[en] Some attractive forces act on negatively charged particles forming simple hexagonal structures of Coulomb crystals in dusty plasmas. In order to elucidate the origin of the forces, optical manipulations have been applied to analyze the behavior of particles in the crystal. It turned out in the analysis that a primary particle in the ion flow caused an attractive force acting on an another particle located in the lower reaches of the flow. This fact implied that the force was due to the wake potential in the flow. Furthermore, a molecular dynamics simulation was employed to examine structures constructed by particles interacting with Coulomb and wake potentials. It was clear in the numerical analysis that the wake potential could form simple hexagonal structures similar to the structure obtained in the experiment. (orig.)