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

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(A 2 Σ + ) 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 H 2 SO 4 preequilibrated with Ar. The light emission spectra exhibit the lines from excited Ar atoms and ionized oxygen O 2 + . Formation of O 2 + species is inconsistent with any thermal process. Furthermore, the SBSL spectra in H 2 SO 4 show 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.

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

“…It was found that gas temperatures inside the bubble are much lower than electron temperatures and do not account for observed emissions. Lower ionisation potentials of the gas provide an increase in electron temperature concurrent with obtained spectra which supports formation of a non-equilibrium plasma [82].…”

“…The effective temperatures for this process were suggested to be near to 900 K. Sufficiently high temperatures can destroy chemically active components within the bubble. This is concurrent with temperature dependant determination of an ionisation or disassociation process, which in turn can govern the number of reactive species produced [17,25,82,84,85]. For a multibubble sonoluminescence system the pressure inside the bubbles has been estimated to be in the order of 300 bar in argon saturated silicone oil, consistent with adiabatic compression [86].…”

Could the study of cavitation luminescence be useful in high dilution research?

“…This type of setting, called Single Bubble SonoLuminescence (SBSL) has been extensively used to observe bubble dynamics and characteristics. 1,51,54,58,59 The spectrum of MBSL in water shows peaks that correspond to excited state OH* (mainly at 310 nm). Solutes can also produce their own lines.…”

“…This could explain the atomic emission lines of noble gases as well as that of non volatile solutes. 59 Most sonoluminescence experiments are conducted in non water liquids, such as concentrated sulphuric and phosphoric acids, glycerine, alkanes, silicone oil, which produce higher light intensities or are more adapted to specific aims, usually further enhanced by noble gas. In contrast, sonoluminescence of untreated water is at least several thousand times weaker.…”

An Attempt to Produce Electrical Discharges in Acoustic Cavitation Bubbles

“…The inside of the cavitation bubble at the collapse has a high pressure and a high temperature because of the roughly adiabatic shrinkage. It is believed that a plasma is produced inside the cavitation bubble at the collapse [18,19] (sometimes it is called "sonoplasma") because of the high temperature. The particular physical parameters at the collapse of the cavitation bubble produces reactive radicals.…”