Theoretical estimation of the temperature and pressure within collapsing acoustical bubbles

Formation of highly reactive species such as (OH)-O-center dot, H-center dot, HO2 center dot and H2O2 due to transient collapse of cavitation bubbles is the primary mechanism of sonochemical reaction. The crucial parameters influencing the formation of radicals are the temperature and pressure achieved in the bubble during the strong collapse. Experimental determinations estimated a temperature of about 5000 K and pressure of several hundreds of MPa within the collapsing bubble. In this theoretical investigation, computer simulations of chemical reactions occurring in an O-2-bubble oscillating in water irradiated by an ultrasonic wave have been performed for diverse combinations of various parameters such as ultrasound frequency (20-1000 kHz), acoustic amplitude (up to 0.3 MPa), static pressure (0.03-0.3 MPa) and liquid temperature (283-333 K). The aim of this series of computations is to correlate the production of (OH)-O-center dot radicals to the temperature and pressure achieved in the bubble during the strong collapse. The employed model combines the dynamic of bubble collapse in acoustical field with the chemical kinetics of single bubble. The results of the numerical simulations revealed that the main oxidant created in an O-2 bubble is (OH)-O-center dot radical. The computer simulations clearly showed the existence of an optimum bubble temperature of about 5200 +/- 200 K and pressure of about 250 +/- 20 MPa. The predicted value of the bubble temperature for the production of (OH)-O-center dot radicals is in excellent agreement with that furnished by the experiments. The existence of an optimum bubble temperature and pressure in collapsing bubbles results from the competitions between the reactions of production and those of consumption of (OH)-O-center dot radicals at high temperatures. (C) 2013 Elsevier B.V. All rights reserved.