Repulsive and Attractive Forces on Dust Particles in a Cluster

The ordered structure of dust particles and forces acting on negatively charged dust particles in a cluster placed in a plasma produced between two electrodes in a microgravity condition is analyzed. Study of the dynamics of a collection of dust particles placed initially at the center of the plasma reveals the expansion of the cluster together with the formation of a certain ordered structure in the presence of collisions with plasma particles. The dynamic expansion of the cluster is a result of the Coulomb repulsion between negatively charged dust particles, while the ordered structure at a later time is a manifestation of the attractive force between a pair of dust particles. An electrostatic force between dust particles in a complex plasma is studied in a context of thermodynamic free energy stored in the dust cluster in the plasma where the overall charge neutrality is conserved. The excessive energy in the system is calculated. An attractive force between a pair of dust particles results from a release of the thermodynamic free energy in charged dust particles