Plasma‐enhanced evaporation and its impact on plasma properties and gaseous chemistry in a pin‐to‐water pulsed discharge

Abstract

The plasma in contact with liquids has led to various novel applications such as plasma biomedicine, material synthesis, and so on. However, the phenomenon of evaporation under plasma treatment and its impact on plasma–liquid interactions has a limited understanding. In this study, the spatially and temporally resolved behavior of water vapor production and its induced influences on plasma properties and gaseous chemistry were studied in detail in an atmospheric pressure pin‐to‐water pulsed He discharge. Diagnostic methods such as laser‐induced fluorescence (LIF) and high‐resolution optical emission spectroscopy (OES) were applied to determine the water vapor and OH radical densities, as well as key plasma parameters such as the gas temperature and electron density. It shows that the physicochemical properties of plasma vary among different discharge regions due to evaporation behavior stimulated during the pulsed discharge‐on phase. In addition, using simulation based on the experimental data, the mechanisms of how water vapor affects the observed spatiotemporal behaviors of OH radicals in different discharge regions are understood. Compared to the pin‐anode and liquid‐cathode sheath regions, proper electron parameters such as density and temperature, as well as water vapor density in the plasma‐positive column, significantly enhance the production of reactive OH radical through the dominant path of electron‐stimulated H2O dissociation. However, higher levels of electron parameters in the intense discharge region near the positive‐pin boundary enhance OH dissociation and finally result in the hollow distribution of OH density. From the global kinetic plasma simulation, the production of reactive hydroxide species playing key roles in plasma medicine treatments, such as O, H, HO2, H2O2, and hydrated ions including H+(H2O)4 and H+(H2O)5, are promoted noticeably as a result of the enhanced water evaporation process.

Plasma–liquid interactions: an experiment and simulation study on plasma dynamic behaviors near the gas–liquid interfacial layer

“…Figure 1(a) schematically depicts the experimental platform used to diagnose plasma dynamics near the gas-liquid interfacial layer. The pulsed pin-to-water discharge device is made of white resin material, and is same as the one used in our previous study [18]. The cross-shaped plasma reactor was equipped with two view windows for fluorescence detection in the front and rear, as well as two Brewster windows for laser beam transmission on both sides.…”

“…Spectral H β line profile emitted from plasma typically follows a Voigt curve, which depends on various broadening and shifting mechanisms such as natural broadening, Doppler broadening, and pressure broadening [24]. The profile of Doppler broadening effects is Gaussian and its full width half maximum (FWHM) could be expressed by [18]:…”

“…f Ni is the equilibrium rotational population of the corresponding level i. Rotational relaxation rate R i and vibrational rotational relaxation rate V i are collision partner-dependent and could be found in [16,18,28]. L ν is the sum of all loss processes of the vibrational level ν, including electronic quenching, radiative and vibrational relaxation.…”

“…L ν is the sum of all loss processes of the vibrational level ν, including electronic quenching, radiative and vibrational relaxation. Detailed calculations of these involved parameters in this fivelevel LIF model are included in our previous work [18].…”

“…Oinuma et al [17] reported an ex situ analysis of the plasma-induced chemistry in the droplet containing formate, droplet trajectory and size measurements, and quantitatively studied the reactivity transfer of OH-radical from the gaseous plasma to the liquid phase. Yang et al [18] reported an obvious evaporation behavior stimulated during the pulsed discharge-on phase and discussed the impact of plasma-enhanced evaporation on gaseous physiochemistry in a pulsed pin-to-water discharge. Winters et al [19] carried out an in situ measurements of OH and H density at a distance of 1 mm from a liquid surface in repetitive nanosecond pulsed discharge.…”