Acoustic Cavitation at 100 kHz the Missing Link between Low- and High-Frequency Ultrasound
Résumé
Acoustic cavitation behavior significantly depends on the ultrasonic frequency [1]. Low and high frequency ultrasounds present opposite physical and chemical characteristics low frequencies (LF, 16-50 kHz) are to be preferred for mechanical effects, high frequencies (HF, 8805;200 kHz) on the contrary show a higher chemical activity [2]. The present work focuses on the less studied but more and more used intermediate frequencies, on the example of 100 kHz. To allow comparison, comprehensive experimental investigations have been carried out at 20 kHz, 100 kHz and 362 kHz in pure water and in aqueous luminol solutions saturated with Ar or Ar/(20 vol.)O2. Sonochemical yields were quantified with H2O2 yields measured at 100 kHz are well in between those observed at 20 and 362 kHz. Photos of sonochemiluminescence confirm this trend and show that the spatial repartition of sonochemical activity is relatively homogeneous at 100 kHz, similarly to HF. The shape of sonoluminescence (SL) spectra at intermediate frequency looks similar to LF. However, a more detailed study of SL indicates a 100 kHz behavior close to HF the addition of O2 in Ar leads to an increase in the SL intensity at 100 kHz, similarly to HF but contrary to LF [3]. This behavior is explained by more effective dissociation of O2 molecules in cavitation bubbles at intermediate and high frequencies compared to 20 kHz. To question how high the electron temperature of the sonochemical plasma is in 100 kHz cavitation bubbles, their ability to dissociate N2 molecule (9.79 eV/bond) was probed by measuring water SL spectra saturated with Ar/(20 vol)N2. Indeed, a previous work [4] showed that in these conditions, SL spectra at HF bear the emission of NH* while 20-kHz spectra do not. This was traced back to N2 dissociation taking place at HF only, due to the higher electron energy. It is shown here that 100 kHz cavitation bubbles lead to N2 dissociation too (Figure 1). Finally, vibrational and rotational temperatures of OH (A2et61523;+) were estimated for all frequencies under Ar and Ar/(20 vol)O2.