Size and Density Evolution of a Single Microparticle Embedded in a Plasma

This article presents two measurement techniques to determine the diameter of a single dust particle during plasma operation. Using long-distance microscopy (LDM), the particle is imaged from outside the plasma chamber. In combination with phase-resolved resonance measurements the development of the volume-averaged particle mass density is measured over several hours. The measurements show a significant decrease of mass density for polymethyl methacrylate (PMMA) particles due to a plasma etching process on the surface. This is explained by a core-shell model and is supported by a surface roughness effect seen in the LDM images, an out-of-focus imaging of the angular Mie scattering pattern and ex-situ laser scattering microscopy measurements.