Collective effects in complex plasma

In complex plasmas together with single particle behavior the study of collective phenomena in large assemblies is an important development. We analyze the interaction among particles in plasma in several cases, starting from clusters consisting of a few particles up to large assemblies. In some cases the formation of the self-built electric field at the edge of the cluster and the screening of an external electric field by the dust cloud are predicted. In the case of small clusters the formation of double layers at the edge of a cluster is considered. The dimension of the cluster plays a role. The competition for charge is crucial in explaining agglomeration of mesoscopic particles. Larger particles, of the size of a Debye length, agglomerate mainly because of the ion 'shadow' force, a small scale aspect of the self-built field. Experiments with an expansion of dust clusters in an external plasma sheath seem to prove the existence of the self-built electric field at the edge of the cloud and the screening of external sheath fields. Other experiments, such as formation of bubbles, lanes, electrorheological properties and 2D dislocations in crystals, enlarge the investigations on collective effects and show their importance in complex plasmas.