  In various types of many-particle systems, bidispersity is frequently used to
avoid spontaneous ordering in particle configuration. In this study, the
relation between bidispersity and disorder degree of particle configuration is
investigated. By using magnetic dipole-dipole interaction, magnet particles are
dispersed in a two-dimensional cell without physical contact between them. In
this magnetic system, bidispersity is introduced by mixing large and small
magnets. Then, the particle system is compressed to produce a uniform particle
configuration. The compressed particle configuration is analyzed by using
Voronoi tessellation for evaluating the disorder degree which strongly depends
on bidispersity. Specifically, standard deviation and skewness of the Voronoi
cell area distribution are measured. As a result, we find that the peak of
standard deviation is observed when the numbers of large and small particles
are almost identical. Although the skewness shows non-monotonic behavior, zero
skewness state (symmetric distribution) can be achieved when the numbers of
large and small particles are identical. In this ideally random (disordered)
state, the ratio between pentagonal, hexagonal, and heptagonal Voronoi cells
become roughly identical, while hexagons are dominant in monodisperse (ordered)
condition. The relation between Voronoi cell analysis and the global bond
orientational order parameter is also discussed.
