An Experimental Study and a Numerical Modelling of Microdroplets Charged in a Helium Atmospheric Pressure Plasma Jet

Abstract

A droplet passing through a low temperature plasma acquires a net negative charge, Q, and the droplet floating potential increases negatively until positive and negative charge fluxes are balanced. Subsequently electrons arrive at the droplet with a very low net electron energy. Low energy electrons (LEE) may play an important role in for example water chemistry, DNA damage (e.g. in radiotherapy) and electron-initiated nanomaterials synthesis [1]. However obtaining true LEE (< 5 eV) sources compatible with liquid water has not been possible to date. In a previous study, we have shown extremely fast and enhanced metal salt reactions within droplets and it is thought LEE reactions from charge on the droplets was the major contributor, in comparison to radiolysis and electron-beam studies [2]. Charging models of collisional high pressure plasmas have received little experimental validation, especially for particles in the size range (microns) where droplets can survive the plasma-induced evaporation. Measuring the small charge on fast moving droplets, however, in a high plasma RF noise environment presents very significant challenges. We present an experimental study of microdroplets charged in a helium atmospheric pressure plasma jet (APPJ) and, along with numerical modelling, determine the dependence of droplet charge, Q, on droplet diameter (D) and plasma conditions e.g. electron density, ne, and temperature, Te. Our measurements were carried out on aerosol droplet streams under various plasma conditions (power and flow) using a 4 mm plate collector. Without droplets, the collector assumed a mean positive potential. This is likely due to the effect of the upstream plasma potential which is estimated at ~12 V, assuming a Te of 2 eV. The background signal variation due to noise and other sources was ~250 mV. With the addition of droplets, the collector mean potential decreased due to the impact of the negative charge flux. At the maximum power, floating potential (VF) is a factor of ~100 greater at 3 mm collector distance compared to 15 mm. On the addition of droplets, the characteristics remain similar at 3 mm but at 15 mm, the potential remains positive until a much higher power. An image charge model [3] was used to obtain charge per droplet estimates from stream of droplets based on droplet rate and size distribution. High resolution imaging was used to obtain droplet statistics at the edge of the capillary. Here, the maximum droplet rate was ~ 5 x 104 s-1 [4]. The voltage – charge calibration factor, obtained from simulation, varies from 4.6 – 7.6 nV per electron, depending on velocity. The average value of charge per droplet can be obtained from the net potential obtained at different powers. The possible influence of the plasma gas flow (Q2) and liquid delivery rate (QL) on collector potential was explored and maximum charging capability was found to occur around a total gas flow of 3.5 slm and for lower QL values.