How to make large, void-free dust clusters in dusty plasma under micro-gravity

Collections of micrometer-sized solid particles immersed in plasma are used to mimic many systems from solid state and fluid physics, due to their strong electrostatic interaction, their large inertia, and the fact that they are large enough to be visualized with ordinary optics. On Earth, gravity restricts the so-called dusty plasma systems to thin, two-dimensional (2D) layers, unless special experimental geometries are used, involving heated or cooled electrons, and/or the use of dielectric materials. In micro-gravity experiments, the formation of a dust-free void breaks the isotropy of 3D dusty plasma systems. In order to do real 3D experiments, this void has somehow to be closed. In this paper, we use a fully self-consistent fluid model to study the closure of a void in a micro-gravity experiment, by lowering the driving potential. The analysis goes beyond the simple description of the 'virtual void', which describes the formation of a void without taking the dust into account. We show that self-organization plays an important role in void formation and void closure, which also allows a reversed scheme, where a discharge is run at low driving potentials and small batches of dust are added. No hysteresis is found this way. Finally, we compare our results with recent experiments and find good agreement, but only when we do not take charge-exchange collisions into account.