362 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
Elastic response of a nematic liquid crystal to an immersed nanowire
We study the immersion of a ferromagnetic nanowire within a nematic liquid
crystal using a lattice Boltzmann algorithm to solve the full three-dimensional
equations of hydrodynamics. We present an algorithm for including a moving
boundary, to simulate a nanowire, in a lattice Boltzmann simulation. The
nematic imposes a torque on a wire that increases linearly with the angle
between the wire and the equilibrium direction of the director field. By
rotation of these nanowires, one can determine the elastic constants of the
nematic.Comment: 10 pages, 8 figure
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
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
Perioperative anaemia management: consensus statement on the role of intravenous iron
A multidisciplinary panel of physicians was convened by Network for Advancement of Transfusion Alternatives to review the evidence on the efficacy and safety of i.v. iron administration to increase haemoglobin levels and reduce blood transfusion in patients undergoing surgery, and to develop a consensus statement on perioperative use of i.v. iron as a transfusion alternative. After conducting a systematic literature search to identify the relevant studies, critical evaluation of the evidence was performed and recommendations formulated using the Grades of Recommendation Assessment, Development and Evaluation Working Group methodology. Two randomized controlled trials (RCTs) and six observational studies in orthopaedic and cardiac surgery were evaluated. Overall, there was little benefit found for the use of i.v. iron. At best, i.v. iron supplementation was found to reduce the proportion of patients requiring transfusions and the number of transfused units in observational studies in orthopaedic surgery but not in cardiac surgery. The two RCTs had serious limitations and the six observational limited by the selection of the control groups. Thus, the quality of the available evidence is considered moderate to very low. For patients undergoing orthopaedic surgery and expected to develop severe postoperative anaemia, the panel suggests i.v. iron administration during the perioperative period (weak recommendation based on moderate/low-quality evidence). For all other types of surgery, no evidence-based recommendation can be made. The panel recommends that large, prospective, RCTs be undertaken to evaluate the efficacy and safety of i.v. iron administration in surgical patients. The implementation of some general good practice points is suggeste
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
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