A combination of imaging techniques reveals a new method of measuring irregular dust in plasmas

Introducing micrometer-sized dust into plasma systems has the potential to examine charging processes in the plasma. While dust particles are spherical in many experiments, that is rarely the case outside of the lab. Comprehensive ways of studying the charging process of irregularly shaped dust particles have remained elusive. Now, researchers have found a way to combine plasma physics tools to begin uncovering the mysteries of these nonspherical particles.

By combining techniques known as long-distance microscopy (LDM) and a phase-resolved resonance method (PRRM), the team demonstrated that the latter technique is capable of measuring charge characteristics of irregularly shaped dust particles in the sheath of radio frequency discharge. Published in Physics of Plasmas, their results show that tetrapod-shaped clusters could be used to measure the charging mechanisms of zinc oxide (ZnO) double tetrapods and compare them to polymethyl methacrylate (PMMA) spheres.

The applicability of the combined technique for a cylindrical zinc oxide particle was verified using LDM to measure size and orientation and PRRM for high-precision measurements of the confinement frequency. The group compared observations of irregular ZnO double tetrapods, which they could fabricate and easily parameterize for LDM, to spherical PMMA particles, which levitated at the same position. This allowed precise comparison of the charging processes for both types of particles.

A capacitor model of particle charge revealed that a corresponding sphere with the same charge as the double tetrapod will have a size comparable to the smallest enclosing sphere of the tetrapod, suggesting that the charging process is independent of particle shape. However, the higher density of collection surfaces in the center of the tetrapod favors the collection of ions in this region, leading to lower negative charges. Molecular dynamics simulations for both the double tetrapod and spherical particle supported the experimental results.

Source: “Charging of an irregularly shaped particle in the sheath of an rf plasma,” by Oguz Han Asnaz, Hendrik Jung, Franko Greiner, and Alexander Piel, Physics of Plasmas (2018). The article can be accessed at https://doi.org/10.1063/1.5038183 .