Formation Of Cavitation Bubbles Research Articles

Modified structure for reduction of cavitation at fluid dynamic bearing of HDD spindle motor under non-operating axial vibration

A hard disk drive (HDD) is subjected to the axial vibration while it is transported long distance or stored in vibrating environments. The cavitation may occur in the lubricant film of the spindle thrust bearing due to the axial vibration. The cavitation bubble may push the lubricating oil out of the bearing clearance so that it may be resulted in system failure of the HDD. The formation of cavitation bubble is studied for a specially designed fluid dynamic thrust bearing (FDTB) for the HDD. A novel design method for the FDTB is proposed, which suppresses cavitation effectively.

A Eulerian–Lagrangian description of cavitating flow

We propose a method of cavitating flow calculation that is based on the Eulerian–Lagrangian description of multiphase flows. The formation and growth of cavitation bubbles are described using the model of heterogeneous volume nucleation. Results of test calculations demonstrate the efficiency of the proposed model.

Synchrotron radiographic studies of ultrasonic melt processing of metal matrix nano composites

Fast synchrotron radiography was used to investigate ultrasonic cavitation bubble formation and their dynamics during liquid metal processing of Al–Cu metal matrix nano composites (MMNC) in comparison with conventional alloys. The experimental observations showed enhanced cavitation potential in MMNC melts, due to the presence of Al2O3 nano particles which were believed to be acting as heterogeneous nuclei for bubble formation. Quantitative image analysis demonstrates that the addition of nano particles increases melt agitation partially, while introducing higher flow velocity variations across the melt. This suggests that the presence of nano particles may substantially alter propensity for ultrasonic treatment effects during solidification processing of MMNCs.

In situ observation and analysis of ultrasonic capillary effect in molten aluminium

An in situ synchrotron radiographic study of a molten Al–10wt% Cu alloy under the influence of an external ultrasonic field was carried out using the Diamond-Manchester Branchline pink X-ray imaging at the Diamond Light Source in UK. A bespoke test rig was used, consisting of an acoustic transducer with a titanium sonotrode coupled with a PID-controlled resistance furnace. An ultrasonic frequency of 30kHz, with a peak to peak amplitude at 140microns, was used, producing a pressure output of 16.9MPa at the radiation surface of the 1-mm diameter sonotrode.This allowed quantification of not only the cavitation bubble formation and collapse, but there was also evidence of the previously hypothesised ultrasonic capillary effect (UCE), providing the first direct observations of this phenomenon in a molten metallic alloy. This was achieved by quantifying the re-filling of a pre-existing groove in the shape of a tube (which acted as a micro-capillary channel) formed by the oxide envelope of the liquid sample. Analytical solutions of the flow suggest that the filling process, which took place in very small timescales, was related to micro-jetting from the collapsing cavitation bubbles. In addition, a secondary mechanism of liquid penetration through the groove, which is related with the density distribution of the oxides inside the groove, and practically to the filtration of aluminium melt from oxides, was revealed. The observation of the almost instantaneous re-filling of a micro-capillary channel with the metallic melt supports the hypothesised sono-capillary effect in technologically important liquids other than water, like metallic alloys with substantially higher surface tension and density.

Targeted microbubbles: a novel application for the treatment of kidney stones.

Kidney stone disease is endemic. Extracorporeal shockwave lithotripsy was the first major technological breakthrough where focused shockwaves were used to fragment stones in the kidney or ureter. The shockwaves induced the formation of cavitation bubbles, whose collapse released energy at the stone, and the energy fragmented the kidney stones into pieces small enough to be passed spontaneously. Can the concept of microbubbles be used without the bulky machine? The logical progression was to manufacture these powerful microbubbles ex vivo and inject these bubbles directly into the collecting system. An external source can be used to induce cavitation once the microbubbles are at their target; the key is targeting these microbubbles to specifically bind to kidney stones. Two important observations have been established: (i) bisphosphonates attach to hydroxyapatite crystals with high affinity; and (ii) there is substantial hydroxyapatite in most kidney stones. The microbubbles can be equipped with bisphosphonate tags to specifically target kidney stones. These bubbles will preferentially bind to the stone and not surrounding tissue, reducing collateral damage. Ultrasound or another suitable form of energy is then applied causing the microbubbles to induce cavitation and fragment the stones. This can be used as an adjunct to ureteroscopy or percutaneous lithotripsy to aid in fragmentation. Randall's plaques, which also contain hydroxyapatite crystals, can also be targeted to pre-emptively destroy these stone precursors. Additionally, targeted microbubbles can aid in kidney stone diagnostics by virtue of being used as an adjunct to traditional imaging methods, especially useful in high-risk patient populations. This novel application of targeted microbubble technology not only represents the next frontier in minimally invasive stone surgery, but a platform technology for other areas of medicine.

Cavitation Bubble Measurement in Tribological Contacts Using Digital Holographic Microscopy

The use of advanced measurement techniques such as the digital holography method described in this paper improves the understanding of the cavitation phenomenon in tribological contacts such as the sliding contact of the piston ring–cylinder liner conjunction. The paper describes the use of digital holography measurement technique to measure cavitation bubble formation and thickness. The position of observed lubricant film rupture preceding the cavitation region is compared with some commonly predicted boundary often used with Reynolds equation. The experimental results indicate that the Reynolds and Elrod boundary conditions are the most suitable for the investigated sliding contact conditions.

The Impact of Acoustic Intensity on Solution Parameters and Decellularization Using Sonication Treatment

A combination method using sonication and chemical treatment to decellularize tissue sample is described. The objective of the study is to determine the acoustic intensity and solution parameters modulating decellularization process. The acoustic intensity distribution of sonication system was first demonstrated by simulation and followed by experimental measurement. The effect of acoustic intensity with solution parameters including dissolved oxygen (DO) concentration, pH, and conductivity was observed using 170 kHz system. From both simulation and experimental results, it is demonstrated that acoustic intensity effects were localised as high cell removal recorded at the center of irradiation. Furthermore, our study demonstrated strong correlation between acoustic intensity with solution parameters. The study indicates positive correlation of DO concentration and pH of SDS detergent with acoustic intensity and negative correlation between conductivity and acoustic intensity. The results further supported by the complete removal of DNA with preservation of extracellular matrix (ECM), collagen and elastin content. In conclusion, the effect of acoustic intensity is localised and influenced decellularization. Our study also indicates strong correlation between acoustic intensity and solution parameters as we suggested the formation of cavitation bubble derived the decellularization process.

Supporting of pristine TiO2 with noble metals to enhance the oxidation and mineralization of paracetamol by sonolysis and sonophotolysis

Pharmaceutical residues in fresh water bodies are of significant environmental concern due to their undesirable effects on aquatic organisms at very low concentrations. The present study is about the elimination and overall degradation of a common anti-inflammatory pharmaceutical-paracetamol (PCT) in water by catalytic oxidation using high-frequency ultrasound, UV-irradiation (254nm) and both. The catalysts were synthesized sonolytically by the reduction of Pd and Au to the zero-valent state followed by immobilization of the nanoparticles on the surface of commercial TiO2 (P-25) to improve the activity of the semiconductor under ultrasonic and UV irradiation. It was found that sonolytic oxidation of PCT during catalysis with Pd–TiO2 and Pd/Au–TiO2 was very effective owing to the significantly smaller size of these particles (than that of Au–TiO2), which promoted the formation and collapse of cavitation bubbles on their surface and the incidence of solute contact with the active sites. The addition of persulfate further enhanced the rate of PCT decay, signifying the role of ultrasound in activating the reagent and that of radical-initiated oxidation reactions in the decomposition of the parent compound. The activity of P-25 was low and almost equal under sonolysis and photolysis, but considerably higher under sono-photolysis, where the unfavorable properties of the semiconductor were minimized via combined effects of the two processes. Hybridization also induced a considerable synergy in carbon-mineralization, which was relatively low with single processes. The results were attributed to minimization of electron-hole pair recombination, surface corrosion and competition for photons and radical species. Au-supported particles exhibited higher activity in the presence of UV-irradiation than ultrasound, which was attributed to the unique activity of gold in the UV–vis region. Finally, Pd–TiO2 was found to be the most stable nanocomposite in terms of the ease of recovery and the efficiency of reuse.

Molecular dynamics simulation on cavitation bubble formation in canonical ensemble

Research on cavitation is very significant for preventing cavitation erosion and for making use of bubbles effectively. Characteristics of cavitation in canonical ensemble are studied by molecular dynamics simulation. Effects of temperature and numerical density on cavitation are analyzed. Comparison with lattice Boltzmann method is also conducted. Simulation results indicate that the temperature and numerical density may affect cavitation remarkably. The formation of cavitation bubbles becomes unstable as the temperature increases, and even hard to occur. A lower numerical density makes cavitation bubble form easier. Moreover, as numerical density reduces, the temperature effect on cavitation becomes less.

Cavitation erosion and hydroabrasion resistance of cold work tool steels produced by powder metallurgy

Carbide-rich cold work tool steels produced by powder metallurgy are designed for applications that are exposed to heavy abrasive wear. Alloying with higher amounts of chromium and molybdenum additionally leads to corrosion resistance. This makes them excellent candidates for use in fluid flow systems with a high abrasive load, e.g. due to liquids containing sand particles. In this study, two different cold work tool steels were examined with focus on wear attack that may occur in combination with flowing liquids. On one hand, hydroabrasive attack results from fluids containing abrasives such as sand. This was simulated using a slurry pot containing a mixture of water and SiO2 particles. On the other hand, changes in flow speed may cause formation of cavitation bubbles that lead to erosive wear of the surfaces of nearby materials. Thus, despite the fact that resistance to surface spalling (e.g. by cavitation) was not a primary goal of alloy development, both alloys were also investigated by means of ultrasonic cavitation testing. The aim was to detect the influences of alloying system, processing route, and heat treatment condition on the wear resistance. The results indicate a positive influence of retained austenite on cavitation resistance, whereas the exact opposite holds true in the case of hydroabrasion. The damage mechanisms were analyzed by means of optical as well as scanning electron microscopy with special focus on the role of carbides during cavitation. Further attention was paid to the manufacturing route by comparing commonly used hot isostatic pressing (HIP) with supersolidus liquid phase sintering (SLPS). The latter permits consolidation of pre-alloyed powders to near net-shape parts or to thick protective layers on lower alloyed substrates. Compared to HIP, SLPS promises lower manufacturing costs combined with comparable mechanical properties.

Effect of multiphoton ionization on performance of crystalline lens.

This Letter presents a model for propagation of a laser pulse in a human crystalline lens. The model contains a transverse beam diffraction effect, laser-induced optical breakdown for the creation of plasma via a multiphoton ionization process, and the gradient index (GRIN) structure. Plasma introduces the nonlinearity in the crystalline lens which affects the propagation of the beam. The multiphoton ionization process generates plasma that changes the refractive index and hence leads to the defocusing of the laser beam. The Letter also points out the relevance of the present investigation to cavitation bubble formation for restoring the elasticity of the eyes.

To Sonicate or Not to Sonicate PM Filters: Reactive Oxygen Species Generation Upon Ultrasonic Irradiation

In aerosol research, a common approach for the collection of particulate matter (PM) is the use of filters in order to obtain sufficient material to undertake analysis. For subsequent chemical and toxicological analyses, in most of cases the PM needs to be extracted from the filters. Sonication is commonly used to most efficiently extract the PM from the filters. Extraction protocols generally involve 10 - 60 min of sonication. The energy of ultrasonic waves causes the formation and collapse of cavitation bubbles in the solution. Inside the collapsing cavities the localised temperatures and pressures can reach extraordinary values. Although fleeting, such conditions can lead to pyrolysis of the molecules present inside the cavitation bubbles (gases dissolved in the liquid and solvent vapours), which results in the production of free radicals and the generation of new compounds formed by reactions with these free radicals. For example, simple sonication of pure water will result in the formation of detectable levels of hydroxyl radicals. As hydroxyl radicals are recognised as playing key roles as oxidants in the atmosphere the extraction of PM from filters using sonication is therefore problematic. Sonication can result in significant chemical and physical changes to PM through thermal degradation and other reactions. In this article, an overview of sonication technique as used in aerosol research is provided, the capacity for radical generation under these conditions is described and an analysis is given of the impact of sonication-derived free radicals on three molecular probes commonly used by researchers in this field to detect Reactive Oxygen Species in PM.

Anomalous Behavior of Cavitation Bubbles Observed in Pulsed Laser Ablation of Ni in Liquid CO<sub>2</sub> near the Critical Point

Pulsed laser ablation of Ni targets in liquid CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> near the critical point (C.P.) (P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">crit</sub> = 7.38 MPa, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">crit</sub> = 304.1 K) was performed and the time evolution of the plasma and the ensuing cavitation bubble analyzed by shadowgraph imaging. Formation of double-layered anomalous cavitation bubbles was observed inside the liquid phase close to the C.P. The size and propagation speed of the cavitation bubbles were measured and it was found that in the experimental range investigated (P = 7.20-7.40 MPa, T = 301-303 K), the size of the cavitation bubble increased by a factor of up to three for increasing pressure and temperature, whereas the propagation speed decreased.

Multiscale model of cavitation bubble formation and breakdown

Cavitation damage is responsible for initial pitting of kidney stone surfaces, damage that is thought to play an important role in shock wave lithotripsy. We introduce a multiscale model of the formation of cavitation bubbles in water, and subsequent breakdown. At a macroscopic, continuum scale cavitation is modeled by the 3D Euler equations with a Tait equation of state. Adaptive mesh refinement is used to provide increased resolution at the liquid/vapor boundary. Cells with both liquid and vapor phases are flagged by the continuum solver for mesoscale, kinetic modeling by a lattice Boltzmann description capable of capturing non-equilibrium behavior (e.g., phase change, energetic jet impingement). Isolated and interacting two-bubble configurations are studied. Computational simulation results are compared with high-speed experimental imaging of individual bubble dynamics and bubble–bubble interaction. The model is used to build a statistical description of multiple-bubble interaction, with input from cavitation cloud imaging. [Work supported by NIH through 5R37DK052985-18.]

Spherical gold nanoparticles and core/shell microparticles under intense femtosecond laser excitation: relaxation dynamics of gold nanoparticles and nanostructuring of borosilicate glass using microparticles

This paper reports surface nanostructuring of borosilicate glass covered with a water layer and the production of diameter pits using core/shell microparticles under excitation with pulses using the optical scheme of an inverted microscope with a objective. We compare the thresholds for hole formation in glass with the use of and uncoated microparticles. The threshold is for and for microparticles, which coincides with the threshold for nanostructuring by a focused femtosecond pulse without microparticles: . Femtosecond pump – probe spectroscopy has been used to study the relaxation dynamics of laser pulse energy absorbed in a Au nanoparticle and the dynamics of energy dissipation to the ambient medium. The threshold for cavitation bubble formation in water with has been determined to be , which is a factor of 30 lower than the bubble formation threshold in the case of uncoated microparticles.

Cavitation erosion damage of scroll steel plates by high-speed gas working fluid

A steel plate is one of the critical components of a scroll expander system that can experience cavitation micro-pitting while in service. The content of the present paper consists of two distinct but interrelated parts. The first part aims to highlight that the use of computational fluid dynamics (CFD) simulations can constitute a potential tool for the prediction of cavitation erosion areas in scroll expander systems. For this purpose, a three-dimensional CFD, steady-state numerical simulation of the refrigerant working fluid is employed. Numerical results revealed the critical areas where cavitation bubbles are formed. These numerical critical areas are in direct qualitative agreement with the actual eroded regions by cavitation, which were found by microscopic observations across the steel plate on an after use, scroll expander system. The second part of the paper aims to further investigate the behaviour and the durability of the steel plate of the studied scroll expander system subjected to cavitation erosion by using an ultrasonic experimental test rig. Scanning electron microscopy and optical interferometer micrographs of the damaged surfaces were observed, showing the nature of the cavitation erosion mechanism and the morphological alterations of the steel plate samples. Experimental results are explained in terms of the cavitation erosion rates, roughness profile, accumulated strain energy, and hardness of the matrix. The experimental study can serve as a valuable input for future development of a CFD numerical model that predicts both cavitation bubbles formation as well as cavitation damage induced by the bubbles that implode on the steels plates.

RETRACTED ARTICLE: Grain refinement of AA5754 aluminum alloy by ultrasonic cavitation: Experimental study and numerical simulation

In this work, an experimental investigation was carried out on the grain refinement of molten AA5754 Aluminum alloy through ultrasonic treatment. The cavitation induced heterogeneous nucleation was suggested as the major mechanism for grain refinement in the AA5754 aluminum alloy. A numerical simulation was performed to predict the formation, growth and collapse of cavitation bubbles in the molten AA5754 Aluminum alloy. Moreover, the acoustic pressure distribution and the induced acoustic streaming by ultrasonic horn reactor were investigated. It is suggested that the streaming by ultrasonic could transport the small bubbles formed in the ultrasonic cavitation zone into the bulk of melt rapidly. These micro-bubbles are collapsed due to acoustic vibrations where the resulting micro-jets are strong enough to break the oxide layer and to wet the impurities. These exogenous particles, intermetallics and oxides could contribute to the formation of fine, uniform and equiaxed microstructure across the treated melt. The experimental results confirmed the simulation predictions.

Size-controlled synthesis of graphite nanoflakes and multi-layer graphene by liquid phase exfoliation of natural graphite

Multi-layer graphene and graphite nanoflakes were produced through graphite liquid exfoliation using organic solvents. The nanoflakes size distribution was statistically analyzed, with the number of measured samples being high enough (from ∼200 to 900) for reliable evaluation of the statistical model. The nanoflakes size data were found to follow a log-normal distribution, with higher fraction of large size flakes as compared to a conventional normal distribution. The same kind of distribution was also obtained for nanoflakes thickness. Based on these findings, the detailed mechanism of the pristine polycrystalline graphite exfoliation in a liquid phase due to formation and collapse of cavitation bubbles was discussed. The high quality of nanoflakes was confirmed by Raman spectroscopy.

Cavitation Measurement during Sonic and Ultrasonic Activated Irrigation

IntroductionThe aims of this study were to quantify and to visualize the possible occurrence of transient cavitation (bubble formation and implosion) during sonic and ultrasonic (UAI) activated irrigation. MethodsThe amount of cavitation generated around several endodontic instruments was measured by sonochemiluminescence dosimetry inside 4 root canal models of human dimensions and varying complexity. Furthermore, the spatial distribution of the sonochemiluminescence in the root canal was visualized with long-exposure photography. ResultsInstrument oscillation frequency, ultrasonic power, and file taper influenced the occurrence and amount of cavitation. In UAI, cavitation was distributed between the file and the wall extending beyond the file and inside lateral canals/isthmuses. In sonic activated irrigation, no cavitation was detected. ConclusionsCavitation was shown to occur in UAI at clinically relevant ultrasonic power settings in both straight and curved canals but not around sonically oscillating instruments, driven at their highest frequency.

Instantaneous and ensemble average cavitation structures in Diesel micro-channel flow orifices

Cavitation developing upstream and inside the micro-channel orifices of transparent multi-hole fuel injector nozzles has been characterised using a high speed visualisation system. Images have been obtained with short exposure time and sufficiently high spatial and temporal resolution, freezing the formation of cavitation bubbles and their further development during every single injection cycle. Post processing of statistically large number of images collected from many successive injection events and numerous identical nozzles has allowed for the first time estimation of the ensemble average cavitation image and its standard deviation. The instantaneous images reveal the formation of a variety of complex and interacting two-phase flow regimes. Vapour bubbles have been found to exist inside the nozzle prior to start of injection. These bubbles originate from the previous injection cycle as they have not been evacuated from the nozzle and hence, they remain trapped, altering the flow of the subsequent injection event. During the opening and closing stages of the needle valve that controls the fuel flow through the nozzle, cavitation is found to form in the valve’s seat area. Subsequently, vortex or ‘string’ cavitation has been recorded to take place in a rather chaotic manner; its life time and most probable location of appearance have been estimated. The ensemble average images reveal the probability of cavitation appearance at a specific location within the nozzle and the micro-channel flow orifice. The standard deviation from the mean reveals locations with significant cycle-to-cycle variations of the flow. These are linked to significant deviations from the mean of the fuel spray dispersion angle forming downstream of the nozzle exit.