Investigation of plasma interactions with microdroplets

Student thesis: Doctoral Thesis


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. Continuous high-accuracy gas temperature measurements of low-temperature atmospheric pressure plasma jets were demonstrated using a small focal spot infrared sensor directed at the outer quartz wall of the plasma. Measured gas temperatures varied from 25 oC – 50 oC, 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 m – 15 m, led to a reduction in gas temperature of up to 10 oC, for the same absorbed power. This is an important parameter in determining droplet evaporation and its impact on plasma chemistry.

Droplets charging has influenced a wide range of applications due to the importance of electrons in reactive species generation and participation in wide range of reactions. The first measurements of particle charge acquired by microparticles in a fully collisional atmospheric pressure low temperature plasma operated in helium is reported here. Using a constant stream of liquid microdroplets and a plate electrode, the maximum average charge per droplet was 2.5 x 106 electrons (400 fC) for a plasma absorbed power 5 W at a distance of 3 mm from the downstream plasma electrode. This represents a 2 – 3 orders of magnitude increase in charge levels compared to other approaches. In addition, double ring electrode collector has been used to estimate the charge value using a simulation model with MATLAB. The estimated charge was 5.0 x 103 which agrees with the range values obtained from the plate electrodes at the equivalent power and distance.

System development of a custom designed chamber (PAD) attached to XPS analysis unit has been achieved. A frozen water sample was obtained and transferred to the preplock chamber which is ready to be transferred to the XPS for analysis. Freezing and analysis the water sample after treatment would help in studying the plasma-liquid interactions which are challenging to study with traditional techniques due to the very short-lifetime of the reactive species. This would provide promising results serving many applications such as plasma medicine.
Date of AwardDec 2022
Original languageEnglish
SupervisorDavide Mariotti (Supervisor) & Paul Maguire (Supervisor)


  • XPS
  • PAD
  • Charged droplets
  • Plasma gas temperature

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