Prediction of plasma-induced reactions in a droplet placed in an atmospheric-pressure helium plasma*

Plasma-induced liquid-phase reactions initially occur at the plasma-liquid interface. Thus, the use of droplets is advantageous to enhance the plasma-liquid interaction due to their large specific surface area. In a uniform plasma, droplets are charged negatively, and the droplet charging controls the electron and ion fluxes from the plasma, which affects the chemical reactions in the droplet. In this work, we have developed a model to simulate chemical reactions in a droplet in a uniform plasma, taking into account the droplet charging and the associated changes in the potential, electron and ion fluxes. First, the behavior of the droplets in the plasma was experimentally studied using a high-speed camera. The droplets with diameter of approximately 120 μm were dispensed into atmospheric-pressure (AP) He DBD driven by a sinusoidal voltage of 100 kHz. The droplets travelled for several ms in the plasma without visually decreasing their size, and were repelled at the sheath edge, indicating negatively charged. This suggests that the droplet evaporation in He DBD is less effective at least for a few ms due to the low temperature. In the modeling, continuity equations for charged/neutral species in both the gas and liquid were solved with Poisson’s equation, in a limited region around a single droplet placed in AP He glow discharge. Boundary conditions for these equations were taken from one-dimensional discharge simulation. The diameter of the droplet was set to 8 μm. The droplet was salt water or silver nitrate solution. In the DC glow discharge with a plasma density of 10¹¹ cm^-3, the droplet charge reached a steady state within 1 us. Similar result was obtained in AP He DBD. Reactions in the droplet were initiated by hydrated electrons or OH at the interface. In the droplet of 10 mM silver nitrate solution, the silver ions were converted to other species by oxidation or reduction within 10 ms.