The cooperative migration dynamics of particles correlates to the nature of hexatic-isotropic phase transition in 2D systems of corner-rounded hexagons.

ABSTRACT

In the two-dimensional (2D) melting transition of colloidal systems, the hexatic-isotropic (H-I) transition can be either first-order or continuous. However, how particle dynamics differs at the single-particle level during these two different melting transitions remains to be disclosed. In this work, by Brownian dynamics (BD) simulations, we have systematically studied the dynamic behavior of corner-rounded hexagons during the H-I transition, for a range of corner-roundness ζ = 0.40 to 0.99 that covers the crossover from the continuous to first-order nature of H-I transition. The results show that hexagons with ζ ≤ 0.5 display a continuous H-I transition, whereas those with ζ ≥ 0.6 demonstrate a first-order H-I transition. Dynamic analysis shows different evolution pathways of the dominant cluster formed by migrating particles, which results in a droplet-like cluster structure for ζ = 0.40 hexagons and a tree-like cluster structure for ζ = 0.99 hexagons. Further investigations on the hopping activities of particles suggest a cooperative origin of migrating clusters. Our work provides a new aspect to understand the dependence of the nature of H-I transition on the roundness of hexagons through particle dynamic behavior.