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 nonequilibrium plasmas.

In this paper radical production in atmospheric pressure water containing plasmas is discussed. As OH is often an important radical in these discharges the paper focuses on OH production.Besides nanosecond pulsed coronas and diffusive glow discharges, several other atmospheric pressure plasmas which are of interest nowadays have a typical electron temperature in the range 1-2 eV and an ionization degree of 10 −5 -10 −4 . These properties are quite different from the typical plasma properties known from low pressure gas discharges.In the plasma physics literature OH production is primarily ascribed to be due to electron, metastable induced or thermal dissociation of water, processes which are dominant in (low pressure) gas discharges and in combustion and hot flames. It is shown in this paper that for several atmospheric pressure plasmas also dissociative recombination can be an effective method of OH radical production. Several examples are presented in detail.This paper provides a basic framework for OH production in atmospheric pressure plasmas and shows that accurate knowledge of n e , T e , T g , the dominant ionic species, radical and neutral species are indispensable to obtain a complete view on the chemical kinetics in these challenging complex atmospheric pressure plasmas. A few relevant plasma diagnostics together with their limitations are also briefly discussed in this context.

The transport of neutral argon atoms in an expanding thermal argon/hydrogen plasma is studied by means of laser-induced fluorescence spectroscopy around 811 nm, on the long living Ar[4s] atoms. Although the Doppler shifted laser-induced fluorescence measurements are performed on argon atoms in the metastable Ar * ( 3 P 2 ) and resonant Ar * ( 3 P 1 ) states, it is argued that in the plasma jet the velocity distribution function of these Ar[4s] atoms images the velocity distribution functions of the ground-state argon atoms. From the results it is inferred that the velocity behaviour of the supersonically expanding argon gas can be predicted from the momentum balance, and the temperature from the adiabatic relation between density and temperature. However, the adiabatic constant is found to be 1.4 ± 0.1, smaller than the adiabatic constant of a neutral argon gas expansion which is 5 3 . Both in the axial and in the radial directions the velocity distributions measured in the shock region show clear departures from thermodynamic equilibrium. From the radial velocity distribution it is concluded that background gas invades the supersonic part of the expanding plasma jet. The results on temperature and velocity in the subsonic region show that the radius of the plasma jet hardly increases after the stationary shock front, indicating that the flow pattern is geometrically determined.

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A combined Thomson-Rayleigh scattering device is discussed. It consists of a Nd:YAG laser as a light source in combination with a multichannel detection technique consisting of a gated light amplifier in combination with an optical multichannel analyzer. Special attention is focused on the analysis of the measured spectra. Including convolution methods and taking into account weak coherent effects increases the dynamic range and the accuracy of the measured electron density n, and temperature T, and neutral particle density no. Accuracies of 1%-4% for n,, 2%-6% for T, and lo%-50% for n, depending on the plasma condition are obtained. The dynamic range for n, is 7 x 1017-1021 m -3, for no is 1020-1023 m -3 and for T, is

Both the axial density and temperature profiles of ground-state nitrogen atoms have been measured in a microwave discharge and its afterglow in the presence of the so-called short-lived afterglow by means of two-photon absorption laser-induced fluorescence (TALIF). The temperature is obtained from the Doppler broadening of the spectral profile, after deconvolution with the laser profile. The N atom temperature decreases from about 1400 K in the end of the discharge zone to about 300 K in the downstream part of the afterglow. The sharp temperature decrease immediately behind the discharge zone can reasonably be explained by heat transfer to the flow tube wall. The absolute N atom density is obtained by calibrating the fluorescence yield with a TALIF signal from krypton atoms. The N density increases from 1.5 × 10 21 m −3 in the discharge zone to about 3.5 × 10 21 m −3 in the late afterglow. However, the N atom flux is conserved along the flow tube, indicating negligible consumption or production of N atoms in the short-lived afterglow.

We investigated the efficiency and formation mechanism of ammonia generation in recombining plasmas generated from mixtures of N 2 and H 2 under various plasma conditions. In contrast to the Haber-Bosch process, in which the molecules are dissociated on a catalytic surface, under these plasma conditions the precursor molecules, N 2 and H 2 , are already dissociated in the gas phase. Surfaces are thus exposed to large fluxes of atomic N and H radicals. The ammonia production turns out to be strongly dependent on the fluxes of atomic N and H radicals to the surface. By optimizing the atomic N and H fluxes to the surface using an atomic nitrogen and hydrogen source ammonia can be formed efficiently, i.e., more than 10% of the total background pressure is measured to be ammonia. The results obtained show a strong similarity with results reported in literature, which were explained by the production of ammonia at the surface by stepwise addition reactions between adsorbed nitrogen and hydrogen containing radicals at the surface and incoming N and H containing radicals. Furthermore, our results indicate that the ammonia production is independent of wall material. The high fluxes of N and H radicals in our experiments result in a passivated surface, and the actual chemistry, leading to the formation of ammonia, takes place in an additional layer on top of this passivated surface.