Plasma Formation during Acoustic Cavitation: Toward a New Paradigm for Sonochemistry

Sergey I. Nikitenko

doi:10.1155/2014/173878

Abstract

The most recent spectroscopic studies of single bubble (SBSL) and multibubble (MBSL) sonoluminescence reveal that the origin of extreme intrabubble conditions is related to nonequilibrium plasma formed inside the collapsing bubbles. Analysis of the relative populations of OH(A2Σ+) vibrational states observed during MBSL in water saturated with noble gases shows that in the presence of argon at low ultrasonic frequency weakly excited plasma is formed. At high-frequency ultrasound the plasma inside the collapsing bubbles exhibits Treanor behavior typical for strong vibrational excitation. Plasma formation during SBSL was observed in concentrated H2SO4preequilibrated with Ar. The light emission spectra exhibit the lines from excited Ar atoms and ionized oxygenO2+. Formation ofO2+species is inconsistent with any thermal process. Furthermore, the SBSL spectra in H2SO4show emission lines from Xe+, Kr+, and Ar+in full agreement with plasma hypothesis. The photons and the “hot” particles generated by cavitation bubbles enable the excitation of nonvolatile species in solutions increasing their chemical reactivity. Secondary sonochemical products may arise from chemically active species that are formed inside the bubble but then diffuse into the liquid phase and react with solution precursors to form a variety of products.

Highlights

When studying the action of 100–500 kHz ultrasound on aqueous solutions, Richards and Loomis [1] discovered that the ultrasonic waves accelerate the hydrolysis of dimethylsulfate and the reduction of potassium iodate by sulfurous acid
This review will be focused on more recent results in sonoluminescence and sonochemistry which provided the new insights into the origin of the processes occurring during acoustic cavitation
In terms of nonequilibrium plasma model, the higher vibronic temperatures in Xe are the result of lower ionization potential of Xe compared to Ar (12.13 eV for Xe, 15.76 eV for Ar)

Summary

Introduction

When studying the action of 100–500 kHz ultrasound on aqueous solutions, Richards and Loomis [1] discovered that the ultrasonic waves accelerate the hydrolysis of dimethylsulfate and the reduction of potassium iodate by sulfurous acid (iodine “clock” reaction). Schmitt et al [2] reported the oxidation of iodide ions in aqueous solutions under the effect of 750 kHz ultrasound. These seminal works have introduced a new field of chemistry called “sonochemistry” by Neppiras [3]. In 1933 Marinesco and Trillat have accidentally observed the darkening of photographic plates submitted to ultrasound in water [12] They attributed this finding to the ultrasonic acceleration of Ag+ chemical reduction at the surface of plates. One year later, Frenzel and Schultes [13] have shown that photographic plate darkening is due to Advances in Physical Chemistry the light emission from sonicated water rather than from chemical reaction. This review will be focused on more recent results in sonoluminescence and sonochemistry which provided the new insights into the origin of the processes occurring during acoustic cavitation

Sonoluminescence as a Probe of Intrabubble Conditions

Conclusions