(Invited) Low Temperature Plasma Electrochemistry with Microscopic Liquid Droplets in Flight
Authors: Maguire PAUL , McQuaid Harold 
Published: July 2021

When microscopic-sized liquid droplets travel through a low temperature RF plasma [1] at atmospheric pressure a number of remarkable and unexpected effects have been observed. After a short flight time, ~0.1ms, there is evidence that chemical reactions induced by the plasma and gas flux proceed at a rate that is significantly faster that observed in plasma – bulk liquid studies and many orders of magnitude faster than in standard bulk chemistry.[2] We suspect this is due to the complex interplay between droplet charge, electric fields, both internal and external to the droplet, and high chemical fluxes arriving at the droplet surface.
There exists a large potential to develop new plasma-liquid processes for medical, chemical, biological, environmental and materials applications, among others and we can highlight some unique features of the plasma – microdroplet system that may provide opportunities for exploitation, namely: (i) a controlled ambient environment, (ii) a large surface area to volume ratio, (iii) small volume, (iv) low droplet temperature, (v) in-flight chemical synthesis and encapsulation, and (iv) remote delivery. These features offer the possibility of delivering high fluxes of active chemical species and nanoparticles remotely and on demand for applications in, for example, plasma-medicine, agriculture and microreaction while keeping the plasma itself at a safe distance.
We have measured reactive oxygen species (ROS) flux variation with distance, up to 150 mm beyond the plasma, along with its effect on bacterial cell viability, DNA and amino acids. We have investigated plasma interactions with single cells, each transported in its own droplet. We have used the individual droplets as chemical microreactors to produce nanoparticles in flight, at rates many orders of magnitude higher that via high energy radiolysis or chemical synthesis. These measurements form the basis for numerical simulation in the gas-plasma and liquid droplet phases. New measurement techniques, based on recently acquired facilities, are being investigated. These include mid-IR absorption studies of droplets and their environment in flight, using tunable supercontinuum and quantum cascade lasers, and freezing plasma-treated droplets in flight for in-situ transfer to XPS surface chemical analysis.
Current theories of microparticle charging in a collisional plasma environment are very limited. While in-flight charge measurements represent a significant challenge, the relatively large size of the droplet (10 – 20 μm diameter) and the limited evaporation over the flight time, offer the prospect of using droplets as a spherical probe to develop enhanced collisional probe theories in the regime where the particle size is greater than Debye lengths or mean free paths. In-flight measurements indicate a minimum net charge of ~105 electrons, considerably higher than that obtained by other charging methods. Analytical – numerical and finite element simulations, in tandem with charge measurements, are being developed to better understand the droplet electrical environment and ultimately to link chemistry and charge in a consistent framework.

References

[1] PD Maguire et al., Appl. Phys. Lett. 106, 224101 (2015); http://dx.doi.org/10.1063/1.4922034

[2] PD Maguire et al., Nano Lett., 17, 1336–1343 (2017) http://dx.doi.org/10.1021/acs.nanolett.6b03440

Figure 1