Cavitation In Liquids Research Articles

Description of ultrasound transducers through wave parameters

High energy ultrasonic waves cause cavitation in liquids. Since many years, a great number of low frequency cavitation effects is well known such as ultrasonic cleaning and biological cell disruption. Recently the interest in high frequency applications has been rising considerably, e.g. in the formation of peroxide in destilled water or the cleaning of wafers. These and other applications require efficient and reliable high frequency transducers. Therefore we are working on the design and optimization of transducers with frequencies of about 100 kHz up to 3 MHz. According to the specific demands of this process, computer simulations are being used in the course of the optimization. The Finite Elements Method, for example, is applied to investigate the forms of vibration. In addition to this, Network Analysis Methods are used to optimize the power transformation and the band width. By these means an average efficiency of about 80 percent is reached. These transducers are developed for laboratories as well as for technical scale applications.

Cavitation phenomena in magnetic liquids

Investigations of acoustic cavitation in magnetic liquids subjected to the action of inhomogeneous magnetic fields were carried out for the first time. Possible mechanisms of the effect of the fields on the dynamics of single bubbles, cavitation threshold, and cavitation-induced erosion are considered. The effect of intense ultrasound vibrations on samples of magnetic liquids with a kerosene base are studied experimentally with and without gradient magnetic fields. It is shown that for magnetic liquids prepared with the optimum content of stabilizing substance and subjected to centrifugation in fields of 6·103g the action of powerful oscillations does not affect adversely their performance properties and structure, which supports the use of magnetic liquids as technological media. An increase-induced in cavitation-induced erosion of test samples after application of a gradient magnetic field is revealed.

Electrical matching of power transducers for acoustic cavitation applications

Over the last 20 years, a large number of studies have been devoted to acoustic cavitation in liquids. High power transducers are needed to generate cavitation. A transducer working at 500 kHz will be presented. This transducer has been electrically matched using a stainless steel layer welded to a piezoelectric element. Optimal thickness of the matching layer, which is considered as a transmission line, has been found using a Mason’s circuit. The transducer conversion efficiency is experimentally evaluated to determine the acoustic pressure amplitude needed to generate cavitation in a liquid. The cavitation noise recorded with a hydrophone is used to characterize the cavitation state in the fluid. The energy of this noise is correlated to the acoustic pressure amplitude. Then the correlation between cavitation noise energy and chemical reactivity in homogeneous chemistry is also made. The bio-effect of cavitation on E. Coli cells is presented as a function of cavitation noise energy. It is evidenced that cavitation noise energy is an efficient tool to characterize a cavitation state in a fluid and to predict its chemical and bacteriological effects.

Liquid-jet impact on liquid and solid surfaces

Experiments were conducted to investigate the mechanisms of the liquid-jet impact on liquid and solid surfaces associated with cavitation damage and rain erosion. In this work, a liquid jet of 3 mm diameter, generated using a single impact jet apparatus, was impacted at ca. 600 m s −1 on the surface of water and polymethyl-methacrylate (PMMA) placed 15 mm below the nozzle exit. The various phenomena which occurred were photographed using an image-converter high-speed camera. These include (i) the shock wave generated at the moment of the liquid-jet impact on the surface, (ii) shock waves reflected at the liquid/air or solid/air interfaces, as release waves, (iii) the damage to the PMMA, (iv) the overlap of release waves on the central axis, causing central damage in PMMA and cavitation in liquids. The cavitation behaviour and the process of the damage is directly related to the behaviour of the compressive wave caused by the impact of the liquid jet.

Classification of lubricants according to cavitation criteria

Cavitation in lubrication liquids has long been known to be detrimental to components in hydraulic systems. Damage has been detected in journal bearings, especially under severe dynamic loading, gears, squeeze film dampers and valves. These findings have led to intensive studies of metal resistance to cavitation erosion, in order to minimize the damage. Results of these studies have been: 1. (a) classification of known materials according to their resistance to cavitation erosion; 2. (b) development of new materials and processes to increase their durability. One of the main achievements in this respect was the establishment of the ASTM G32-92 Standard Method of Vibratory Cavitation Erosion Test. However, very little was done with respect to the liquid phase, e.g. the lubricants. As a consequence there is no standard procedure for testing of lubricants for their cavitation properties and no relevant specifications in national and international standards. This study includes theoretical and experimental investigations. The theoretical approach examines the lubricant in elastohydrodynamically lubricated (EHL) contacts. Using numerical simulations, based on Reynolds equation and elastic deformation theory, the pressure profile and film shape have been computed. It is further investigated how the operating conditions affect the properties, e.g. “cavitation energy” of zones of sub-ambient pressure values and if a correlation between these results and cavitation erosion criteria can be found. The experimental approach includes testing of 20 liquid lubricants, belonging to the following four groups: mineral oils, mineral-based oils, bio degradable oils and synthetic oils. Testing was performed by vibrating a standard aluminium tip in each oil and periodically recording the gravimetric results. These results enabled the classification of the lubricants according to their cavitance, which is inversely proportional to the mass of solid material eroded by a cavitating liquid under controlled conditions. The results of both approaches can be combined into an engineering tool in the future. This tool may serve the designer to improve the use of existing lubricants and the lubrication industry as an aid for the development of new lubricants with increased cavitance in hydraulic systems.

Dynamics of Laser Induced Morphological Changes of Liquids Part I. Cavitation and Explosive Vaporization of Liquids.

Intense laser irradiation to liquids can induce morphological changes such as cavitation, plume ejection, and so on with a distinct laser fluence threshold. The phenomena are classified into three types in viewpoints of their fundamental mechanisms. When a liquid is transparent to laser light, breakdown followed by plasma formation brings about cavitation. When IR laser light excites molecular vibration of a liquid, explosive vaporization is induced. A morphological change of a liquid can be induced also by UV laser light which excites liquid molecules electronically. In this review of Part I, the history and outline of the former two phenomena are summarized, and the third morphological change of a liquid will be described in Part II.

The propagation of ultrasonic waves through a bubbly liquid into tissue: a linear analysis

The steady-state response induced by a harmonically driven, ultrasonic wave in a structure comprised of two layers, the first a bubbly liquid, and the second a viscoelastic solid with a rigid boundary, is studied in the linear approximation. This structure is intended to model a cavitating liquid in contact with tissue. The upper surface of the liquid is driven harmonically and models the source. The lower surface of the solid is rigid and models the bone. While cavitation is inherently nonlinear, the propagation process is approximated as linear. The transient response is not calculated. The model of the bubbly liquid is a simple continuum one, supplemented by allowing for a distribution of different equilibrium bubble radii and for the relaxation of the oscillations of each bubble. The model contains three functions, the probability distribution describing the distribution of bubble radii, and two functions modeling the mechanical response of the individual bubble and the tissue, respectively. Numerical examples are worked out by adapting data taken from various published sources to deduce the parameters of these functions. These examples permit an assessment of the overall attenuation of the structure, and of the magnitudes of the pressure and particle velocity in the bubbly liquid and of the traction and the particle displacement in the tissue. >

Transient, high-pressure solidification associated with cavitation in water

The very high pressures (≳1 GPa) that occur during the final stages of collapse of a cavitation bubble force the water in the vicinity of the bubble wall briefly (∼1 ns) into a metastable state of subcooling, relative to the equilibrium phase diagram. Estimates show that the subcooling can fall below the critical temperature for homogeneous nucleation of freezing and that high-pressure ice particles form at a sufficient rate to affect the collapse. Because of the greater density of high-pressure ice, a sudden drop in pressure occurs that triggers a shock wave that converges at the center of the compressed gas in the bubble. Such microshocks are believed to be the cause of the extremely short duration of the flashes of sonoluminescence (<50 ps) that have been observed from single cavitation bubbles. The occurrence of transient, high-pressure solidification can explain different phenomena associated with cavitation, specifically the decrease in cavitation erosion and the increase in sonoluminescence as the overall water temperature approaches 0 °C, together with the nucleation of freezing by cavitation in subcooled liquids. A single explanation for such diverse effects provides strong support for the solidification hypothesis.

A general theory for the formation of a new phase. Static cavitation in a nonvolatile liquid

The rate of formation of cavities in a nonvolatile liquid subjected to negative pressure is calculated. The pressure is assumed to be sufficiently moderate, so that the critical bubble (hole) can be treated macroscopically. If this condition is satisfied one may apply the Zel'dovich saddle-point method. However, in distinction from that method, the present paper makes use of a construction of Gibbs grand canonical ensemble, which allows one to avoid the ambiguities that appear in the calculation of the microscopic initial stage of formation of the hole and yields a more accurate value of the kinetic (preexponential) factor. A more accurate value of the work required to form a critical hole, the value which occurs in the exponent, is obtained by taking into account the dependence of the surface tension of the hole on the curvature of the surface. For this purpose we use an expansion of the surface tension in powers of the curvature of the surface of the hole. Since the surface tension occurs in the exponent of the expression for the probability or rate of cavitation, it is necessary to retain the expansion terms which are linear or quadratic in the curvature, whereas the correction factors to the cavitation rate, due to higher-order terms tend to unity as the expansion of the liquid decreases, in contradistinction from the first terms. It is important that for large expansions, and hence for small curvatures of the surface of the critical hole, the series for the surface tension diverges.

The influence of water air content on cavitation erosion in distilled water

The influence of increased air content of the cavitating liquid (distilled water) was studied in rotating disk test rig. A rise in the total air content (including dissolved and entrained air) of the water in the undersaturated range resulted in bubble collapse cushioning and reduction of cavitation damage. When the water was oversaturated with air, large air bubbles formed and cavitation damage was drastically reduced, probably due to both bubble collapse cushioning and shock wave attenuation.

Multidimensional Theory of Homogeneous Boiling of Pure Liquids

The stationary rate of homogeneous formation of bubbles, which are described by two variables, the radius and the inner pressure, is obtained. The problem is considered on the basis of multidimensional stochastic equations. It is shown that the diffusion tensor for this problem appears to be an auxiliary value. It (or the diffusion coefficient for one-dimensional treatment) can be calculated, as has been done previously by different authors, but it does not have independent significance when equations of time dependence for the variables and their equilibrium distribution are known. The result obtained is valid for a whole range of variation of the viscosity η of a liquid. For the case of cavitation of a nonviscous liquid, the theory developed here corrects the previous result of Zeldovich (Acta Physicochim., USSR, 18, 1 (1943)), which gives an infinite rate for the process when η → 0. Certain corrections in the equation for bubble growth due to evaporation of the liquid were obtained. A new expression for the equilibrium distribution function of bubbles is also proposed.

Ultrasound cavitation in sonochemistry: Decomposition of carbon tetrachloride in aqueous solutions of potassium iodide

An introductory description of the main phenomena connected with ultrasonic induced cavitation is given, which mainly describes the motion of microbubbles present in a cavitating liquid. Then, the sonolysis of carbon tetrachloride is described in aqueous solutions of potassium iodide. Yields of free iodine are given for several experimental conditions of the ultrasonic power and irradiation time.

Cavitation and Tension in Liquids. By D. H. TREVENA. Adam Hilger, 1987. 125 pp. £22.50

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Martin Greenspan's work in acoustical physics

Martin (Moe) Greenspan's long career at the National Bureau of Standards began in 1936 in the general area of the strength of engineering materials and structures. His theory of stress distribution in perforated plates was an important contribution to modern linear elastic fracture mechanics. In 1947 he began to work in physical acoustics and continued in this until 1987. His landmark accomplishments include the theoretical analysis and measurement of sound propagation in rarefied gases, development of an instrument for measurement of sound velocity in sea water, and determination of the important physical influences leading to cavitation in liquids by ultrasound. In recent years, he worked on methods for measuring accurately the acoustical emissions (stress waves) generated by physically stressed materials of engineering. The influences of his work on sound in rarefied gases persist today; the database he obtained in the late 1940s–1950s was far in advance of the analytical capabilities for sound propagation in rarefied gases at that time. The data are still used for evaluation of new theoretical developments. Moe's work has always been characterized by an excellent coordination between theoretical‐mathematical development and laboratory experiment.

A model of bubble cavitation in a real liquid

Present-day notions about the mechanism of the evolution of a cavitation cluster, i.e., a swarm of minute vapor-gas bubbles, in a sound field (e.g., in the focal region of a "concentrator" or acoustic velocity transformer) are based on the instability of the shape of the bubbles during their explosionlike expansion and rapid collapse (implosion). It is assumed that instability induces bubble disintegration and, hence, an avalanche-type multiplication of cavitation centers [I]. It has been postulated, on the basis of an analysis of high-speed motion pictures of the process [i], in particular, that the number of bubbles in the visible cavitation zone can be many orders of magnitude greater than the number of initial cavitation nuclei as determined, e.g., according to Gavrilov's procedure [2]. Experiments indicate that this effect is dynamic: During the first few periods after application of the sound field, the number of bubbles is consistent with the number of nuclei expected according to the state of the liquid, but then it increases and arrives at a steady state, which is governed by the characteristics of the field and the liquid [i]~

Applications of ultrasound to organic chemistry

Ultrasound promotes efficient mixing in solid-liquid and liquid-liquid mixtures and it has also been used in several phase transfer catalysed reactions, e.g. N-alkylation of amines, synthesis of ethers and esters and hydrolysis of esters. In this Paper preliminary results are presented which support the view that the presence of oxygen in a cavitating liquid generates singlet oxygen in addition to peroxy radicals.

Observation of electrical effects with cavitating liquid flow

Observations have been made under several different circumstances of the increased development of streaming potential with cavitating flow of high purity water. Two separate mechanism for this increase are proposed and shown to be consistence with the observations. The magnitude of these electrical effects and previous work with non-cavitating flow suggests that these effects could be important to erosion-corrosion problems with cavitating flow.

Acoustic cavitation in low-temperature liquids

This paper is a review of acoustic cavitation in liquid helium, hydrogen, nitrogen and other low-temperature liquids. Cavitation nuclei and the tensile strength of these liquids are considered. The dynamics of cavities and bubble growth thresholds in the acoustic field are analysed. Experimental and theoretical results are discussed.

Rectified heat transfer in vapor bubbles

Some experimental findings on the dynamics of vapor bubbles in ultrasonic fields are reviewed. In particular, attention is drawn to a phenomenon which might be described as ultrasonically stimulated boiling. There is also a summary of certain pertinent facts about the threshold of cavitation in liquids with low foreign gas content, such as liquid nitrogen or liquid helium. It is emphasized that threshold pressure amplitudes can be less than the static pressure. It is proposed that an explanation of these observations could lie in a net positive heat transfer into a pulsating bubble. The minimum amplitude required to cause such a ‘‘rectified’’ heat transfer is calculated.

Free Radical Generation by Ultrasound in Aqueous and Nonaqueous Solutions

The physical principles underlying the oscillatory behavior of minute gas bubbles in liquids exposed to ultrasound are reviewed. Results from mathematical analyses suggest that these oscillations sometimes become unstable leading to transient cavitation in which a bubble violently collapses during a single acoustic half-cycle producing high temperatures and pressures. The role that micronuclei, resonant bubble size, and rectified diffusion play in the initiation of transient cavitation is explained. Evidence to support these theoretical predictions is presented with particular emphasis on sonoluminescence which provides some non-chemical evidence for the formation of free radicals. Acoustic methods for conducting sonochemical investigations are discussed. In aqueous solutions transient cavitation initially generates hydrogen atoms and hydroxyl radicals which may recombine to form hydrogen and hydrogen peroxide or may react with solutes in the gas phase, at the gas-liquid boundary or in the bulk of the solution. The analogies and differences between sonochemistry and ionizing radiation chemistry are explored. The use of spin trapping and electron spin resonance to identify hydrogen atoms and hydroxyl radicals conclusively and to detect transient cavitation produced by continuous wave and by pulsed ultrasound is described in detail. The study of the chemical effects of cavitation in organic liquids is a relatively unexplored area which has recently become the subject of renewed interest. Examples of the decomposition of solvent and solute, of ultrasonically initiated free-radical polymerization and polymer degradation are presented. Spin trapping has been used to identify radicals in organic liquids, in polymer degradation and in the decomposition of organometallic compounds.