Plasma Charging of Micron-Size Droplets
Authors: Hendawy Nourhan , McQuaid Harold , Mariotti Davide , Maguire PAUL 
Published: July 2021

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 in the range 70 µV to 20 μV as the plasma – collector distance is increased. 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 μV. With the addition of droplets, the collector mean potential decreased due to the impact of the negative charge flux. The calculated total current versus distance varied from 100 – 10 pA, figure 1. To obtain charge per droplet estimates requires knowledge of 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 leading to a lower bound average charge value of > 104 electrons per droplet.[3] However the number of droplets reaching the downstream collector could not be measured directly and instead aerosol-specific frequencies were extracted from measured signal FFTs and found to be in the range 100 Hz – 1000 Hz. These are likely to be the larger droplets with diameters up to 60 mm. Analysis of filtered time-domain signals are underway in an attempt to extract individual droplet size and charge from image charge models.[4]

Figure
1
Total droplet-induced current versus collector distance from the plasma

[1] Léon Sanche, ‘Cancer Treatment: Low-Energy Electron Therapy’, Nature Materials, 14.9 (2015), 861–63 https://doi.org/10.1038/nmat4333

[2] Paul Maguire et al., ‘Continuous In-Flight Synthesis for On-Demand Delivery of Ligand-Free Colloidal Gold Nanoparticles’, Nano Letters, 17.3 (2017), 1336–43 https://doi.org/10.1021/acs.nanolett.6b03440

[3] Paul Maguire et al., ‘Controlled Microdroplet Transport in an Atmospheric Pressure Microplasma’, Applied Physics Letters, 106.22 (2015) https://doi.org/10.1063/1.4922034

[4] E. Borzabadi and A. G. Bailey, ‘The Measurement of Charge on Microscopic Particles’, Journal of Physics E: Scientific Instruments, 12.12 (1979), 1137–38 https://doi.org/10.1088/0022-3735/12/12/005

Figure 1