Pressure Influence on Acoustic Cavitation Phenomenonin Ionic Liquids: Electrochemical Study

Abstract

The propagation of an acoustic wave into a solution can lead to the break of liquid cohesion strengths. This results in the nucleation, growth and collapse of cavitation bubbles feed by solvent vapors and dissolved gases. This collapse may be beneficial as in the case of cleaning processing, but is clearly detrimental in some application as in the case of therapeutic treatments such as tumor ablation, or for the ultrasonic power transmission in coupling fluids. For the electrochemical processes assisted by ultrasound, its control is mandatory. Then, this phenomenon must be reduced by the application of over-pressures into the reactors [1]. To be able to determine the respective contributions of dissolved gases and of vapors into the cavitation bubble collapse, the present work will seek the effect of a limitation in dissolved gases into a sonoreactor by controlling the pressure in a low vapor pressure media: ionic liquids. Those solvents are constituted of a dissymmetric anion and cation, are liquid at room temperature, and show excellent properties for our tests such as a great conductivity. As their vapor pressure is exceptionally low, only the dissolved gases will contribute to the cavitation: as the content in dissolved gas is driven by the pressure, its reduction will result in a reduction of the cavitation activity. In order to quantify this level, the diffusion study of electrochemical species was used as mass transfer sensor [2]. An electroactive species (Ferrocene) forming a rapid and reversible system is present in the solution at low concentration, and in presence of ultrasound, two contribution to the current in steady state conditions can be observed: one depending of the time and the other independent. Both are respectively proportional to the flow generated by the acoustic convection and to the bubble collapse at the electrode interface. The results show an important decrease of the cavitation phenomenon for argon pressure lower than 0.6 bar. A mathematical treatment of the curves allows the extrapolation up to the absolute cavitation quenching, at which the single contribution comes from convection. [1] Parag R. Gogate, Aniruddha B. Pandit. 2005, Ultrasonic Sonochemistry , 12 , 21_27. [2] Pollet B.G.,Hihn J-Y.,Doche M.L., Mandroyan A., Lorimer J-P., Mason T.J. 2007 , Journal of Electrochemistry Society, 154 , 131-138. Figure 1