid-to-gas phase transition. The critical surface concentration of dissolved N2 required for nanobubble nucleation, CN2,critical(s), obtained from the faradaic current at the moment just prior to bubble formation, is measured to be ∼0.11 M and is independent of the electrode radius and the bulk N2H4 concentration. Our results suggest that the size of stable gas bubble nuclei depends only on the local concentration of N2 near the electrode surface, consistent with previously reported studies of the electrogeneration of H2 nanobubbles. CN2,critical(s) is ∼160 times larger than the N2 saturation concentration at room temperature and atmospheric pressure. The residual current for N2H4 oxidation after formation of a stable N2 nanobubble at the electrode surface is proportional to the N2H4 concentration as well as the nanoelectrode radius, indicating that the dynamic equilibrium required for the existence of a stable N2 nanobubble is determined by N2H4 electrooxidation at the three phase contact line.

(Allen J. Bard Award) The Electrochemical Nucleation and Physical Behavior of Hydrogen Nanobubbles

Gas nanobubbles formed at solid/liquid interfaces have received significant attention during the past decade due to their remarkable properties. For instance, interfacial nanobubbles may exist for hours or days, in contrast to their expected short lifetime (< 1 s, based on the theoretical rate of gas dissolution from a nanoscale bubble). We have developed an electrochemical approach for investigating the formation and properties of a single nanobubble of H2 with a radius between 5 and 50 nm. To create a H2 nanobubble, a Pt nanodisk electrode, shrouded in a glass sheath, is used to reduce H+ in a concentrated acid solution, creating a supersaturated solution of H2 adjacent to the electrode surface. As the electrode potential is scanned towards negative potentials, the current arising from H2 generation increases exponentially and then suddenly decreases to near background levels, signaling a liquid-to-gas phase transformation associated with the formation of a single nanobubble at the electrode surface. The critical concentration of dissolved H2 required for nanobubble nucleation, obtained from the electrochemical response, is measured to be ~0.25 M, corresponding to ~310-fold supersaturation of H2. We estimate the critical size of the smallest stable H2 gas bubble nucleus to be ~4 nm; this nucleus grows and covers the Pt nanodisk surface until it becomes pinned at the Pt/glass interface. Thermodynamic relations indicate that the internal pressure within these nanobubbles is of the order of 100 atm. The nanobubble experiments also provide insight into the structure and chemical dynamics of electrochemical three-phase solid/liquid/gas boundaries. We demonstrate that a steady-state residual current of ~100 pA, measured after bubble formation, results from continuous electrogeneration of H2 at the Pt/electrolyte/H2 gas interface; steady-state reduction of H+ is required to balanced the rate of H2 dissolution from the nanobubble into the solution. These experiments allow unique measurements of H+ reduction rates at a three-phase boundary, as well as estimation of the width of the three-phase boundary. We also report additional experiments that demonstrate that the stability and shape of H2 nanobubbles are sensitive to the applied pressure and to the presence of surfactants. Finally, we show that individual H2 bubbles can be electrochemically nucleated within nanopores.

Formation of toroidal bubbles from acoustic droplet vaporization

Acoustic droplet vaporization (ADV) is the selective vaporization of liquid microdroplets using ultrasound to produce stable gas bubbles. ADV is the primary mechanism in an ultrasound based cancer therapy, called gas embolotherapy, where the resulting bubbles are used to create localized occlusions leading to tumor necrosis. In this investigation, early time scale events including phase change are directly visualized using ultra-high speed imaging. Modulating elevated acoustic pressure or pulse length resulted in toroidal bubbles. For sufficiently short pulses (4 cycles at 7.5 MHz), toroidal bubble formation could be avoided, regardless of acoustic pressures tested.

Microstructural changes in sclera under thermo‐mechanical effect of 1.56 µm laser radiation increasing transscleral humor outflow

Pores in the sclera are a candidate pathway for aqueous transport and therefore can be utilized to decrease the intraocular pressure (IOP) in glaucomatous eyes. Since pore formation is a well-known mechanism for stress relaxation in solids, laser-induced creation of pores in cartilage increases hydraulic permeability and promotes tissue regeneration. The aim of this paper is to demonstrate the thermo-mechanical effect of non-destructive laser irradiation on microstructural changes in sclera, in particular pore formation, resulting in substantial increase of water permeability of eye tissues that can be a novel approach to normalize the IOP. Experiments were performed ex vivo on eight eyes of four mini-pigs and in vivo on eight eyes of four rabbits using pulse repetitive laser radiation of 1.56 µm in wavelength. Twenty laser spots of 0.6 mm in diameter with laser settings (power 0.9 W, pulse duration of 200 milliseconds, pulse repetition rate of 2 Hz) resulting in substantial increase of sclera hydraulic permeability were applied on the sclera at 1-2 mm from the eye limb. Sclera and underlying structures (choroid and ciliary body) of the rabbits' eyes were examined histologically in 1 and 45 days after laser irradiation, atomic force microscope (AFM) was applied before and after laser irradiation. Histological and AFM examinations have clearly recognized laser-assisted stable structural alterations: rarefication of the collagen structure in the laser irradiated zone and formation of sub-micron pores. Laser-induced alterations in the structure of ciliary bodies were small in size and mainly reversible. We have proposed a possible mechanism of the arising pores stabilization due to formation of small stable gas bubbles in sclera tissue. It is shown, for the first time, that thermo-mechanical effect of pulse repetitive laser irradiation results in pores formation in sclera. That can be a basis of a novel, safe, and effective technique for IOP normalization due to enhancing of uveoscleral outflow under non-destructive laser irradiation of the sclera.

Bubston structure of water and aqueous solutions of electrolytes

Theoretical concepts of the existence of stable gas bubbles (bubstons) in pure water and aqueous solutions of electrolytes that are in equilibrium with an external gas medium have been developed. A theoretical model of ion adsorption on the water surface is proposed, and the double electric layer forming due to this absorption is quantitatively described within this model. These results allowed us to make a thermodynamic description of the bubston structure in the water-external gas medium system. It is shown that the formation of this structure at certain temperatures and concentrations of dissolved impurity ions is a first-order phase transition. The unique role of helium as an external gas medium in the problem under consideration is established: bubston structures in this case do not arise at any initial ion concentrations, whereas the helium solubility increases with temperature. The mechanism of formation for experimentally observed bubston clusters is considered.

Irradiation behavior of ground U(Mo) fuel with and without Si added to the matrix

In the framework of the IRIS-TUM irradiation program, several full size, flat dispersion fuel plates containing ground U(Mo) fuel kernels in an aluminum matrix, with and without addition of silicon (2.1 wt.%), have been irradiated in the OSIRIS reactor. The highest irradiated fuel plate (with an Al–Si matrix) reached a local maximum burnup of 88.3% 235U LEU-equivalent and showed a maximum thickness increase of 323 μm (66%) but remained intact. This paper reports the post irradiation examination results obtained on four IRIS-TUM plates. The evolution of the fission gas behavior in this fuel type from homogeneously dispersed nanobubbles to the eventual formation of large but apparently stable fission gas bubbles at the interface of the interaction layer and the fuel kernel is illustrated. It is also shown that the observed moderate, but positive effect of Si as inhibitor for the U(Mo)–Al interaction is related to the dispersion of this element in the interaction layer, although its concentration is very inhomogeneous and appears to be too low to fully inhibit interaction layer growth.

Generalizations of the Young–Laplace equation for the pressure of a mechanically stable gas bubble in a soft elastic material

The Young-Laplace equation for the pressure of a mechanically stable gas bubble is generalized to include the effects of both surface tension and elastic forces of its surroundings. The latter are taken to be comprised of a soft isotropic material. Generalizations are derived for conditions of constant external pressure and constant system volume. The derived equations are formally exact for a spherical bubble surrounded by a spherical shell of isotropic material, provided that the bubble is sufficiently large for the surface tension to be treated macroscopically, and that the bubble radius is much larger than the thickness of the bubble/soft material interface. The underlying equations are also used to derive a simple expression for the Gibbs free energy of deformation of an elastic medium that surrounds a gas bubble. The possible relevance of this expression to some recently published ideas on decompression sickness ("the bends") is discussed.

Drag Reduction by Dimples on Surfaces in Plane–Plane Contact Lubrication

An experimental system was designed to evaluate the drag reduction by dimples on surfaces in plane–plane contact lubrication. Because of the dimples on the surfaces, in some of the experiments, it was observed that stable gas bubbles existed in the lubricant film. The results of the experiments showed that the existence of gas was decided by the depth of dimples and the volume of lubricant supply; the deeper the dimples were and the less the lubricant supply, the higher the probability of gas existence. The reason for the existence of gas, based on a model analysis, was the meniscus force of the gas–liquid interface, which caused the gas to be enclosed in the dimples. When a small amount of lubricant was supplied, the friction force was reduced because of a mixed lubrication film of oil and gas. For different amounts of lubricant supply, an optimal depth of dimples existed in plane–plane contact lubrication, which could obviously reduce the friction force. An optimization model for full film lubrication was presented to determine the optimal depth of dimples. Based on the model, the optimal dimple depth is approximately proportional to the distance between the two surfaces. The theoretical analysis agreed with the experiments.

Towards Aberration Correction of Transcranial Ultrasound Using Acoustic Droplet Vaporization

We report on the first experiments demonstrating the transcranial acoustic formation of stable gas bubbles that can be used for transcranial ultrasound aberration correction. It is demonstrated that the gas bubbles can be formed transcranially by phase-transitioning single, superheated, micron-size, liquid dodecafluoropentane droplets with ultrasound, a process known as acoustic droplet vaporization (ADV). ADV was performed at 550 kHz, where the skull is less attenuating and aberrating, allowing for higher-amplitudes to be reached at the focus. Additionally, it is demonstrated that time-reversal focusing at 1 MHz can be used to correct for transcranial aberrations with a single gas bubble acting as a point beacon. Aberration correction was performed using a synthetic aperture approach and verified by the realignment of the scattered waveforms. Under the conditions described below, time-reversal aberration correction using gas bubbles resulted in a gain of 1.9 ± 0.3 in an introduced focusing factor. This is a small fraction of the gain anticipated from complete transmit-receive of a fully-populated two-dimensional array with sub-wavelength elements. (E-mail: khaworth@umich.edu)

Low-temperature irradiation behavior of uranium–molybdenum alloy dispersion fuel

Irradiation tests have been conducted to evaluate the performance of a series of high-density uranium–molybdenum (U–Mo) alloy, aluminum matrix dispersion fuels. Fuel plates incorporating alloys with molybdenum content in the range of 4–10 wt% were tested. Two irradiation test vehicles were used to irradiate low-enrichment fuels to approximately 40 and 70 at.% 235U burnup in the advanced test reactor at fuel temperatures of approximately 65 °C. The fuel particles used to fabricate dispersion specimens for most of the test were produced by generating filings from a cast rod. In general, fuels with molybdenum contents of 6 wt% or more showed stable in-reactor fission gas behavior, exhibiting a distribution of small, stable gas bubbles. Fuel particle swelling was moderate and decreased with increasing alloy content. Fuel particles with a molybdenum content of 4 wt% performed poorly, exhibiting extensive fuel–matrix interaction and the growth of relatively large fission gas bubbles. Fuel particles with 4 or 6 wt% molybdenum reacted more rapidly with the aluminum matrix than those with higher-alloy content. Fuel particles produced by an atomization process were also included in the test to determine the effect of fuel particle morphology and microstructure on fuel performance for the U–10Mo composition. Both of the U–10Mo fuel particle types exhibited good irradiation performance, but showed visible differences in fission gas bubble nucleation and growth behavior.

Observation and quantification of gas bubble formation on a mechanical heart valve.

Clinical studies using transcranial Doppler ultrasonography in patients with mechanical heart valves (MHV) have detected gaseous emboli. The relationship of gaseous emboli release and cavitation on MHV has been a subject of debate in the literature. To study the influence of cavitation and gas content on the formation and growth of stable gas bubbles, a mock circulatory loop, which employed a Medtronic-Hall pyrolytic carbon disk valve in the mitral position, was used. A high-speed video camera allowed observation of cavitation and gas bubble release on the inflow valve surfaces as a function of cavitation intensity and carbon dioxide (CO2) concentration, while an ultrasonic monitoring system scanned the aortic outflow tract to quantify gas bubble production by calculating the gray scale levels of the images. In the absence of cavitation, no stable gas bubbles were formed. When gas bubbles were formed, they were first seen a few milliseconds after and in the vicinity of cavitation collapse. The volume of the gas bubbles detected in the aortic track increased with both increased CO2 and increased cavitation intensity. No correlation was observed between O2 concentration and bubble volume. We conclude that cavitation is an essential precursor to stable gas bubble formation, and CO2, the most soluble blood gas, is the major component of stable gas bubbles.

Heavy Oil Flow Under Solution-gas Drive: Pressure Depletion Tests

Abstract The formation of foamy oil has been postulated to be an important factor contributing to the success in primary production of heavy oil under solution-gas drive in Alberta and Saskatchewan. This paper presents the results of pressure depletion or blowdown tests on unconsolidated oil sand cores which were conducted to investigate the foamy oil flow phenomenon. Experimental results suggest that foamy oil flow is likely dominant between the bubble point and the critical gas saturation for fast tests (one-hour step duration), but may be much less important as free gas is produced via gas channels at gas saturation greater than the critical saturation. Foamy oil flow is also a function of step duration, and for slow tests (greater than 24-hour step duration), foamy oil flow may be absent. Tests also reveal that the total bitumen recovery was extremely high, ranging from 35 - 50%, and no obvious temperature effects were observed. Both features are perhaps caused by high pressure decline rates resulting from the step pressure decline. Whether foamy oil flow occurs or not, the solution-gas drive mechanisms in heavy oil reservoirs are similar to those in conventional oil. The liberation of solution gas and the expansion of free gas are the main pressure maintenance sources below the bubble point. Introduction In the primary production of heavy oil, the drive mechanism is generally believed to be solution-gas drive. It is also an important mechanism for cyclic steam stimulation. A better understanding of heavy oil reservoir performance under solution-gas drive is essential to both processes. The concept of foamy oil flow has been used to explain the anomalous performance of heavy oil reservoirs under solution-gas drive(1,2). Foamy oil flow refers to the simultaneous flow of gas bubbles and oil in a pseudo-continuous phase. Ward et al.(3) illustrated theoretically that in a closed volume, the size of the stable gas bubbles formed by a pressure drop depends upon the number of gas bubbles. The diameter of gas bubbles could be as small as 3 µm and as many as ten million bubbles could be formed in one cubic centimetre of live oil. This hypothesis prompted Smith(1) to postulate that gas bubbles could perhaps flow through the pore throat, unaffected by capillary forces. He assumed that the flow of the oil-gas mixture in the reservoir could be approximated by a single-phase flow. The average parameters for this single-phase mixture flow were estimated from the individual parameters of bitumen and gas. He reported that this single-phase model successfully predicted the pressure response observed in Lloydminster heavy oil reservoirs. Sarma and Maini et al.(4,5) studied experimentally the role of gas nucleation in the primary production of heavy oil under solution- gas drive. They concluded that the nucleation of gas bubbles was unlikely to increase the mobility of heavy oil, and that the heavy oil-gas mixture was flowing in the form of an oil-continuous foam. Poon and Kisman(6) investigated the effects of non-Newtonian fluid on the primary production of heavy oil.