545 research outputs found
Hydrodynamics of topological defects in nematic liquid crystals
We show that back-flow, the coupling between the order parameter and the
velocity fields, has a significant effect on the motion of defects in nematic
liquid crystals. In particular the defect speed can depend strongly on the
topological strength in two dimensions and on the sense of rotation of the
director about the core in three dimensions.Comment: 4 pages including two figure
Simulations of collision times in gravity driven granular flow
We use simulations to investigate collision time distributions as one
approaches the static limit of steady-state flow of dry granular matter. The
collision times fall in a power-law distribution with an exponent dictated by
whether the grains are ordered or disordered. Remarkably, the exponents have
almost no dependence on dimension. We are also able to resolve a disagreement
between simulation and experiments on the exponent of the collision time
power-law distribution.Comment: 7 pages, 5 figure
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
Hydrodynamics of domain growth in nematic liquid crystals
We study the growth of aligned domains in nematic liquid crystals. Results
are obtained solving the Beris-Edwards equations of motion using the lattice
Boltzmann approach. Spatial anisotropy in the domain growth is shown to be a
consequence of the flow induced by the changing order parameter field
(backflow). The generalization of the results to the growth of a cylindrical
domain, which involves the dynamics of a defect ring, is discussed.Comment: 12 revtex-style pages, including 12 figures; small changes before
publicatio
Numerical calculations of the phase diagram of cubic blue phases in cholesteric liquid crystals
We study the static properties of cubic blue phases by numerically minimising
the three-dimensional, Landau-de Gennes free energy for a cholesteric liquid
crystal close to the isotropic-cholesteric phase transition. Thus we are able
to refine the powerful but approximate, semi-analytic frameworks that have been
used previously. We obtain the equilibrium phase diagram and discuss it in
relation to previous results. We find that the value of the chirality above
which blue phases appear is shifted by 20% (towards experimentally more
accessible regions) with respect to previous estimates. We also find that the
region of stability of the O5 structure -- which has not been observed
experimentally -- shrinks, while that of BP I (O8-) increases thus giving the
correct order of appearance of blue phases at small chirality. We also study
the approach to equilibrium starting from the infinite chirality solutions and
we find that in some cases the disclination network has to assemble during the
equilibration. In these situations disclinations are formed via the merging of
isolated aligned defects.Comment: 16 pages, 5 figures. Accepted for publication in Phys. Rev.
Domain Walls and Phase Transitions in the Frustrated Two-Dimensional XY Model
We study and compare the critical properties of the two-dimensional (2D) XY
model in a transverse magnetic field with magnetic filling factors f=1/3 and
f=2/5. In addition to the spin waves, the low energy excitations of the system
consist of various domain walls between degenerate ground states. The lowest
energy domain wall has a similar structure for both f=1/3 and f=2/5 and its
properties dictate the nature of the phase transition. For f=2/5 these lowest
energy walls have a negative energy for binding to each other, giving rise to a
branching domain-wall structure and leading to a first order phase transition.
For f=1/3 this binding energy is positive, resulting in a linear critical
interface. In order to make a comparison to recent experiments, we investigate
the effect of small quenched bond disorder for f=2/5. A finite-size scaling
analysis of extensive Monte Carlo simulations strongly suggests that the
critical exponents of the phase transition for f=1/3, and for f=2/5 with
disorder, fall into the universality class of the two-dimensional Ising model.Comment: 5 pages, 3 eps figures, REVTEX, revised version with new figure
Hydroclimate variability from western Iberia (Portugal) during the Holocene: insights from a composite stalagmite isotope record
© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Thatcher, D. L., Wanamaker, A. D., Denniston, R. F., Asmerom, Y., Polyak, V. J., Fullick, D., Ummenhofer, C. C., Gillikin, D. P., & Haws, J. A. Hydroclimate variability from western Iberia (Portugal) during the Holocene: insights from a composite stalagmite isotope record. Holocene, (2020): 095968362090864, doi:10.1177/0959683620908648.Iberia is predicted under future warming scenarios to be increasingly impacted by drought. While it is known that this region has experienced multiple intervals of enhanced aridity over the Holocene, additional hydroclimate-sensitive records from Iberia are necessary to place current and future drying into a broader perspective. Toward that end, we present a multi-proxy composite record from six well-dated and overlapping speleothems from Buraca Gloriosa (BG) cave, located in western Portugal. The coherence between the six stalagmites in this composite stalagmite record illustrates that climate (not in-cave processes) impacts speleothem isotopic values. This record provides the first high-resolution, precisely dated, terrestrial record of Holocene hydroclimate from west-central Iberia. The BG record reveals that aridity in western Portugal increased secularly from 9.0 ka BP to present, as evidenced by rising values of both carbon (δ13C) and oxygen (δ18O) stable isotope values. This trend tracks the decrease in Northern Hemisphere summer insolation and parallels Iberian margin sea surface temperatures (SST). The increased aridity over the Holocene is consistent with changes in Hadley Circulation and a southward migration of the Intertropical Convergence Zone (ITCZ). Centennial-scale shifts in hydroclimate are coincident with changes in total solar irradiance (TSI) after 4 ka BP. Several major drying events are evident, the most prominent of which was centered around 4.2 ka BP, a feature also noted in other Iberian climate records and coinciding with well-documented regional cultural shifts. Substantially, wetter conditions occurred from 0.8 ka BP to 0.15 ka BP, including much of the ‘Little Ice Age’. This was followed by increasing aridity toward present day. This composite stalagmite proxy record complements oceanic records from coastal Iberia, lacustrine records from inland Iberia, and speleothem records from both northern and southern Spain and depicts the spatial and temporal variability in hydroclimate in Iberia.The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported, in part, by the US National Science Foundation (Grants: #1804528 to ADW; #1804635 to RD; #1804132 to CCU; #1806025 to YA and VP; #1805163 to DPG; BCS-0455145, BCS-0612923, and BCS-1118155 to JAH)
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
- …