1,144 research outputs found
Hard discs under steady shear: comparison of Brownian dynamics simulations and mode coupling theory
Brownian dynamics simulations of bidisperse hard discs moving in two
dimensions in a given steady and homogeneous shear flow are presented close to
and above the glasstransition density. The stationary structure functions and
stresses of shear-melted glass are compared quantitatively to parameter-free
numerical calculations of monodisperse hard discs using mode coupling theory
within the integration through transients framework. Theory qualitatively
explains the properties of the yielding glass but quantitatively
overestimatesthe shear-driven stresses and structural anisotropies.Comment: 1. The original Phil. Trans. R. Soc. contains an error in the caption
of the y-axes of the upper left panel in figure 9: There's a factor
\dot{\gamma} missing in the denominato
Bond formation and slow heterogeneous dynamics in adhesive spheres with long--ranged repulsion: Quantitative test of Mode Coupling Theory
A colloidal system of spheres interacting with both a deep and narrow
attractive potential and a shallow long-ranged barrier exhibits a prepeak in
the static structure factor. This peak can be related to an additional
mesoscopic length scale of clusters and/or voids in the system. Simulation
studies of this system have revealed that it vitrifies upon increasing the
attraction into a gel-like solid at intermediate densities. The dynamics at the
mesoscopic length scale corresponding to the prepeak represents the slowest
mode in the system. Using mode coupling theory with all input directly taken
from simulations, we reveal the mechanism for glassy arrest in the system at
40% packing fraction. The effects of the low-q peak and of polydispersity are
considered in detail. We demonstrate that the local formation of physical bonds
is the process whose slowing down causes arrest.
It remains largely unaffected by the large-scale heterogeneities, and sets
the clock for the slow cluster mode. Results from mode-coupling theory without
adjustable parameters agree semi-quantitatively with the local density
correlators but overestimate the lifetime of the mesoscopic structure (voids).Comment: 10 pages, 8 figure
Thermodynamics of Blue Phases In Electric Fields
We present extensive numerical studies to determine the phase diagrams of
cubic and hexagonal blue phases in an electric field. We confirm the earlier
prediction that hexagonal phases, both 2 and 3 dimensional, are stabilized by a
field, but we significantly refine the phase boundaries, which were previously
estimated by means of a semi-analytical approximation. In particular, our
simulations show that the blue phase I -- blue phase II transition at fixed
chirality is largely unaffected by electric field, as observed experimentally.Comment: submitted to Physical Review E, 7 pages (excluding figures), 12
figure
Structure of Blue Phase III of Cholesteric Liquid Crystals
We report large scale simulations of the blue phases of cholesteric liquid crystals. Our results suggest a structure for blue phase III, the blue fog, which has been the subject of a long debate in liquid crystal physics. We propose that blue phase III is an amorphous network of disclination lines, which is thermodynamically and kinetically stabilised over crystalline blue phases at intermediate chiralities}. This amorphous network becomes ordered under an applied electric field, as seen in experiments
Scaling soft matter physics to thousands of graphics processing units in parallel
We describe a multi-graphics processing unit (GPU) implementation of the Ludwig application, which specialises in simulating a variety of complex fluids via lattice Boltzmann fluid dynamics coupled to additional physics describing complex fluid constituents. We describe our methodology in augmenting the original central processing unit (CPU) version with GPU functionality in a maintainable fashion. We present several optimisations that maximise performance on the GPU architecture through tuning for the GPU memory hierarchy. We describe how we implement particles within the fluid in such a way to avoid a major diversion of the CPU and GPU codebases, whilst minimising data transfer at each time step. We detail our halo-exchange communication phase for the code, which exploits overlapping to allow efficient parallel scaling to many GPUs. We present results showing that the application demonstrates excellent scaling to at least 8192 GPUs in parallel, the largest system tested at the time of writing. The GPU version (on NVIDIA K20X GPUs) is around 3.5-5 times faster that the CPU version (on fully utilised AMD Opteron 6274 16-core CPUs), comparing equal numbers of CPUs and GPUs
Rheology of Lamellar Liquid Crystals in Two and Three Dimensions: A Simulation Study
We present large scale computer simulations of the nonlinear bulk rheology of
lamellar phases (smectic liquid crystals) at moderate to large values of the
shear rate (Peclet numbers 10-100), in both two and three dimensions. In two
dimensions we find that modest shear rates align the system and stabilise an
almost regular lamellar phase, but high shear rates induce the nucleation and
proliferation of defects, which in steady state is balanced by the annihilation
of defects of opposite sign. The critical shear rate at onset of this second
regime is controlled by thermodynamic and kinetic parameters; we offer a
scaling analysis that relates the critical shear rate to a critical "capillary
number" involving those variables. Within the defect proliferation regime, the
defects may be partially annealed by slowly decreasing the applied shear rate;
this causes marked memory effects, and history-dependent rheology. Simulations
in three dimensions show instead shear-induced ordering even at the highest
shear rates studied here. This suggests that the critical shear rate shifts
markedly upward on increasing dimensionality. This may in part reflect the
reduced constraints on defect motion, allowing them to find and annihilate each
other more easily. Residual edge defects in the 3D aligned state mostly point
along the flow velocity, an orientation impossible in two dimensions.Comment: 18 pages, 12 figure
Ordering dynamics of blue phases entails kinetic stabilization of amorphous networks
The cubic blue phases of liquid crystals are fascinating and technologically
promising examples of hierarchically structured soft materials, comprising
ordered networks of defect lines (disclinations) within a liquid crystalline
matrix. We present the first large-scale simulations of their domain growth,
starting from a blue phase nucleus within a supercooled isotropic or
cholesteric background. The nucleated phase is thermodynamically stable; one
expects its slow orderly growth, creating a bulk cubic. Instead, we find that
the strong propensity to form disclinations drives the rapid disorderly growth
of a metastable amorphous defect network. During this process the original
nucleus is destroyed; re-emergence of the stable phase may therefore require a
second nucleation step. Our findings suggest that blue phases exhibit
hierarchical behavior in their ordering dynamics, to match that in their
structure.Comment: 11 pages, 5 figures, 2 supplementary figures, 2 supplementary tables,
accepted by PNA
Domain growth in cholesteric blue phases: hybrid lattice Boltzmann simulations
Here we review a hybrid lattice Boltzmann algorithm to solve the equations of
motion of cholesteric liquid crystals. The method consists in coupling a
lattice Boltzmann solver for the Navier-Stokes equation to a finite difference
method to solve the dynamical equations governing the evolution of the liquid
crystalline order parameter. We apply this method to study the growth of
cholesteric blue phase domains, within a cholesteric phase. We focus on the
growth of blue phase II and on a thin slab geometry in which the domain wall is
flat. Our results show that, depending on the chirality, the growing blue phase
is either BPII with no or few defects, or another structure with hexagonal
ordering. We hope that our simulations will spur further experimental
investigations on quenches in micron-size blue phase samples. The computational
size that our hybrid lattice Boltzmann scheme can handle suggest that large
scale simulations of new generation of blue phase liquid crystal device are
within reach.Comment: 22 pages, 4 figure
Parental Distress, Parenting Practices, and Child Adaptive Outcomes Following Traumatic Brain Injury
Moderate and severe pediatric traumatic brain injuries (TBI) are associated with significant familial distress and child adaptive sequelae. Our aim was to examine the relationship between parental psychological distress, parenting practices (authoritarian, permissive, authoritative), and child adaptive functioning 12–36 months following TBI or orthopedic injury (OI). Injury type was hypothesized to moderate the relationship between parental distress and child adaptive functioning, demonstrating a significantly stronger relationship in the TBI relative to OI group. Authoritarian parenting practices were hypothesized to mediate relationship between parental distress and child adaptive functioning across groups. Groups (TBI n=21, OI n=23) did not differ significantly on age at injury, time since injury, sex, race, or SES. Parents completed the Brief Symptom Inventory, Parenting Practices Questionnaire, and Vineland-II. Moderation and mediation hypotheses were tested using hierarchical multiple regression and a bootstrapping approach, respectively. Results supported moderation and revealed that higher parental psychological distress was associated with lower child adaptive functioning in the TBI group only. Mediation results indicated that higher parental distress was associated with authoritarian parenting practices and lower adaptive functioning across groups. Results suggest that parenting practices are an important area of focus for studies attempting to elucidate the relationship between parent and child functioning following TBI
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