Plasma-liquid interactions: a review and roadmap

Plasma-liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas

Spectroscopy of Sonoluminescence and Sonochemistry in Water Saturated with N₂–Ar Mixtures

Sonoluminescence spectra in relation with sonochemical activity of water sparged with Ar/N₂ gas mixtures were systematically studied at two ultrasonic frequencies (20 and 359 kHz). At 20 kHz, solely the molecular emission of OH (A²Σ⁺–X²Π_i) is observed in addition to a broad continuum typical for multibubble sonoluminescence. On the contrary, at high frequency a second emission band is present around 336 nm which is assigned to the NH (A³Π–X³Σ^–) system. In addition, the sonolysis of a 0.2 M NH₃·H₂O solution at 359 kHz in the presence of pure Ar yields the emission bands of NH (A³Π – X³Σ^–) (336 nm) and NH (C¹Π–A¹Δ) (322 nm) systems confirming the sonochemical production of NH radicals. The N₂ (C³Π_u–B³Π_g) emission band is absent at both frequencies. This uncommon phenomenon can be explained by the quenching of the N₂ (C³Π_u) excited state with water molecules inside the bubbles. The sonoluminescence of NH radicals at 359 kHz indicates more effective intrabubble dissociation of N₂ molecules at high ultrasonic frequency compared to low-frequency (20 kHz) ultrasound. Its absence at 20 kHz may also be related to strong quenching, e.g., by water molecules. The kinetic study of the formation of principal sonochemical products (H₂, H₂O₂, HNO₃, HNO₂) confirmed the more drastic conditions produced during bubble collapse at higher ultrasonic frequency

Influence of Ultrasonic Frequency on Swan Band Sonoluminescence and Sonochemical Activity in Aqueous <i>tert</i>-Butyl Alcohol Solutions

The multibubble sonoluminescence (MBSL) spectra of <i>t</i>-BuOH aqueous solutions submitted to power ultrasound at 20, 204, 362, and 613 kHz show emissions for the Δυ = −1 to Δυ = +2 vibrational sequences of C₂* Swan system (d³Π_g → a³Π_u). The Δυ=+2 emission overlaps with the CH­(A–X) emission band. The maximal Swan band emission is observed when the MBSL of water itself is almost completely quenched. In general, MBSL is more intense at high-frequency compared to 20 kHz ultrasound. However, in the presence of Xe, the MBSL of C₂* at 20 kHz is so bright that it can be seen by the unaided eye as a blue glow in the close vicinity of the ultrasonic tip. The intensity of the C₂* band emission exhibits a maximum vs <i>t</i>-BuOH concentration: 0.1–0.2 M at 20 kHz and (1–8) × 10^–3 M at high-frequency ultrasound. Such a huge difference is attributed to a much smaller bubble size at high ultrasonic frequency or, in other words, to a much higher bubble surface/volume ratio providing more efficient saturation of the bubble interior with <i>t</i>-BuOH vapors and to the fact that high frequency bubbles remain active for many more cycles than 20 kHz ones, thus accumulating more hydrocarbon decomposition products. Simulation of the emission spectra using Specair software demonstrated the absence of thermal equilibrium for C₂* radicals (<i>T</i>_v > <i>T</i>_r), where <i>T</i>_v and <i>T</i>_r are the vibrational and the rotational temperature, respectively. In Ar, <i>T</i>_v decreases with increasing <i>t</i>-BuOH concentration reaching a steady value in the concentration domain that corresponds to C₂* emission maximum intensity. In the presence of Xe an extremely high <i>T</i>_v is obtained, which is explained by the relatively low ionization potential of Xe providing a higher electron temperature of nonequilibrium plasma generated during bubble collapse. Analysis of the gaseous products of <i>t</i>-BuOH sonolysis reveals a significant sonochemical activity even at high <i>t</i>-BuOH concentration when MBSL is totally quenched, indicating that drastic conditions could be produced also within nonsonoluminescing cavitation bubbles

Luminescence of Trivalent Lanthanide Ions Excited by Single-Bubble and Multibubble Cavitations

This article focuses on the possibility of exciting some lanthanides (Ce³⁺, Tb³⁺, Gd³⁺, and Eu³⁺) by ultrasound in aqueous solutions. Depending on the lanthanide ions and on the acoustic cavitation conditions (single-bubble or multibubble systems), the excitation mechanism is shown to be photoexcitation (e.g., for Ce³⁺) or collision-induced excitation (e.g., for Tb³⁺). The sonoluminescence of Tb³⁺ is studied in detail at various ultrasonic frequencies, allowing quantification of the amount of quenching. The latter is much stronger in sonoluminescence than in photoluminescence due to the particular properties of acoustic cavitation. Complexation with citrate ions enhances manifold sonoluminescence of lanthanides due to reduction of intra- and inner-molecular quenching

Nonequilibrium Vibrational Excitation of OH Radicals Generated During Multibubble Cavitation in Water

The sonoluminescence (SL) spectra of OH­(A²Σ⁺) excited state produced during the sonolysis of water sparged with argon were measured and analyzed at various ultrasonic frequencies (20, 204, 362, 609, and 1057 kHz) in order to determine the intrabubble conditions created by multibubble cavitation. The relative populations of the OH­(<i>A</i>²Σ⁺) <i>v′</i> = 1–4 vibrational states as well as the vibronic temperatures (<i>T</i>_v, <i>T</i>_e) have been calculated after deconvolution of the SL spectra. The results of this study provide evidence for nonequilibrium plasma formation during sonolysis of water in the presence of argon. At low ultrasonic frequency (20 kHz), a weakly excited plasma with Brau vibrational distribution is formed (<i>T</i>_e ∼ 0.7 eV and <i>T</i>_v ∼ 5000 K). By contrast, at high-frequency ultrasound, the plasma inside the collapsing bubbles exhibits Treanor behavior typical for strong vibrational excitation. The <i>T</i>_e and <i>T</i>_<i>v</i> values increase with ultrasonic frequency, reaching <i>T</i>_e ∼ 1 eV and <i>T</i>_v ∼ 9800 K at 1057 kHz