Pulsed Sonochemistry

Abstract

Very few papers are devoted to the study of the chemical effects of pulsed ultrasound in the low-frequency range. The present work consists of a systematic experimental study of the effects of pulsed ultrasound in the 20 kHz range using an immersed titanium horn. The light scattered by the bubble cloud, the acoustic pressure, and the sonochemical activity were measured. The sonochemical activity was studied by measuring the light emitted by a fast chemiluminescent reaction (oxidation of luminol). The chemiluminescence behavior observed at 20 kHz was compared with the behavior observed at 1.7 MHz. The chemiluminescence takes time to install when sonication starts and, at 20 kHz, the luminescence intensity decreases monoexponentially when sonication stops. Interestingly, at 1.7 MHz, the luminescence intensity decreases biexponentially with an important fast component. The interpretation of these various behaviors requires that the acoustical characteristics of the ultrasound generator and of the vessel, and also the properties of the bubble field, be considered.

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

“…Pulsed, sweeping or bi-modal signals can increase active cavitation compared to continuous wave output [117,[171][172][173][174][175][176][177][178][179][180]. Continuous wave ultrasound becomes inefficient due to limited spatial distribution, coalescence-induced loss of applied pressure amplitude and bubble growth beyond the region of active cavitation [172].…”

“…The shorter P ON T the shorter the lifetime of the active bubbles once the signal is switched off. That is, if P ON T is short enough a bubble may dissolve completely during P OFF T before the next P ON T pulse, or subsequent P ON T pulses may not cause remaining bubbles to become active [117,176,179]. Conversely, a longer P ON T time can result in the bubbles becoming too large to dissolve to an active size during P OFF T. This may cause bubbles to become inactive degas bubbles primarily via coalescence [176,181].…”

“…In the P OFF T phase there is a clear decrease in bubble volume due to the action of surface tension. However the bubbles may not completely dissolve or become inactive during this phase, this is due to residual pressure amplitude greater than the cavitation threshold which is said to remain during a short P OFF T [172,176,179]. If a P OFF T is chosen that gives large inactive bubbles chance to dissolve to an active size there are two benefits; firstly, the bubbles are now active and secondly, a reduction in bubble size may cause a decrease in coalescence and inhibition of the generation of degas bubbles [172,182].…”

“…The increase in sonochemical activity may be due to the conditions at the start of sonication which deliver increased violent volume pulsations of the bubbles, therefore, repeated pulsing of this period can result in increased disassociation inside the bubble and aid in fragmentation [65,179]. This is especially true of large degas bubbles that can be forced to fragment and produce new bubbles that may continue the cavitation cycle [176].…”

Enhancing sonochemical activity in aqueous media using power-modulated pulsed ultrasound: an initial study

“…Although the use of ultrasonic pulses to engage in chemical activity has been studied for almost 40 years [7], it has only been in the past decade that a systematic study has been performed [8][9][10][11][12][13]. The pulse trains used in this earlier work were square wave pulses such that the maximum amplitude, P 0 , was equal to the maximum amplitude employed in continuous wave (CW) experiments.…”