Controlling gas temperature via continuous monitoring is essential in various plasma applications especially for biomedical treatments and nanomaterial synthesis but traditional techniques have limitations due to low accuracy, high cost or experimental complexity. We demonstrate continuous high-accuracy gas temperature measurements of low-temperature atmospheric pressure plasma jets using a small focal spot infrared sensor directed at the outer quartz wall of the plasma. The impact of heat transfer across the capillary tube was determined using calibration measurements of the inner wall temperature. Measured gas temperatures varied from 25 °C–50 °C, increasing with absorbed power and decreased gas flow. The introduction into the plasma of a stream (∼105 s−1) of microdroplets, in the size range 12 μm–15 μm, led to a reduction in gas temperature of up to 10 °C, for the same absorbed power. This is an important parameter in determining droplet evaporation and its impact on plasma chemistry.

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.

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

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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 and is shown in Figure 1 for low and high 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. [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] 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 [4] 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 Figure 1