Collective Attraction of Equal-Sign Charged Grains in Plasmas

It is shown that, in the presence of many grains embedded in a plasma, any two grains with the same charge sign can attract each other. The attraction is caused by collective effects. Both the strength of attraction and the distance at which the attraction is located depend on the average dust density. In the limit of strong collective interaction, the potential energy of interaction is found to be equal to the Coulomb interaction with an amplitude periodically changing its sign at a sequence of interdust distances. The condition for collective effects to dominate lead to a threshold condition that is fulfilled in existing experiments. The effect of collective attraction is applied for the physical interpretation of the observed phenomenon of the formation of dust crystals in laboratory experiments.