Single bubble cavitation

Flannigan DJ, Suslick KS (2005) Plasma formation and temperature measurement during single-bubble cavitation. Nature (London) 434 52-55... 

Koda S, Tanaka K, Sakamoto H, Matsuoka T, Nomura H (2004) Sonochemical efficiency during single-bubble cavitation in water. J Phys Chem A 108 11609-11612... 

Y. T. Didenko, K. S. Suslick (2002) The energy efficiency of formation of photon, radicals and ions during single bubble cavitation, nature, 418 394-397... 

The most plausible explanation for the differences between MBSL and SBSL is simply the degree of compression and the extent of consequent local heating. In SBSL, the bubble collapse is much more spherical than is likely in the complex acoustics of a bubble cloud. As a consequence, the effective temperature reached in single-bubble cavitation is probably sufficiently high to induce significant ionization and plasma formation. In these circumstances, SBSL will be dominated by featureless bremstrahlung emission, rather than bands from atomic or molecular emission as in MBSL. 

One of the simplest approaches to quantify the pressure field downstream of the constriction used to generate cavitation is to assume linear pressure recovery profile. Yan et al. [8] have used similar approach also considering a single bubble to be existing independent of the other bubbles. Such an approach may be adequate when the intensity of turbulence is quite low i.e. for a venturi type constriction or any other constriction with a smooth variation in the cross-sectional flow area. The pressure recovery from the point at which cavitation starts to any downstream pipe position can be approximated by a linear expression with respect to the distance downstream of the constriction. In such a case, the local pressure at any downstream position can be estimated as ... 

Figure 1. Cavitation Near a Surface. The sequence of a single bubble collapsing follows from left to right, top to bottom.

Sonoluminescence in general may be considered a special case of homogeneous sonochemistry however, recent discoveries in this field have heightened interest in the phenomenon in and by itself.11,12 Under conditions where an isolated, single bubble undergoes cavitation, recent studies on the duration of the sonoluminescence flash suggest that a shock wave may be created within... 

In order to study the mechanism by which a cavitation bubble may stimulate ice crystallization, a standing wave system was used to levitate a single bubble in a fixed location within pure water. It was found that the nucleation of ice could only be stimulated at a higher nucleation temperature in the presence of the oscillating cavitation bubble (Chow et al., 2004). 

Air bubbles can be introduced in a microfluidic mixer in order to enhance the mixing process. The surface of an air bubble in a liquid medium exposed to a sound field can act as a vibrating membrane. The membrane vibration causes a bulk fluid movement at the air-hquid interface. This effect, known as cavitation microstreaming, has been applied in microfluidic micromixers using a single bubble [181] (Fig. 19) or an array of bubbles [178, 179]. 

The process of cavitation has since been the subject of several intensive studies, for example. Cum et al., 1990, 1992 Atchley et al., 1988 Alippi et al., 1992 Leighton, 1995. One of the theories is that cavitation is the result of events that occur within a cloud of bubbles and not just within a single bubble (as assumed in the above equations), but reaction engineering approaches taken so far have been based on the classical single-bubble theories. 

In the case of high nuclei densities Bode, Laake and Meier (1987) observed a bubble selection mechanism. The different sizes of bubbles in a cavitation cluster cause an interaction of the bubbles by pressure and velocity waves propagating between the single bubbles, so that the smaller bubbles tend to collapse earlier than the larger ones because of differences in the surface tension ( Meier 1987 ). All these effects occur simultaneously, therefore an accurate interpretation of the results is difficult. 

A follow up work by Brotchie et al. [42] noted that the resonance sizes of sono-luminescence and sonochemically active bubbles are different. The sonochemilu-minescence, resulting from the reaction between OH radicals generated within cavitation bubbles and luminol molecules, intensity was used to determine the sono-ehemically active (SCL) bubbles. The resonance size of SL bubbles are found to be relatively larger than that of SCL bubbles. In addition, Eq. (1.2) shows that the resonance size decreases with an increase in ultrasonic frequency. Brotchie et al. [42] have also confirmed this experimentally. The sizes were found to be 3.9, 3.2, 2.9, 2.7 and 2 pm at 213, 355, 647, 875 and 1056 kHz frequency, respectively. Another important aspect that needs to be mentioned is the difference between theoretical and experimentally determined resonance sizes of the cavitation bubbles. Equation (1.2) provides a theoretical value of 14 pm at 213 kHz whereas the experimental value is found to be 3.9 pm. This is also known from single bubble work at 20 kHz where the experimental resonance size was found to be about 5 pm compared to the theoretical value of 150 pm [43]. The difference between the resonance size determined by Eq. (1.2) and experimental value is due to the fact that Eq. (1.2) is a very simplified one that does not consider the physical properties of the liquid or bubble contents. 

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... 

Single-bubble sonoluminescence (SBSL) Emission of light from a single cavitating bubble in a liquid, usually water. 

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