194 research outputs found
Bistable defect structures in blue phase devices
Blue phases (BPs) are liquid crystals made up by networks of defects, or
disclination lines. While existing phase diagrams show a striking variety of
competing metastable topologies for these networks, very little is known as to
how to kinetically reach a target structure, or how to switch from one to the
other, which is of paramount importance for devices. We theoretically identify
two confined blue phase I systems in which by applying an appropriate series of
electric field it is possible to select one of two bistable defect patterns.
Our results may be used to realise new generation and fast switching
energy-saving bistable devices in ultrathin surface treated BPI wafers.Comment: 4 pages, 3 figures. Accepted for publication in Phys. Rev. Let
Stabilising the Blue Phases
We present an investigation of the phase diagram of cholesteric liquid
crystals within the framework of Landau - de Gennes theory. The free energy is
modified to incorporate all three Frank elastic constants and to allow for a
temperature dependent pitch in the cholesteric phase. It is found that the
region of stability of the cubic blue phases depends significantly on the value
of the elastic constants, being reduced when the bend elastic constant is
larger than splay and when twist is smaller than the other two. Most
dramatically we find a large increase in the region of stability of blue phase
I, and a qualitative change in the phase diagram, in a system where the
cholesteric phase displays helix inversion.Comment: 15 pages, 6 figure
Identification and Calculation of the Universal Maximum Drag Reduction Asymptote by Polymers in Wall Bounded Turbulence
Drag reduction by polymers in wall turbulence is bounded from above by a
universal maximal drag reduction (MDR) velocity profile that is a log-law,
estimated experimentally by Virk as . Here
and are the mean streamwise velocity and the distance from the
wall in "wall" units. In this Letter we propose that this MDR profile is an
edge solution of the Navier-Stokes equations (with an effective viscosity
profile) beyond which no turbulent solutions exist. This insight rationalizes
the universality of the MDR and provides a maximum principle which allows an
ab-initio calculation of the parameters in this law without any viscoelastic
experimental input.Comment: 4 pages, 1 fig. Phys. Rev. Letts., submitte
Drag Reduction by Polymers in Wall Bounded Turbulence
We address the mechanism of drag reduction by polymers in turbulent wall
bounded flows. On the basis of the equations of fluid mechanics we present a
quantitative derivation of the "maximum drag reduction (MDR) asymptote" which
is the maximum drag reduction attained by polymers. Based on Newtonian
information only we prove the existence of drag reduction, and with one
experimental parameter we reach a quantitative agreement with the experimental
measurements.Comment: 4 pages, 1 fig., included, PRL, submitte
The geometry and thermodynamics of dissipative quantum systems
Dirac's method of classical analogy is employed to incorporate quantum
degrees of freedom into modern nonequilibrium thermodynamics. The proposed
formulation of dissipative quantum mechanics builds entirely upon the geometric
structures implied by commutators and canonical correlations. A lucid
formulation of a nonlinear quantum master equation follows from the
thermodynamic structure. Complex classical environments with internal structure
can be handled readily.Comment: 4 pages, definitely no figure
Thermodynamically guided nonequilibrium Monte Carlo method for generating realistic shear flows in polymeric systems
A thermodynamically guided atomistic MonteCarlo methodology is presented for simulating systems beyond equilibrium by expanding the statistical ensemble to include a tensorial variable accounting for the overall structure of the system subjected to flow. For a given shear rate, the corresponding tensorial conjugate field is determined iteratively through independent nonequilibrium molecular dynamics simulations. Test simulations for the effect of flow on the conformation of a C50H102 polyethylene liquid show that the two methods (expanded MonteCarlo and nonequilibrium molecular dynamics) provide identical results.open181
Rheology of distorted nematic liquid crystals
We use lattice Boltzmann simulations of the Beris--Edwards formulation of
nematodynamics to probe the response of a nematic liquid crystal with
conflicting anchoring at the boundaries under shear and Poiseuille flow. The
geometry we focus on is that of the hybrid aligned nematic (HAN) cell, common
in devices. In the nematic phase, backflow effects resulting from the elastic
distortion in the director field render the velocity profile strongly
non-Newtonian and asymmetric. As the transition to the isotropic phase is
approached, these effects become progressively weaker. If the fluid is heated
just above the transition point, however, another asymmetry appears, in the
dynamics of shear band formation.Comment: 7 pages, 4 figures. Accepted for publication in Europhys. Let
Lattice Boltzmann Simulations of Liquid Crystal Hydrodynamics
We describe a lattice Boltzmann algorithm to simulate liquid crystal
hydrodynamics. The equations of motion are written in terms of a tensor order
parameter. This allows both the isotropic and the nematic phases to be
considered. Backflow effects and the hydrodynamics of topological defects are
naturally included in the simulations, as are viscoelastic properties such as
shear-thinning and shear-banding.Comment: 14 pages, 5 figures, Revte
Steady-state hydrodynamic instabilities of active liquid crystals: Hybrid lattice Boltzmann simulations
We report hybrid lattice Boltzmann (HLB) simulations of the hydrodynamics of
an active nematic liquid crystal sandwiched between confining walls with
various anchoring conditions. We confirm the existence of a transition between
a passive phase and an active phase, in which there is spontaneous flow in the
steady state. This transition is attained for sufficiently ``extensile'' rods,
in the case of flow-aligning liquid crystals, and for sufficiently
``contractile'' ones for flow-tumbling materials. In a quasi-1D geometry, deep
in the active phase of flow-aligning materials, our simulations give evidence
of hysteresis and history-dependent steady states, as well as of spontaneous
banded flow. Flow-tumbling materials, in contrast, re-arrange themselves so
that only the two boundary layers flow in steady state. Two-dimensional
simulations, with periodic boundary conditions, show additional instabilities,
with the spontaneous flow appearing as patterns made up of ``convection
rolls''. These results demonstrate a remarkable richness (including dependence
on anchoring conditions) in the steady-state phase behaviour of active
materials, even in the absence of external forcing; they have no counterpart
for passive nematics. Our HLB methodology, which combines lattice Boltzmann for
momentum transport with a finite difference scheme for the order parameter
dynamics, offers a robust and efficient method for probing the complex
hydrodynamic behaviour of active nematics.Comment: 18 eps figures, accepted for publication in Phys. Rev.
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