Interferometric droplet imaging for in situ aerosol characterization in an inductively coupled plasma

Abstract

Size, velocity and evaporation rate of droplets in an Ar inductively coupled plasma (ICP) are simultaneously measured for the first time using a novel laser based imaging technique. In interferometric droplet imaging (IDI), an interference pattern created by the reflected and refracted rays from a droplet are collected in an out-of-focus image. The droplet diameter is determined by counting the number of fringes in the collected interference pattern. Combination of IDI and particle tracking velocimetry (PTV) provides the capability of monitoring droplet properties during the journey inside ICP. Using a demountable-direct injection high efficiency nebulizer, droplets in the range of 3–30 μm in diameter traveling at 15–70 m s⁻¹ are observed in the analytical zone of the ICP. The upper velocity threshold for surviving droplets is determined by the nebulizer gas flow rate, whereas the lower threshold is mainly influenced by thermal expansion of the plasma gas. Droplet evaporation rates (0.26–0.36 mm² s⁻¹) are in good agreement with other reports and theoretical simulations for droplets in a 3000 K Ar environment.