SONOCHEMISTRY - SOME FACTORS THAT DETERMINE THE ABILITY OF A LIQUID TO CAVITATE IN AN ULTRASONIC-FIELD

Aqueous luminol solutions are irradiated with a 5-ms pulse train of 1-MHz ultrasound (on:off ratio 1:10), and the luminescence pulse train generated is recorded. The first pulses of the train produce chemiluminescence less efficiently than the later ones. The critical number of pulses, N(crit), which has to be applied until the full luminescence signal is developed increases moderately with increasing preirradiation by continuous ultrasound until a very steep increase occurs at a critical time, t(crit). At this point, the solution is practically devoid of nuclei capable of inducing cavitation. Similarly, N(crit) increases moderately with decreasing gas content of the solution (produced by evacuation, but irradiation under full atmospheric pressure) until at about 80% gas concentration a steep increase in N(crit) occurs again. The results are explained in terms of nuclei X(n)(g) (gas pockets stabilized in crevices of microscopic solid dust particles) which are needed for the initiation of chemically effective cavitation. These nuclei are very efficiently destroyed by ultrasound. They are also removed by decreasing the concentration of molecularly dissolved gas in the liquid. However, the ultrasound itself also produces short-lived nuclei X(u) (free microbubbles) which can initiate cavitation.