Cavitation stable

There are two types in acoustic cavitation. One is transient cavitation and the other is stable cavitation [14, 15]. There are two definitions in transient cavitation. One is that the lifetime of a bubble is relatively short such as one or a few acoustic cycles as a bubble is fragmented into daughter bubbles due to its shape instability. The other is that bubbles are active in light emission (sonoluminescence (SL)) or chemical reactions (sonochemical reactions). Accordingly, there are two definitions in stable cavitation. One is that bubbles are shape stable and have a long lifetime. The other is that bubbles are inactive in SL and chemical reactions. There exist... 

In Fig. 1.1, the parameter space for transient and stable cavitation bubbles is shown in R0 (ambient bubble radius) - pa (acoustic amplitude) plane [15]. The ambient bubble radius is defined as the bubble radius when an acoustic wave (ultrasound) is absent. The acoustic amplitude is defined as the pressure amplitude of an acoustic wave (ultrasound). Here, transient and stable cavitation bubbles are defined by their shape stability. This is the result of numerical simulations of bubble pulsations. Above the thickest line, bubbles are those of transient cavitation. Below the thickest line, bubbles are those of stable cavitation. Near the left upper side, there is a region for bubbles of high-energy stable cavitation designated by Stable (strong nf0) . In the brackets, the type of acoustic cavitation noise is indicated. The acoustic cavitation noise is defined as acoustic emissions from... 

Fig. 1.1 The regions for transient cavitation bubbles and stable cavitation bubbles when they are defined by the shape stability of bubbles in the parameter space of ambient bubble radius (R0) and the acoustic amplitude (p ). The ultrasonic frequency is 515 kHz. The thickest line is the border between the region for stable cavitation bubbles and that for transient ones. The type of bubble pulsation has been indicated by the frequency spectrum of acoustic cavitation noise such as nf0 (periodic pulsation with the acoustic period), nfo/2 (doubled acoustic period), nf0/4 (quadrupled acoustic period), and chaotic (non-periodic pulsation). Any transient cavitation bubbles result in the broad-band noise due to the temporal fluctuation in the number of bubbles. Reprinted from Ultrasonics Sonochemistry, vol. 17, K.Yasui, T.Tuziuti, J. Lee, T.Kozuka, A.Towata, and Y. Iida, Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles, pp. 460-472, Copyright (2010), with permission from Elsevier...

From Fig. 1.1, it is seen that stable cavitation bubbles are tiny bubbles of a few pm in ambient radius or relatively large bubbles of about 10 pm or more in radius at 515 kHz. The range of ambient radius for transient cavitation bubbles becomes... 

Apart from the classification based on the mode of generation of cavities, cavitation can also be classified as transient cavitation and stable cavitation [3]. The classification is based on the maximum radius reached (resonant size), life time of cavity (which decides the extent of collapse) in the bulk of liquid and the pattern of cavity collapse. Generation of transient or stable cavitation usually depends on the set of operating parameters and constitution of the liquid medium. Depending on the specific application under question, it is very important to select particular set of operating conditions such that maximum effects are obtained with minimum possible energy consumption. 

The bubble formed in stable cavitation contains gas (and very small amount of vapor) at ultrasonic intensity in the range of 1-3 W/cm2. Stable cavitation involves formation of smaller bubbles with non linear oscillations over many acoustic cycles. The typical bubble dynamics profile for the case of stable cavitation has been shown in Fig. 2.3. The phenomenon of growth of bubbles in stable cavitation is due to rectified diffusion [4] where, influx of gas during the rarefaction is higher than the flux of gas going out during compression. The temperature and pressure generated in this type of cavitation is lower as compared to transient cavitation and can be estimated as ... 

Fig. 2.3 Radius and pressure profiles in the case of Stable Cavitation (Typical profile at frequency of irradiation = 300 kHz, intensity of irradiation = 5 W/m2 and initial radius of the nuclei = 5 mm)...

It has been generally observed that the mechanical effects due to cavitational events are more responsible for the microbial disinfection and the chemical and heat effects play only a supporting role [56]. Microstreaming resulting from stable cavitation has been shown to produce stresses, sufficient to disrupt cell membranes... 

In addition, it should be noted that the experimental setup is different between the 20 kHz and the high frequency systems. Therefore, the type of the cavitation would be different between them. It has been reported that the 20 kHz system consisting of a hom type sonicator generates predominantly transient cavitation whereas the systems used for higher frequencies generate stable cavitation [34]. 

Sehgal C, Sutherland RG, Verrall RE (1980) Optical spectra of sonoluminescence from transient and stable cavitation in water saturated with various gases. J Phys Chem 84 388-395... 

On the other hand stable cavitation (bubbles that oscillate in a regular fashion for many acoustic cycles) induce microstreaming in the surrounding liquid which can also induce stress in any microbiological species present [5]. This type of cavitation may well be important in a range of applications of ultrasound to biotechnology [6]. An important consequence of the fluid micro-convection induced by bubble collapse is a sharp increase in the mass transfer at liquid-solid interfaces. In microbiology there are two zones where this ultrasonic enhancement of mass transfer will be important. The first is at the membrane and/or cellular wall and the second is in the cytosol i. e. the liquid present inside the cell. 

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. 

The first and second derivatives of R with respect to time are represented by R and R, respectively. Solving this equation for different values of R0 can be quite illustrative on the complex nonlinear dynamics of cavitation bubbles. Fig. 2 shows two different cases when a frequency of 20 kHz and an intensity equivalent to Py — 2.7 bar are used. In the first case (Fig. 2a), a relatively large (R0 = 2 mm) bubble couples with the sonic field through small-amplitude growth and compression cycles (stable cavitation). In contrast, a smaller bubble (P0 = 20 pm) experiences resonant coupling, which results... 

It is widely thought that the high pressure emitted from a "transient" cavitation bubble is responsible for the nucleation process (Hickling, 1994) however, experiments utilizing a single oscillating bubble have shown that ice can be initiated by a "stable" cavitation bubble. The mechanism of nucleation may be related to the asymmetric bubble shape, the flow field associated with the cavitation bubble, or the production of microbubbles. 

Figure 8.1.4 Stable cavitation. Time-dependent radius of a bubble with 1 pm rest radius Rq.

A more illustrative description of the bubble motion in an acoustic field can be introduced by the concept of cavitation thresholds for different types of bubble behavior. Depending on the nature of the motion four basic types of cavitating voids are distinguished stable cavitation, rectified diffusion, dissolving bubbles, and transient cavitation. 

Only bubbles with the attributes stable or transient are Ukely to exist in a steady-state cavitating sound field. The lower size of a bubble at a given sound pressure is defined by the lower limit for stable cavitation. Any bubble smaller than this size would dissolve rather quickly under the action of its own surface tension. 

Figure 8.1.13 Regions of stable cavitation, transient cavitation and dissolving bubbles for a frequency of 20 kHz in water at ambient temperature and a static pressure of 1 bar. Bubble radius R is normalized by the resonant bubble radius R,.

Pf is the final recovered pressure, which depends on the pressure loss arising due to the presence of the constriction. The typical radius history obtained for such a case is exactly similar to stable cavitation, as discussed earlier (Fig. 8.2.2). Such low magnitude pressure pulses are unlikely to bring about the physical/chemical effects observed in the case of hydrodynamic-cavitation reactors (Save et al., 1997, Suslick et al., 1997, Vichare et al., 2000b) and hence this approach is not suitable and the predictions cannot be relied upon. 

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