Cavitation ultrasound frequency

When the ultrasound frequency used changes, the following factors would change (1) the temperature and pressure inside the collapsing cavitation bubbles, (2) the number and distribution of bubbles, (3) the size and lifetime of bubbles, (4) the... 

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. 

Since cavitational effects in fluids vary inversely with ultrasound frequency, it is likely that cavitational effects should play an even more important role in low-frequency sonophoresis. Tachibana et al. hypothesized that application of low-frequency ultrasound results into acoustic streaming in the hair follicles and sweat ducts of the skin, thus leading to enhanced transdermal transport. Mitragotri et al. hypohesized that transdermal transport during low-frequency sonophoresis occurs across the keratinocytes rather than hair follicles. They provided the following hypothesis for the higher efficacy of low-frequency sonophoresis. 

Recently Perusich and Alkire [105] have proposed a mathematical model to determine the reaction and transport between liquid microjets and a reactive solid surface. Conditions were established under which oxide depassivation and repassivation occurs as a function of ultrasonic intensity, surface film thickness, and fluid microjet surface coverage. The model was based on the concept that cavitation induces sufficient momentum and mass transfer rates (water hammer pressures as described earlier) at a surface to create oxide film stresses leading to depassivation. The model was used to evaluate experimental data on the corrosion behavior of iron in sulfuric acid [106,107], Focused ultrasound was used to investigate processes that influence depassivation and repassivation phenomena on pure and cast iron in 2N H2S04 at two ultrasound frequencies and at power intensities of up to 7.8 kW/cm2. 

Fig. 2 The cavitation process growth and collapse of a microbubble as a function of time and ultrasound frequency. (View this art in color at www. dekker.com.)...

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 Frequency. The frequency of ultrasoimd has a significant effect on the cavitation process. At very high frequencies (>1 MHz), the cavitation effect is reduced as the inertia of a cavitation bubble becomes too high to react to fast changing pressures. Most ultrasoimd-induced reactions are therefore carried out at frequencies between 20 and 900 kHz. The optimum ultrasoimd effect as a function of frequency depends on the reaction system eg, water dissociation has an optimum frequency at approximately 500 kHz (21). For bulk pol5mierizations the maximum radical formation rate is obtained at 20 kHz. At this frequency the highest strain rates are produced, which results in a high radical formation rate by polymer scission (22). 

Keywords Ultrasound Acoustic cavitation Cavitation bubbles Ultrasound frequency Bubble temperature Sonochemistry... 

Fig. 1.2 Numerically simulated frequency spectra of the hydrophone signal due to acoustic cavitation noise. The driving ultrasound is 515 kHz in frequency and 2.6 bar in pressure amplitude, (a) For stable cavitation bubbles of 1.5 pm in ambient radius, (b) For transient cavitation bubbles of 3 pm in ambient radius. Reprinted from Ultrasonics Sonochemistry, vol. 17, K. Yasui, T. Tuziuti, J. Lee, T. Kozuka, A. Towata, and Y. lida, Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles, pp. 460-472, Copyright (2010), with permission from Elsevier...

In the sonochemical reactors, selection of suitable operating parameters such as the intensity and the frequency of ultrasound and the vapor pressure of the cavitating media is an essential factor as the bubble behavior and hence the yields of sonochemical transformation are significantly altered due to these parameters. It is necessary that both the frequency and intensity of irradiation should not be increased beyond an optimum value, which is also a function of the type of the application and the equipment under consideration. The liquid phase physicochemical properties should be adjusted in such a way that generation of cavitation events is eased and also large number of smaller size cavities are formed in the system. 

Tatake PA, Pandit AB (2002) Modeling and experimental investigation into cavity dynamics and cavitational yield Influences of dual frequency ultrasound sources. Chem Eng Sci 57 4987 1995... 

Moholkar et al. [11] studied the effect of operating parameters, viz. recovery pressure and time of recovery in the case of hydrodynamic cavitation reactors and the frequency and intensity of irradiation in the case of acoustic cavitation reactors, on the cavity behavior. From their study, it can be seen that the increase in the frequency of irradiation and reduction in the time of the pressure recovery result in an increment in the lifetime of the cavity, whereas amplitude of cavity oscillations increases with an increase in the intensity of ultrasonic irradiation and the recovery pressure and the rate of pressure recovery. Thus, it can be said that the intensity of ultrasound in the case of acoustic cavitation and the recovery pressure in the case of hydrodynamic cavitation are analogous to each other. Similarly, the frequency of the ultrasound and the time or rate of pressure recovery, are analogous to each other. Thus, it is clear that hydrodynamic cavitation can also be used for carrying out so called sonochemical transformations and the desired/sufficient cavitation intensities can be obtained using proper geometric and operating conditions. 

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

Sonochemistry is strongly affected by a variety of external parameters, including acoustic frequency, acoustic intensity, bulk temperature, static pressure, choice of ambient gas, and choice of solvent. These are important considerations in the effective use of ultrasound to influence chemical reactivity, and are also easily understandable in terms of the cavitational hot-spot mechanism. A summary of these effects is given in Table II. 

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