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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