Orientational glass in mixtures of elliptic and circular particles: Structural heterogeneities, rotational dynamics, and rheology

Abstract

Using molecular dynamics simulation with an angle-dependent Lennard-Jones potential, we study orientational glass with quadrupolar symmetry in mixtures of elliptic particles and circular impurities in two dimensions. With a mild aspect ratio (= 1.2) and a mild size ratio (= 1.2), we realize a plastic crystal at relatively high temperature T . With further lowering T , we find a structural phase transition for very small impurity concentration c and pinned disordered orientations for not small c. The ellipses are anchored by the impurities in the planar alignment. With increasing c, the orientation domains composed of isosceles triangles gradually become smaller, resulting in orientational glass with crystal order. In our simulation, the impurity distribution becomes heterogeneous during quenching from liquid, which then produces rotational dynamic heterogeneities. We also examine rheology in orientational glass to predict a shape memory effect and a superelasticity effect, where a large fraction of the strain is due to collective orientation changes.

Self-organization into ferroelectric and antiferroelectric crystals via the interplay between particle shape and dipolar interaction

“…For small η, this potential well represents interacting spheroids and produces a ferroelastic phase (62). See SI Appendix, A for further details.…”

“…Motion and Numerical Units. The numerical integration of particle positions and orientations is obtained by solving the equation of motion for uniaxial molecules (43,62,63), mr i = −∂U/∂r i and I(…”

“…· ∂U/∂n i under an appropriate ensemble (below). Here, ↔ 1 is the unit tensor and the inertia moment I is assumed to be that of corresponding spheroids: I = σ 2 (1 + p)m/20 with p = (1 + 2η) 1/6 being the aspect ratio (43,62). All of the quantities in this article are presented in dimensionless form, where the units of energy, length, and mass are , σ, and m, respectively.…”

Ferroelectric glass of spheroidal dipoles with impurities: polar nanoregions, response to applied electric field, and ergodicity breakdown

“…At lower T , an orientational phase transition occurs, where the crystal structure becomes noncubic. In nondilute mixtures, this transition is diffuse with slow relaxations, where the orientations and the strains are strongly coupled, both exhibiting nanoscale heterogeneities [55][56][57][58] . Some polymers also undergo ferroelectric transitions due to alignment of permanent dipoles 13,[59][60][61] .…”

“…In electrostatics, we use an Ewald scheme includ-ing image dipoles and applied electric field 68,69 , which has been used to study water between electrodes [70][71][72] . To prepare a mixed crystal, we cool a liquid mixture from high T ; then, our impurity distribution at low T is naturally formed during crystallization 57,58 .…”