Transient cavitation threshold

Choice of liquid Vapor pressure Surface tension Viscosity Chemical reactivity Intensity of collapse Transient cavitation threshold Transient cavitation threshold Primary or secondary sonochemistry... 

Figure 8.1.5 Onset of transient cavitation after passing the transient cavitation threshold.

The upper size limit is made up by the transient cavitation threshold. Smaller bubbles may exist as stable oscillating bubbles that eventually grow in size by rectified diffusion or their path into regions with higher sound pressures. Bigger bubbles than this will collapse and create smaller daughter bubbles. The bubble size Rq is rather small and is of the order of several micrometers. 

For example, at 3 MHz, 80% of the bubbles undergoing transient collapse do so because they are above the transient threshold immediately at the start of sonication (initial bubble radii = 1.03 to 1.115 pm). The likelihood of a scenario where a bubble grows by rectified diffusion and then undergoes transient collapse decreases with increasing frequency. Indeed, the rectified-diffusion threshold increases more rapidly with increasing frequency than does the transient-cavitation threshold. 

The temperature is higher than 5000 K for bubbles of 0.28 3.5 pm in ambient radius. For the linear resonance radius of 11 pm, it is only about 1000 K. In Fig. 1.9b, the rate of production of chemical species is shown. It is above 10s s 1 for bubbles of 0.28 8 pm in ambient radius. The minimum ambient radius coincides with the Blake threshold radius (Rgiake) f°r transient cavitation (active bubbles). It is calculated by the following formula [6]. 

Cavitation threshold, Intensity of cavitation, rate of chemical reaction Transient threshold Size of the nuclei (cavitation threshold)... 

Fig. 3. Thresholds of cavitation. Region A Bubble growth through rectified diffusion only. Region B Bubble growth through transient cavitation. RD, Threshold for rectified diffusion Rlt threshold for predomination of inertial effects RB, Blake threshold for transient cavitation. [After R. E. Apfel (S).]...

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. 

The last piece for the model is the bubble-size distribution function and the limits for the rest radii of bubbles in the sound field. The cavitation thresholds as a function of applied sound pressure indicate the upper and lower size limits for bubbles in a cavitating sound field. A simplifying point of view would differentiate between a) transient bubbles, b) stable bubbles and c) dissolving bubbles. 

As we will see later, the Rayleigh-Plesset description closely matches the actual radial behavior of a bubble as long as the non-radial deformations are small (or of short duration). Since the behavior of a bubble depends on the applied acoustic pressure, Apfel estimated the threshold associated to transient cavitation. A part of this threshold is, of course, common to the Blake threshold (explosive growth of a cavitation nucleus. Fig. 14). 

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