Structural transitions in two-dimensional hard-sphere systems

We spread randomly noncharged steel particles (diameter, 1.59 mm) on a silicon wafer to form a two-dimensional hard-sphere system. The particle structure versus the particle coverage was monitored. We observed the particle structural transition from liquidlike to triangular-lattice crystal-like with increasing particle coverage by analyzing the particle structure factor. The particle coverage at which the structural transition occurs was quantified by the curves of S(max) (A) and G6 (A); S(max) is the amplitude of the first peak of the structure factor (depicting the particle positional order), and G6 is the bond orientation order parameter. We also conducted a Monte Carlo simulation study. The Monte Carlo simulation results show good agreement with the experimental results at low particle area fractions. However, at high area fractions, the experimentally observed particle structure is less organized than that generated by simulations.