663 research outputs found
Spinodal decomposition to a lamellar phase: effects of hydrodynamic flow
Results are presented for the kinetics of domain growth of a two-dimensional
fluid quenched from a disordered to a lamellar phase. At early times when a
Lifshitz-Slyozov mechanism is operative the growth process proceeds
logarithmically in time to a frozen state with locked-in defects. However when
hydrodynamic modes become important, or the fluid is subjected to shear, the
frustration of the system is alleviated and the size and orientation of the
lamellae attain their equilibrium values.Comment: 4 Revtex pages, 4 figures, to appear in Physical Review Letter
Lattice Boltzmann Algorithm for three-dimensional liquid crystal hydrodynamics
We describe a lattice Boltzmann algorithm to simulate liquid crystal
hydrodynamics in three dimensions. 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 effects
such as shear-thinning and shear-banding. We describe the implementation of
velocity boundary conditions and show that the algorithm can be used to
describe optical bounce in twisted nematic devices and secondary flow in
sheared nematics with an imposed twist.Comment: 12 pages, 3 figure
Rheology of cholesteric blue phases
Blue phases of cholesteric liquid crystals offer a spectacular example of
naturally occurring disclination line networks. Here we numerically solve the
hydrodynamic equations of motion to investigate the response of three types of
blue phases to an imposed Poiseuille flow. We show that shear forces bend and
twist and can unzip the disclination lines. Under gentle forcing the network
opposes the flow and the apparent viscosity is significantly higher than that
of an isotropic liquid. With increased forcing we find strong shear thinning
corresponding to the disruption of the defect network. As the viscosity starts
to drop, the imposed flow sets the network into motion. Disclinations break-up
and re-form with their neighbours in the flow direction. This gives rise to
oscillations in the time-dependent measurement of the average stress.Comment: 4 pages, 4 figure
Phase ordering of two-dimensional symmetric binary fluids: a droplet scaling state
The late-stage phase ordering, in dimensions, of symmetric fluid
mixtures violates dynamical scaling. We show however that, even at 50/50 volume
fractions, if an asymmetric droplet morphology is initially present then this
sustains itself, throughout the viscous hydrodynamic regime, by a
`coalescence-induced coalescence' mechanism. Scaling is recovered (with length
scale , as in ). The crossover to the inertial hydrodynamic
regime is delayed even longer than in ; on entering it, full symmetry is
finally restored and we find , regardless of the initial state.Comment: 4 pages, three figures include
An H-Theorem for the Lattice Boltzmann Approach to Hydrodynamics
The lattice Boltzmann equation can be viewed as a discretization of the
continuous Boltzmann equation. Because of this connection it has long been
speculated that lattice Boltzmann algorithms might obey an H-theorem. In this
letter we prove that usual nine-velocity models do not obey an H-theorem but
models that do obey an H-theorem can be constructed. We consider the general
conditions a lattice Boltzmann scheme must satisfy in order to obey an
H-theorem and show why on a lattice, unlike the continuous case, dynamics that
decrease an H-functional do not necessarily lead to a unique ground state.Comment: 6 pages, latex, no figures, accepted for publication in Europhys.
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Lattice Boltzmann simulations of lamellar and droplet phases
Lattice Boltzmann simulations are used to investigate spinodal decomposition
in a two-dimensional binary fluid with equilibrium lamellar and droplet phases.
We emphasise the importance of hydrodynamic flow to the phase separation
kinetics. For mixtures slightly asymmetric in composition the fluid phase
separates into bulk and lamellar phases with the lamellae forming distinctive
spiral structures to minimise their elastic energy.Comment: 19 pages, 5 figure
Natural marine and terrestrial compounds as modulators of matrix metalloproteinases-2 (MMP-2) and MMP-9 in alzheimer’s disease
Several studies have reported neuroprotective effects by natural products. A wide range of natural compounds have been investigated, and some of these may play a beneficial role in Alzheimer’s disease (AD) progression. Matrix metalloproteinases (MMPs), a family of zinc-dependent endopeptidases, have been implicated in AD. In particular, MMP-2 and MMP-9 are able to trigger several neuroinflammatory and neurodegenerative pathways. In this review, we summarize and discuss existing literature on natural marine and terrestrial compounds, as well as their ability to modulate MMP-2 and MMP-9, and we evaluate their potential as therapeutic compounds for neurodegenerative and neuroinflammatory diseases, with a focus on Alzheimer’s disease
Forcing Adsorption of a Tethered Polymer by Pulling
We present an analysis of a partially directed walk model of a polymer which
at one end is tethered to a sticky surface and at the other end is subjected to
a pulling force at fixed angle away from the point of tethering. Using the
kernel method, we derive the full generating function for this model in two and
three dimensions and obtain the respective phase diagrams.
We observe adsorbed and desorbed phases with a thermodynamic phase transition
in between. In the absence of a pulling force this model has a second-order
thermal desorption transition which merely gets shifted by the presence of a
lateral pulling force. On the other hand, if the pulling force contains a
non-zero vertical component this transition becomes first-order.
Strikingly, we find that if the angle between the pulling force and the
surface is beneath a critical value, a sufficiently strong force will induce
polymer adsorption, no matter how large the temperature of the system.
Our findings are similar in two and three dimensions, an additional feature
in three dimensions being the occurrence of a reentrance transition at constant
pulling force for small temperature, which has been observed previously for
this model in the presence of pure vertical pulling. Interestingly, the
reentrance phenomenon vanishes under certain pulling angles, with details
depending on how the three-dimensional polymer is modeled
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