Study on Plasma-Induced Liquid-Phase Reactions in a Droplet as Reaction Field

Plasma-induced liquid-phase reactions are initiated by electrons/ions/radicals from the plasma to the liquid surface. Therefore, the use of droplets is advantageous to enhance the plasma liquid interaction due to their large specific surface area. From the application point of view, the droplet injection into the plasma has been applied to the treatment of water [1] and gas [2] containing harmful substances, and the energy efficiency for these treatments has been improved. In materials synthesis, monodisperse gold particles have recently been reported to be produced by injecting droplets of HAuCl₄ aqueous solution into Ar plasma [3]. In this case, the particle synthesis is controlled, in principle, by the droplet diameter, the type and concentration of the chemical in the droplet, the plasma density, and the interaction time between the plasma and the droplet. Therefore, quantitative studies of plasma-droplet interactions, including chemical reactions in droplets, are necessary. This knowledge will also be useful for resource conservation. In this work, we have developed a model to simulate chemical reactions in a droplet in a uniform plasma, taking into account the electron/ion/radical fluxes from the plasma to the droplet surface, droplet charging and the associated changes in the potential distribution. Continuity equations for charged/neutral species in both gas and liquid phases were solved with Poisson’s equation, in a limited region of approximately 0.5 mm around a single droplet placed in atmospheric-pressure helium glow discharge. Boundary conditions for these equations were taken from the results of gas discharge simulation. Therefore, analysis conditions can be set for a variety of plasmas. In this study, homogeneous plasmas in DC glow discharge and dielectric barrier discharge (DBD) in atmospheric pressure helium were studied. We considered a droplet of aqueous silver nitrate solution with diameter of 8 μm, which is typical value in ultrasonic atomization.<br/>In the plasma, the droplet is charged negatively due to the difference in mobility and diffusion coefficient of electrons and ions, similar to the case of dust plasma. The droplet charging is an important factor because it controls the electron/ion fluxes on the droplet surface.<br/>In the DC glow discharge with a plasma density of 10¹¹ cm^-3, the droplet charging reached a steady state within 1 μs. In our model, electrons and ions are assumed to be converted to hydrated electrons, OH and H⁺, respectively, at the droplet surface, and the liquid-phase reactions are initiated from these species. Ag⁺ at the droplet surface is reduced by the hydrated electron, or oxidized by OH radical, and starts to decrease. The concentration of active species is higher only near the droplet surface because they react there before penetrating into the droplet by diffusion or drift. The reaction time of Ag⁺ is an important factor to focus on. From the present simulation, Ag⁺ was converted to other species within approximately 10 ms when AgNO₃ concentration was 10 mM. It was confirmed that this reaction time could be estimated from the flux of electrons and ions incident on the droplet. Therefore, we emphasize the importance of the electron/ion/radical fluxes to the droplet surface in controlling the reactions in the droplet. Similar simulation was performed for atmospheric-pressure helium DBD. In DBD, the electric field, electron and ion densities vary slightly with time. However, the droplet charging was determined by the averaged plasma density, and the calculated results were almost similar to those of DC glow discharge if the plasma density was the same. These results provide a basic idea for the use of droplets in plasmas.<br/>This work is supported by JSPS KAKENHI No. 18H01207.<br/>[1] Y. Minamitani <i>et al</i>. IEEE Trans. Plasma Sci. <b>36</b>, 2586 (2008), [2] S. Daito <i>et al</i>., Jpn. J. Appl. Phys. <b>39</b>, 4914 (2000), [3] Nitta <i>et al</i>. J. Phys. D <b>54</b>, 33LT01 (2021)