Heterodyne I: Enhancing sonochemical efficiency through application of the heterodyne effect: An initial study

Relatively little is known concerning the application of concurrent and coaxial multi-frequency ultrasound on the facilitation of chemical reactivity. When two or more acoustic frequencies are input into a solution, frequencies corresponding to the sum of the frequencies with an amplitude related to the difference between the two frequencies (both known in acoustics as heterodyne frequencies) are produced. Given the fact that cavitation dynamics are dependent upon frequency and power among other variables, the generation of additional waves should produce a situation where the cavitation bubbles produced by the two coincident frequencies are out of resonance with the heterodyne frequency, resulting in an increase in cavitation collapse efficiency. The resulting cavitation efficiency is expected to be higher than that which is observed from the simple sum of the acoustic energy transferred from each single frequency. As such, the energy and cavitation efficiency of this technique for bond breakage and formation is expected to be significantly higher than using either a single ultrasonic frequency or two frequencies acting separately. In this study we have embarked on a comparison of the efficacy of heterodyne sonochemistry with more traditional single-frequency or dual frequency ultrasound where the heterodyne frequency is zero. As a prototypical reaction, we have explored the effects of multi-frequency ultrasound on the degradation of acid orange, a common industrial colorant. An increase in the degradation rate by a factor of 500% was observed using incident frequencies of 305 and 310 kHz heterodyne ultrasound (5 kHz heterodyne) compared with what would be expected theoretically were the two frequencies to act independently. Reasons for the increase in efficiency are discussed.