Charging of dust in plasmas: Electrical capacity cannot be approximated by the smallest enclosing sphere—tetrapods revisited

The charging behavior of micrometer-scale ZnO tetrapods within the sheath of a low-pressure rf-driven argon plasma was examined with a high-precision resonance method. Taking into account recent measurements on the electron sticking coefficient of polymethyl methacrylate (PMMA, [C5O2H8]n), the experimental results presented by Asnaz et al., [Phys. Plasmas 25, 073702 (2018)] regarding the charge of tetrapods being modeled by the smallest enclosing sphere approximation were determined to be erroneous. Rather than this approach, the correct interpretation involves using the tetrapod's electrical capacity, which can be approximated with reasonable precision by its orientation-averaged equivalent sphere, to accurately depict the charging of the tetrapods.