Random packing of binary hard discs

Random dense packing of binary hard discs generated under the infinitesimal gravity protocol is investigated by a Monte Carlo simulation in the entire region of the size ratio and the concentration. When the size difference of the two species is small, the packing fraction of disc assembly becomes lower than that of the monodisperse system. Defining the adjacent neighbours of a disc through the radical tessellation, we show that the packing fraction can be related to the distance of adjacent discs and that the suppressed packing fraction is caused by an increase of the adjacent neighbours which are not seen in the monodisperse system.