222 Destructive Effects of High Intensity Ultrasound /I2 : 2 



2. Cavitation 



As was mentioned in the previous section, when cavitation occurs in an 

 intense ultrasonic field, biological cells can be fractured. If cavitation is 

 suppressed, no cells are broken (unless the suspending medium is allowed 

 to heat to a lethal temperature). Accordingly, it is appropriate to 

 examine the action of cavitation in more detail. The following discus- 

 sion presupposes a minimum knowledge of acoustics. It is suggested 

 that the student feeling unsure in this field first reread Section 2 of 

 Chapter 1 and Section 4 of Chapter 1 1 . 



Associated with every sound wave, audible or not, is a variation in 

 both the local pressure P and the local particle velocity v. The acoustic 

 pressure p has been defined as 



P = P~Po 



where P is the average or atmospheric pressure. The absolute pressure 

 P in a gas cannot be negative. Therefore, the acoustic pressure ampli- 

 tude must always be less than the atmospheric pressure. 



In contrast, liquids have a definite volume and can sustain negative 

 pressures or tensions. Pressure amplitudes of hundreds of atmospheres 

 can be generated in water. However, when the pressure becomes low 

 enough (that is, the tension becomes great enough), the liquid will 

 fracture. The liquid breaks by forming small, more or less spherical 

 holes, called cavities. 



Various physical theories have attempted to predict the negative 

 pressure at which pure water will fracture. The most straightforward of 

 these predicts about — 1 5,000 atm. This theory essentially calculates the 

 work to pull two planes apart. A somewhat more sophisticated theory 

 employs both the spherical nature of the holes and another general semi- 

 empirical theory called "absolute rate theory." This combination, 

 named nucleation theory, predicts cavitation thresholds of about 

 -1,500 atm. 



No one has ever reached this value. The lowest threshold reported for 

 cavitation (that is, liquid rupture) in water is - 350 atm. Apparently, 

 dirt on the sides of the vessel, or suspended in the medium, even in 

 triply distilled water, always limits the tension the liquid can withstand 

 before rupturing. For dirty liquids, saturated with gas and tiny gas 

 nuclei, cavitation may occur at positive pressures. Indeed, cellular 

 disruption is sometimes found with pressure amplitudes of only 0.1 

 atm, that is, at positive pressures of 0.9 atm. 



Cavitation can be observed by various techniques. Intense cavitation 

 is readily visible to the naked eye. A hissing sound is often heard. A 

 probe microphone out of the main sound field will pick up noise radiated 



