关键词: polydispersity, ordering, phase diagram, dynamics

Abstract: We study the effect of particle size polydispersity ($\delta$) on the melting transition ($T^*$), local ordering, solid-liquid coexistence phase and dynamics of two-dimensional Lennard-Jones fluids up to moderate polydispersity by means of computer simulations. The particle sizes are drawn at random from the Gaussian (G) and uniform (U) distribution functions. For these systems, we further consider two different kinds of particles, viz., particles having the same mass irrespective of size, and in the other case the mass of the particle scales with its size. It is observed that with increasing polydispersity, the value of $T^\ast$ initially increases due to improved packing efficiency ($\phi$) followed by a decrease and terminates at $\delta\approx 8%$ (U-system) and $14%$ (G-system) with no significant difference for both mass types. The interesting observation is that the particular value at which $\phi$ drops suddenly coincides with the peak of the heat capacity $(C_{P})$ curve, indicating a transition. The quantification of local particle ordering through the hexatic order parameter ($Q_6$), Voronoi construction and pair correlation function reveals that the ordering decreases with increasing $\delta$ and $T$. Furthermore, the solid-liquid coexistence region for the G-system is shown to be comparatively wider in the $T$-$\delta$ plane phase diagram than that for the U system. Finally, the study of dynamics reveals that polydisperse systems relax faster compared to monodisperse systems; however, no significant qualitative differences, depending on the distribution type and mass polydispersity, are observed.

Key words: polydispersity, ordering, phase diagram, dynamics