CREATING NEW PHASE TRANSITIONS AS BRIDGES FOR BROKEN ERGODICITY IN CONFINED COLLOIDAL PRISMS

Abstract

Recent advances in the synthesis and fabrication of faceted sub-micron particles with different shapes have spurred interest in using these particles as building blocks for the assembly of targeted complex structures having enhanced optical properties. Several tunable parameters like particle shape, inter-particle interactions and geometry of the assembly, allow the design of a wide range of morphologies and material properties. In this work, Monte Carlo simulations are used to study the entropy-driven assembly of space-filling convex prism shapes; namely, square and hexagonal prisms under parallel slit-confinement, with either hard or soft-repulsive wall potentials. Phases with diverse structural order arise due to the anisotropy associated with the prismatic particle shape and the restriction of the entropic degrees of freedom of these particles by the wall potentials. In the hard-wall slit-confinement model, the wall separation were varied to explore the 2D and quasi-2D phase behavior of square and hexagonal prisms. Our simulation results for hexagonal prisms revealed two types of first order phase transitions at the quasi-2D confinement separations: 1) solid-solid transition (6-fold symmetry solid to 4-fold symmetry solid) occurring through lattice symmetry breaking, and 2) solid-dense liquid-solid (6-fold symmetry solid-no order-4-fold symmetry solid). The predicted dense liquid has a density intermediate between those of the two solid phases and has high translational/orientational disorder and mobility. For square prisms, we observe a solid-polycrystalline-solid phase transition where a lattice spacing rearrangement gives rise to the polycrystalline phase having multiple locally ordered domains. The unusual phase transitions predicted in this work for the hard confinement model are attributed to the broken ergodicity associated with a dynamically disconnected rotational phase space accessible to the particles. Indeed, for a narrow range of slit separations, particles have two distinct and dynamically disconnected rotational states: unflipped (with prism face parallel to wall plane) and flipped (with prism side parallel to wall plane), which cast distinct projection areas over the wall plane and lead to different 2D tessellating lattices. As an experimentally viable strategy to dynamically bridge those rotational states but still retain the observed hard-slit phase behavior, a soft-repulsive wall model was also investigated.