Nature of the force field in plasma wakes

Plasma wakes are relevant in a number of applications. Here the attention is focused on complex plasmas formed by dust particles suspended in partially inonized plasmas. Two forces are observed to contribute significantly in a wake created by a suspended particle immersed in a plasma with supersonic flowing ions. First, the well-known electrostatic force caused by the macroscopic electric field produced by charge accumulation in the wake. Second, the ion flow field is distorted in the wake and creates a flow-induced force caused by the momentum transfer from ion particles. The first force (electrostatic) has been proposed to cause the alignment of dust particles observed in experimental conditions in complex plasmas. The present article provides evidence that actually the second force (ion flow), not previously considered, is a more likely candidate that can explain the alignment observed in experiments.