Cavitation

Cavitation is the formation and subsequent collapse, within a liquid, of cavities or bubbles that contain vapor or a mixture of vapor and gas. The ultrasonic field must have sufficient intensity to initiate the cavitation. The collapse of these bubbles causes locally intense pressures and temperatures.

For a horn with a circular face, the required cavitation power can be estimated from the following graph.

Figure 1. Cavitation intensity at 20 kHz

Table 1. References

Reference #	Reference	Notes
1	Perkins (1), figure 5, p. 6	Method 1
2a	Culp (0)	Method 1
2b	Culp (0)	Method 1 Spool horn (face uniformity = 0.99)
3	Sonics - private communication to Culp (0)	Method 1
4	Peshkovsky (1), figure 9, p. 321	Method 2

Notes:

Method 1 = Wattmeter measuring net power
Method 2 = Calorimetric
For the Perkins data with a Ø3.0 mm horn, the net power was 9.5 watts. This small value might have significant error (e.g., due to wattmeter calibration errors) so this datum should perhaps be regarded with some suspicion, especially since it deviates significantly from other data.
If the horn's face amplitude is not completely uniform then the average face amplitude should be used in figure 1.

Example

Assume a 20 kHz Ø50 mm horn with an average face amplitude of 100 microns peak‑to‑peak.

From the above graph at Ø50 mm (Ø5 cm), the cavitation intensity is 0.4 watt/cm²/micron. The horn's face area is 19.63 cm² so the estimated cavitation power is 785 watts (round up to 800 watts). This is the net cavitation power, exclusive of any additional power that would be needed to drive the ultrasonic stack alone.