Ultrasound cavitational collapse

Maximum disruption is obtained in a zone close to the probe tip and the biological cells must be kept here for sufficient time to allow disruption to take place. A delicate balance must therefore be struck between the power of the probe and the disruption rate since power ultrasound, with its associated cavitational collapse energy and bulk heating effect, can denature the contents of the cell once released. Indeed for this type of usage it is important to keep the cell sample cool during sonication. The method is very effective and continues to be an important tool in microbiology and biochemistry research. 

Cavitation is the formation of gaseous cavities in a medium upon ultrasound exposure. The primary cause of cavitation is ultrasound-induced pressure variation in the medium. Cavitation involves either the rapid growth and collapse of a bubble (inertial cavitation) or the slow oscillatory motion of a bubble in an ultrasound field (stable cavitation). Collapse of cavitation bubbles releases a shock wave that can cause structural alteration in the surrounding tissue [13]. Tissues contain air pockets trapped in the fibrous structures that act as nuclei for cavitation upon ultrasound exposure. The cavitational effects vary inversely with ultrasound frequency and directly with ultrasound intensity. Cavitation might be important when low-frequency ultrasound is used, when gassy fluids are exposed, or when small gas-filled spaces are exposed. 

When a liquid-solid interface is subjected to ultrasound, cavitation still occurs, but with major changes in the nature of the bubble collapse. If the surface is significantly larger than the cavitating bubble (% 100 pm at 20 kHz), spherical implosion of the cavity no longer occurs, but instead there is a markedly asymmetric collapse which generates a jet of liquid directed at the surface, as seen directly in high speed... 

Depending on the particular type of bubbles, ultrasound cavitation can be transient or stable. In the transient type, also known as inertial cavitation, bubbles are either voids or vapour bubbles, which are believed to be produoed by intensities above 10 W/cm. They exist for one, or at most a few aooustic cycles, and expand to a radius of at least twice their initial size before collapsing abruptly on oompression and often disintegrating into small bubbles. The smaller bubbles formed can act as nuclei for further bubbles or, if their radius is sufficiently small, they can simply dissolve into the bulk solution under the aotion of the very large surface tension forces present. The lifetime of transient bubbles is believed to be too short to allow any mass flow by diffusion of gas into or out of the bubbles by contrast, evaporation and condensation of liquid are believed to ocour freely. In the absence of gas to cushion the implosion, the bubbles will collapse highly abruptly. 

The effect of ultrasound is ascribed to promotion of cavitation, which is the rapid generation and collapse of microbubbles within the medium this cavitational collapse results in dramatic pressure and thermal differentials on a microscopic scale, which accelerate mass transport and enhance energy transfer [16]. The enhanced mass transport has also been used to increase the sensitivity of voltammetric analysis [17]. Besides the enhanced mass transport, heating and interfacial cleaning due to the asymmetric collapse of the bubbles at the solid/liquid interface may influence electrolysis. 

We will conclude this survey of the synthesis of nanomaterials by sonochemical methods by mentioning that the most important material of the last decade, carbon nanotubes, were also synthesized by ultrasound radiation [144]. The carbon nanotube is produced by applying ultrasound to liquid chlorobenzene with ZnCl2 partides and to o-dichlorobenzene with ZnCh and Zn particles. It is considered that the polymer and the disordered carbon, which are formed by cavitational collapse in homogeneous liquid, are annealed by the inter-particle collision induced by the turbulent flow and shockwaves. 

A very important point occurs in the transmission of acoustic power into a liquid which is termed the cavitation threshold. When very low power ultrasound is passed through a liquid and the power is gradually increased, a point is reached at which the intensity of sonication is sufficient to cause cavitation in the fluid. It is only at powers above the cavitation threshold that the majority of sonochemical effects occur because only then can the great energies associated with cavitational collapse be released into the fluid. In the medical profession, where the use of ultrasonic scanning techniques is widespread, keeping scanning intensities below the cavitation threshold is of vital importance. As soon as the irradiation power used in the medical scan rises above this critical value, cavitation is induced and, as a consequence, unwanted even possibly hazardous chemical reactions may occur in the body. Thus, for both chemical and medical reasons there is a considerable drive towards the determination of the exact point at which cavitation occurs in liquid media, particularly in aqueous systems. Historically, therefore, the determination of the cavitation threshold was one of the major drives in dosimetry. 

A practically useful approach is to compare the rate constants of a reaction with and without ultrasound. Such studies (Lorimer et al., 1991) show that the rate constant with ultrasound us is not equal to the sum of the rate constants without ultrasound k and the rate constant associated with cavitational collapse bub. that is. 

A major factor in ultrasound-induced processes is the presence of cavitation both in the bulk solution and at interfaces. The phenomenon caused by voids or gas bubbles in the solution phase being coupled to the oscillating pressure field, is responsible for hot spot processes and microjetting. There are different types of cavitation, notably stable cavitation (violently oscillating bubbles), or transient cavitation (collapsing bubbles) [34]. The ultrasound frequency and intensity determine the type and violence of the process. Cavitation occurs more readily in the vicinity of the electrode surface... 

Ultrasound is particularly effective in surface decontamination where the cleaning action induced by cavitational collapse near a surface will... 

Because of the extreme conditions during a cavitation event, radicals can be formed. Several parameters affect cavitation and thereby the polymerization reaction, since the radical formation rate is directly influenced by the cavitational collapse. The number of radicals formed due to sonification is a function of the number of cavities created and the number of radicals that are formed per cavitation bubble. The bubble wall velocity during collapse and the hot-spot temperature determine the rate at which radicals are formed, both inside and outside a single bubble. These two parameters depend on the physical properties of the liquid as well as on the physical and chemical processes occurring around the cavity. The most important properties and processes occurring in a cavitation bubble are depicted schematically in Figure 21.10. The number of cavities is determined, for instance, by the impurities in the liquid, the static pressure, the ultrasound intensity, and the vapor pressure. This emphasizes the complexity of the influences on the overall... 

The use of ultrasonic (US) radiation (typical range 20 to 850 kHz) to accelerate Diels-Alder reactions is undergoing continuous expansion. There is a parallelism between the ultrasonic and high pressure-assisted reactions. Ultrasonic radiations induce cavitation, that is, the formation and the collapse of microbubbles inside the liquid phase which is accompanied by the local generation of high temperature and high pressure [29]. Snyder and coworkers [30] published the first ultrasound-assisted Diels-Alder reactions that involved the cycloadditions of o-quinone 37 with appropriate dienes 38 to synthesize abietanoid diterpenes A-C (Scheme 4.7) isolated from the traditional Chinese medicine, Dan Shen, prepared from the roots of Salvia miltiorrhiza Bunge. 

Abstract Acoustic cavitation is the formation and collapse of bubbles in liquid irradiated by intense ultrasound. The speed of the bubble collapse sometimes reaches the sound velocity in the liquid. Accordingly, the bubble collapse becomes a quasi-adiabatic process. The temperature and pressure inside a bubble increase to thousands of Kelvin and thousands of bars, respectively. As a result, water vapor and oxygen, if present, are dissociated inside a bubble and oxidants such as OH, O, and H2O2 are produced, which is called sonochemical reactions. The pulsation of active bubbles is intrinsically nonlinear. In the present review, fundamentals of acoustic cavitation, sonochemistry, and acoustic fields in sonochemical reactors have been discussed. 

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