519 research outputs found
Field-driven dynamics of nematic microcapillaries
Polymer-dispersed liquid crystal (PDLC) composites have long been a focus of
study for their unique electro-optical properties which have resulted in
various applications such as switchable (transparent/translucent) windows.
These composites are manufactured using desirable "bottom-up" techniques, such
as phase separation of a liquid crystal/polymer mixture, which enable
production of PDLC films at very large scales. LC domains within PDLCs are
typically spheroidal, as opposed to rectangular for an LCD panel, and thus
exhibit substantially different behaviour in the presence of an external field.
The fundamental difference between spheroidal and rectangular nematic domains
is that the former results in the presence of nanoscale orientational defects
in LC order while the latter does not. Progress in the development and
optimization of PDLC electro-optical properties has progressed at a relatively
slow pace due to this increased complexity. In this work, continuum simulations
are performed in order to capture the complex formation and electric
field-driven switching dynamics of approximations of PDLC domains. Using a
simplified elliptic cylinder (microcapillary) geometry as an approximation of
spheroidal PDLC domains, the effects of geometry (aspect ratio), surface
anchoring, and external field strength are studied through the use of the
Landau--de Gennes model of the nematic LC phase.Comment: 22 pages, 9 figures, Physical Review
Pair creation, motion, and annihilation of topological defects in 2D nematics
We present a novel framework for the study of disclinations in
two-dimensional active nematic liquid crystals, and topological defects in
general. The order tensor formalism is used to calculate exact multi-particle
solutions of the linearized static equations inside a uniformly aligned state.
Topological charge conservation requires a fixed difference between the number
of half charges. Starting from a set of hydrodynamic equations, we derive a
low-dimensional dynamical system for the parameters of the static solutions,
which describes the motion of a half-disclination pair, or of several pairs.
Within this formalism, we model defect production and annihilation, as observed
in experiments. Our dynamics also provide an estimate for the critical density
at which production and annihilation rates are balanced
Defect unbinding in active nematics
We formulate the statistical dynamics of topological defects in the active
nematic phase, formed in two dimensions by a collection of self-driven
particles on a substrate. An important consequence of the non-equilibrium drive
is the spontaneous motility of strength +1/2 disclinations. Starting from the
hydrodynamic equations of active nematics, we derive an interacting particle
description of defects that includes active torques. We show that activity,
within perturbation theory, lowers the defect-unbinding transition temperature,
determining a critical line in the temperature-activity plane that separates
the quasi-long-range ordered (nematic) and disordered (isotropic) phases. Below
a critical activity, defects remain bound as rotational noise decorrelates the
directed dynamics of +1/2 defects, stabilizing the quasi-long-range ordered
nematic state. This activity threshold vanishes at low temperature, leading to
a re-entrant transition. At large enough activity, active forces always exceed
thermal ones and the perturbative result fails, suggesting that in this regime
activity will always disorder the system. Crucially, rotational diffusion being
a two-dimensional phenomenon, defect unbinding cannot be described by a
simplified one-dimensional model.Comment: 15 pages (including SI), 4 figures. Significant technical
improvements without changing the result
Cross-talk between topological defects in different fields revealed by nematic microfluidics
Topological defects are singularities in material fields that play a vital
role across a range of systems: from cosmic microwave background polarization
to superconductors, and biological materials. Although topological defects and
their mutual interactions have been extensively studied, little is known about
the interplay between defects in different fields -- especially when they
co-evolve -- within the same physical system. Here, using nematic
microfluidics, we study the cross-talk of topological defects in two different
material fields -- the velocity field and the molecular orientational field.
Specifically, we generate hydrodynamic stagnation points of different
topological charges at the center of star-shaped microfluidic junctions, which
then interact with emergent topological defects in the orientational field of
the nematic director. We combine experiments, and analytical and numerical
calculations to demonstrate that a hydrodynamic singularity of given
topological charge can nucleate a nematic defect of equal topological charge,
and corroborate this by creating , and topological defects in
, , and arm junctions. Our work is an attempt toward understanding
materials that are governed by distinctly multi-field topology, where disparate
topology-carrying fields are coupled, and concertedly determine the material
properties and response.Comment: 18 pages, 9 figure
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