Study of the Kinetics of Droplets Size Evolution and the Mechanisms of Solid Particles Creation in an Atmospheric Plasma Jet, Using a Laser Beam Diffraction Technique

The kinetics of droplets size evolution in an atmospheric plasma jet in which a precursor is injected in liquid phase through a nebulizer to deposit thin films is studied using laser diffraction. In the absence of plasma, droplets reach the substrate (at 75 mm from the nebulizer), but when a pure helium discharge is ignited all the droplets transform into gaseous phase before reaching the substrate. Theoretical and experimental analysis of the system show that the size of these droplets evolve due to evaporation in the plasma and depends upon the plasma gas composition and gas flow rate. Other possible mechanisms like particle charging have been found to play a minor role in the droplet size evolution. In high energy pure helium plasmas when the droplets interact with plasma, small size SiO x particles (1 μm maximum diameter compared to droplet of size up to 70 μm) are formed. Increasing the power dissipated in the discharge increases the number of particles formed but not their size suggesting that nucleation depends on the density of active plasma species. The particle size however depends on the time of residence and precursor concentration in the high energy zone, hence particle size growth depends upon the local concentration and transport of the precursor species.