arXiv subjects

Robin L. B. Selinger

Publications and source records attributed to Robin L. B. Selinger.

17 recordsLinked to original sources

arXiv2026

Chaos-generating periodic orbits of topological defects in confined active nematics

Active nematics in two dimensions stir themselves efficiently through internally generated chaotic flows, largely driven by motile $+1/2$ disclinations. We investigate how this tendency toward chaotic fluid stirring can, counterintuitively, produce certain ordered, periodic flows in confinement, characterized by stable periodic orbits of $+1/2$ disclinations. We computationally study two-dimensional active nematics in systems with boundary conditions requiring a prescribed number $n$ of excess $+1/2$ disclinations, using Beris-Edwards nematohydrodynamics simulations alongside an agent-based simulation approach. We find that when confinement is sufficiently strong to prevent defect pair-nucleation, but not strong enough to arrest all flow, then $n=3$ defects generically follow a "golden braid" orbit as observed recently in experiments, and we predict a "silver braid" orbit of $n=4$ defects. For these results and for greater numbers of defects, we show that the periodic or chaotic nature of the dynamics is determined by a balance between the number of defects and the number of vortices in the flow field, suggesting a new design criterion for ordered flows in active nematics.

cond-mat.soft↗

arXiv2026

Chirality Amplification and Chiral Segregation in Liquid Crystals

Liquid crystal mesophases of achiral molecules are normally achiral, yet in a few materials they spontaneously segregate and form right- and left-handed chiral domains. One mechanism that drives chiral segregation is molecular shape fluctuations between axial chiral conformations, where molecular interactions favor matching chirality and promote helical twist. Cooperative chiral ordering may also play a role in chirality amplification, as when a tiny fraction of chiral dopant drives a nematic phase to become cholesteric. We present a model of cooperative chiral ordering in liquid crystals using Maier-Saupe theory, and predict a phase diagram with a segregated cholesteric phase with alternating domains of left- and right-handed chiral twist, with opposite enantiomeric excess, in addition to racemic nematic and isotropic phases. Our model also demonstrates how chiral molecular fluctuations influence the helical twisting power of dopants in the nematic phase, which may be observed even in materials where the segregated cholesteric phase is preempted by a transition to another phase. We compare these results with Monte Carlo simulation studies of the switchable chiral Lebwohl-Lasher model, where each spin switches between right- and left-handed chiral states. Simulation results validate the predicted phase diagram, demonstrate chiral amplification in the racemic nematic phase, and reveal complex coarsening dynamics in the segregated cholesteric phase. These results suggest that molecular fluctuations between degenerate chiral configurations may be a common mechanism to produce cooperative chiral order in achiral liquid crystals.

cond-mat.soft↗

arXiv2025

Multistable polar textures in geometrically frustrated nematic liquid crystals

The ability to manipulate polar entities with multiple external fields opens exciting possibilities for emerging functionalities and novel applications in spin systems, photonics, metamaterials, and soft matter. Liquid crystals (LCs), exhibiting both a crystalline structure and liquid fluidity, represent a promising platform for manipulating phases with polar molecular order, notably ferroelectric ones. However, achieving a polar symmetry is challenging with rod-shaped LC molecules, which form predominantly apolar nematic phases. We report an approach in which a geometric lattice confinement of nematic LCs is used to induce planar polar order on the scale of a mesoscopic metamaterial. We confine the nematic LC in a micropillar array, forming topological defect-pillar pairs of elastic dipoles with a free top interface in contact with an immiscible fluid. The resulting dipole lattice configurations can be programmed rheologically by flowing the top fluid and maintained even after flow cessation, a phenomenon attributed to orientational multistability of the dipoles. This multi-memory effect enables the encoding and reconfiguration of directional information. Overall, this research establishes a foundational understanding of topological dipoles under confinement and shear flow, enabling the detection, tracking, and recording of flow profiles and paving the way for future advances in soft matter physics and stimuli-responsive materials.

cond-mat.soft↗

arXiv2024

Frank-Read Mechanism in Nematic Liquid Crystals

In a crystalline solid under mechanical stress, a Frank-Read source is a pinned dislocation segment that repeatedly bows and detaches, generating concentric dislocation loops. We demonstrate that in nematic liquid crystals, an analogous Frank-Read mechanism can generate concentric disclination loops. Using experiment, simulation, and theory, we study a disclination segment pinned between surface defects on one substrate in a nematic cell. Under applied twist of the nematic director, the pinned segment bows and emits a new disclination loop which expands, leaving the original segment intact; loop emission repeats for each additional 180$^\circ$ of applied twist. We present experimental micrographs showing loop expansion and snap-off, numerical simulations of loop emission under both quasistatic and dynamic loading, and theoretical analysis considering both free energy minimization and the balance of competing forces. We find that the critical stress for disclination loop emission scales as the inverse of segment length, and changes as a function of strain rate and temperature, in close analogy to the Frank-Read source mechanism in crystals. Lastly, we discuss how Frank-Read sources could be used to modify microstructural evolution in both passive and active nematics.

cond-mat.soft↗

arXiv2020

Geometry and mechanics of disclination lines in 3D nematic liquid crystals

In 3D nematic liquid crystals, disclination lines have a range of geometric structures. Locally, they may resemble $+1/2$ or $-1/2$ defects in 2D nematic phases, or they may have 3D twist. Here, we analyze the structure in terms of the director deformation modes around the disclination, as well as the nematic order tensor inside the disclination core. Based on this analysis, we construct a vector to represent the orientation of the disclination, as well as tensors to represent higher-order structure. We apply this method to simulations of a 3D disclination arch, and determine how the structure changes along the contour length. We then use this geometric analysis to investigate three types of forces acting on a disclination: Peach-Koehler forces due to external stress, interaction forces between disclination lines, and active forces. These results apply to the motion of disclination lines in both conventional and active liquid crystals.

cond-mat.soft↗

arXiv2019

Three-dimensional modeling of chiral nematic texture evolution under electric switching

Chiral nematic liquid crystals exhibit both a helical planar ground state with uniform twist and a metastable defect-rich focal conic texture, and can be switched between the two microstructures via application of transient voltage pulses. In this work, we model these electrically-induced texture transitions using finite difference methods to examine resulting microstructural evolution, the first time this transition has been modeled in three dimensions. We analyze the planar to focal conic, focal conic to planar, and planar to planar transitions depending on voltage pulse magnitude. We consider first the special case of chiral nematics with matched twist and bend elastic constants. Results show a variety of defect-rich morphologies in the disordered focal conic texture and demonstrate a fast recovery of the planar ground state on switching without formation of a transient planar state. We evaluate both texture microstructural evolution as well as cell capacitance. Beyond the single elastic constant approximation, we evaluate the planar to transient-planar as well as the planar to Helfrich-deformed transitions in simulations of a liquid crystal compound with different elastic constants. Our methods represent the evolving microstructure as a uniaxial director field, with relaxation dynamics calculated from a tensor representation so that half charge disclination defects are not suppressed. We discuss potential application of these computationally efficient three-dimensional modeling approaches for design and optimization of chiral nematic devices.

cond-mat.mtrl-sci↗

arXiv2016

Gradient-driven diffusion and pattern formation in crowded mixtures

Gradient-driven diffusion in crowded, multicomponent mixtures is a topic of high interest because of its role in biological processes such as transport in cell membranes. In partially phase-separated solutions, gradient-driven diffusion affects microstructure, which in turn affects diffusivity; a key question is how this complex coupling controls both transport and pattern formation. To examine these mechanisms, we study a two-dimensional multi-component lattice gas model, where "tracer" molecules diffuse between a source and a sink separated by a solution of sticky "crowder" molecules that cluster to form dynamically evolving obstacles. In the high temperature limit, crowders and tracers are miscible and transport may be predicted analytically. At intermediate temperatures, crowders phase separate into clusters that drift toward the tracer sink. As a result, steady-state tracer diffusivity depends non-monotonically on both temperature and crowder density and we observe a variety of complex microstructures. In the low temperature limit, crowders rapidly aggregate to form obstacles that are kinetically arrested; if crowder density is near the percolation threshold, resulting tracer diffusivity shows scaling behavior with the same scaling exponent as the random resistor network model. Though highly idealized, this simple model reveals fundamental mechanisms governing coupled gradient-driven diffusion, phase separation, and microstructural evolution in crowded solutions.

cond-mat.soft↗

arXiv2014

Spontaneous chiral symmetry breaking in model bacterial suspensions

Chiral symmetry breaking is ubiquitous in biological systems, from DNA to bacterial suspensions. A key unresolved problem is how chiral structures may spontaneously emerge from achiral interactions. We study a simple model of bacterial suspensions in three dimensions that effectively incorporates active motion and hydrodynamic interactions. We perform large-scale molecular dynamics simulations (up to $10^6$ particles) and describe stable (or long-lived metastable) collective states that exhibit chiral organization although the interactions are achiral. We elucidate under which conditions these chiral states will emerge and grow to large scales. We also study a related equilibrium model that clarifies the role of orientational fluctuations.

cond-mat.soft↗

arXiv2013

Lipid Segregation on Cylindrically and Spherically Curved Membranes

We investigate how an externally imposed curvature influences lipid segregation on two-phase-coexistent membranes. We show that the bending-modulus contrast of the two phases and the curvature act together to yield a reduced effective line tension. On largely curved membranes, a state of multiple domains (or rafts) forms due to a mechanism analogous to that causing magnetic-vortex formation in type-II superconductors. We determine the criterion for such multi-domain state to occur; we then calculate respectively the size of the domains formed on cylindrically and spherically curved membranes.

cond-mat.soft↗

arXiv2013

Shape and Chirality Transitions in Off-Axis Twist Nematic Elastomer Ribbons

Using both experiments and finite element simulations, we explore the shape evolution of off-axis nematic elastomer ribbons as a function of temperature. The elastomers are prepared by cross-linking the mesogens with planar anchoring of the director at top and bottom surfaces with a 90 degree left-handed twist. Shape evolution depends sensitively on the off-axis director orientation at the sample mid-plane. When the director at midplane is parallel to either the ribbon's long or short axes, ribbons form either helicoids or spirals depending on the aspect ratio and temperature. If the director at midplane is more than 5 degrees off-axis, then they form only spiral ribbons. Samples in all these geometries show a remarkable transition from right- to left-handed chiral shapes on change of temperature. Simulation studies provide insight into the mechanisms driving shape evolution and enable engineering design of these materials for future applications.

cond-mat.soft↗

arXiv2011

Coarse-Grained Modeling of a Deformable Nematic Vesicle

We develop a coarse-grained particle-based model to simulate membranes with nematic liquid-crystal order. The coarse-grained particles form vesicles which, at low temperature, have orientational order in the local tangent plane. As the strength of coupling between the nematic director and the vesicle curvature increases, the vesicles show a morphology transition from spherical to prolate and finally to a tube. We also observe the shape and defect arrangement around the tips of the prolate vesicle.

cond-mat.soft↗

arXiv2009

Modeling elastic instabilities in nematic elastomers

Liquid crystal elastomers are cross-linked polymer networks covalently bonded with liquid crystal mesogens. In the nematic phase, due to strong coupling between mechanical strain and orientational order, these materials display strain-induced instabilities associated with formation and evolution of orientational domains. Using a 3-d finite element elastodynamics simulation, we investigate one such instability, the onset of stripe formation in a monodomain film stretched along an axis perpendicular to the nematic director. In our simulation we observe the formation of striped domains with alternating director rotation. This model allows us to explore the fundamental physics governing dynamic mechanical response of nematic elastomers and also provides a potentially useful computational tool for engineering device applications.

cond-mat.soft↗

arXiv2006

Size effects and dislocation patterning in two-dimensional bending

We perform atomistic Monte Carlo simulations of bending a Lennard-Jones single crystal in two dimensions. Dislocations nucleate only at the free surface as there are no sources in the interior of the sample. When dislocations reach sufficient density, they spontaneously coalesce to nucleate grain boundaries, and the resulting microstructure depends strongly on the initial crystal orientation of the sample. In initial yield, we find a reverse size effect, in which larger samples show a higher scaled bending moment than smaller samples for a given strain and strain rate. This effect is associated with source-limited plasticity and high strain rate relative to dislocation mobility, and the size effect in initial yield disappears when we scale the data to account for strain rate effects. Once dislocations coalesce to form grain boundaries, the size effect reverses and we find that smaller crystals support a higher scaled bending moment than larger crystals. This finding is in qualitative agreement with experimental results. Finally, we observe an instability at the compressed crystal surface that suggests a novel mechanism for the formation of a hillock structure. The hillock is formed when a high angle grain boundary, after absorbing additional dislocations, becomes unstable and folds to form a new crystal grain that protrudes from the free surface.

cond-mat.mtrl-sci↗

arXiv2004

Shape Selection in Chiral Self-Assembly

Many biological and synthetic materials self-assemble into helical or twisted aggregates. The shape is determined by a complex interplay between elastic forces and the orientation and chirality of the constituent molecules. We study this interplay through Monte Carlo simulations, with an accelerated algorithm motivated by the growth of an aggregate out of solution. The simulations show that the curvature changes smoothly from cylindrical to saddle-like as the elastic moduli are varied. Remarkably, aggregates of either handedness form from molecules of a single handedness, depending on the molecular orientation.

cond-mat.soft↗

arXiv2003

Simulations of Helix Unwinding in Ferroelectric Liquid Crystals

In bulk ferroelectric liquid crystals, the molecular director twists in a helix. In narrow cells, this helix can be unwound by an applied electric field or by boundary effects. To describe helix unwinding as a function of both electric field and boundary effects, we develop a mesoscale simulation model based on a continuum free energy discretized on a two-dimensional lattice. In these simulations, we determine both the director profile across the cell and the net electrostatic polarization. By varying the cell size, we show how boundary effects shift the critical field for helix unwinding and lower the saturation polarization. Our results are consistent with experimental data.

cond-mat.soft↗

arXiv2001

Dynamics and Patterning of Screw Dislocations in Two Dimensions

To understand how dislocations form ordered structures during the deformation of metals, we perform computer simulation studies of the dynamics and patterning of screw dislocations in two dimensions. The simulation is carried out using an idealized atomistic model with anti-plane displacements only; we show that this system is an analog of the two-dimensional XY rotor model. Simulation studies show that under a constant applied shear strain rate, the flow of dislocations spontaneously coalesces to form narrow dislocation-rich channels separated by wide dislocation-free regions, so that the applied strain is localized into slip bands. We argue that this pattern formation represents a phase separation into ordered and disordered phases associated with the XY model, and conjecture that thermodynamic forces drive strain localization.

cond-mat.mtrl-sci↗

arXiv1998

Fingering Instability of Dislocations and Related Defects

We identify a fundamental morphological instability of mobile dislocations in crystals and related line defects. A positive gradient in the local driving force along the direction of defect motion destabilizes long-wavelength vibrational modes, producing a ``fingering'' pattern. The minimum unstable wavelength scales as the inverse square root of the force gradient. We demonstrate the instability's onset in simulations of a screw dislocation in Al (via molecular dynamics) and of a vortex in a 3-d XY ``rotator'' model.

cond-mat.mtrl-sci↗