Uma análise da transição sólido-fluido em coloides bidimensionais polimórficos

Abstract

The melting of two-dimensional solids differs from that observed in three-dimensional systems. While the solid-liquid transition in three dimensions is a discontinuous transition, the melting in two dimensions is usually described by the KTHNY theory, where symmetry breaks lead to a intermediate state corresponds to a hexatic phase, such that melting occurs through a solid-hexatic transition. continuous and a discontinuous hexatic-liquid transition. However, some colloidal particles with a softened core may show different behaviors in the melting of the triangular solid phase, where the solid-hexatic transition is discontinuous. Also, softened core colloids have polymorphism, having more than one solid phase. So, a natural question is: how does the crystalline structure influence the melting scenario? In order to understand the solid-liquid transition scenario, in this work we analyze the structural behavior of a two-dimensional system described in monomeric form in a softened core model using Molecular Dynamics. This material presents polymorphism, presenting solid, crystalline and amorphous phases. Here, we focus our study on extensive simulations of melting and freezing along three isobars, one in the low-density triangular phase, another in the Stripes phase, and a third in the Kagome phase. We observed the influence on the structural symmetry for the behavior of its transition, with continuous or discontinuous transition orientations depending on the translational and orientational order parameters. We observed the existing hysteresis in the transitions, and how the analysis of the freezing and heating curves helped to observe the existence of a hexatic phase for the three studied symmetries. Our results show an expressive relationship between the crystalline structure and the behavior of the system, especially if analyzed in the light of the KTHNY theory, and help to understand the complex scenario of two-dimensional melting.