2,553 research outputs found
Liquid crystals boojum-colloids
Colloidal particles dispersed in a liquid crystal lead to distortions of the
director field. The distortions are responsible for long-range effective
colloidal interactions whose asymptotic behaviour is well understood. The short
distance behaviour of the interaction, however, is sensitive to the structure
and dynamics of the topological defects nucleated near the colloidal particles
in the strong anchoring regime. The full non-linear theory is required in order
to determine the interaction at short separations. Spherical colloidal
particles with sufficiently strong planar degenerate anchoring nucleate a pair
of antipodal surface topological defects, known as boojums. We use the
Landau-de Gennes formalism in order to resolve the mesoscopic structure of the
boojum cores and to determine the pairwise colloidal interaction. We compare
the results in three (3D) and two (2D) spatial dimensions. The corresponding
free energy functionals are minimized numerically using finite elements with
adaptive meshes. Boojums are always point-like in 2D, but acquire a rather
complex structure in 3D which depends on the combination of the anchoring
potential, the radius of the colloid, the temperature and the LC elastic
anisotropy. We identify three types of defect cores in 3D which we call single,
double and split core boojums, and investigate the associated structural
transitions. In the presence of two colloidal particles there are substantial
re-arrangements of the defects at short distances, both in 3D and 2D. These
re-arrangements lead to qualitative changes in the force-distance profile when
compared to the asymptotic quadrupole-quadrupole interaction. In line with the
experimental results, the presence of the defects prevents coalescence of the
colloidal particles in 2D, but not in 3D systems.Comment: 18 pages, 21 figure
Colloidal interactions in two dimensional nematics
The interaction between two disks immersed in a 2D nematic is investigated
(i) analitically using the tensor order parameter formalism for the nematic
configuration around isolated disks and (ii) numerically using finite element
methods with adaptive meshing to minimize the corresponding Landau-de Gennes
free energy. For strong homeotropic anchoring, each disk generates a pair of
defects with one-half topological charge responsible for the 2D quadrupolar
interaction between the disks at large distances. At short distance, the
position of the defects may change, leading to unexpected complex interactions
with the quadrupolar repulsive interactions becoming attractive. This short
range attraction in all directions is still anisotropic. As the distance
between the disks decreases their preferred relative orientation with respect
to the far-field nematic director changes from oblique to perpendicular.Comment: 7 pages, 7 figure
Complex fluids at complex surfaces simply complicated
We study wetting and filling of patterned surfaces by a nematic liquid crystal. We focus on three important classes of periodic surfaces: triangular, sinusoidal and rectangular. The results highlight the similarities and differences of nematic wetting of these surfaces and wetting by simple fluids. The interplay of geometry, surface and elastic energies can lead to the suppression of either filling or wetting. The periodic rectangular surface displays re-entrant transitions, with a sequence dry-filled-wet-filled, in the relevant region of parameter space
Filling and wetting transitions of nematic liquid crystals on sinusoidal substrates
Close to sinusoidal substrates, simple fluids may undergo a filling
transition, in which the fluid passes from a dry to a filled state, where the
interface remains unbent but bound to the substrate. Increasing the surface
field, the interface unbinds and a wetting transition occurs. We show that this
double-transition sequence may be strongly modified in the case of ordered
fluids, such as nematic liquid crystals. Depending on the preferred orientation
of the nematic molecules at the structured substrate and at the
isotropic-nematic interface, the filling transition may not exist, and the
fluid passes directly from a dry to a complete-wet state, with the interface
far from the substrate. More interestingly, in other situations, the complete
wetting transition may be prevented, and the fluid passes from a dry to a
filled state, and remains in this configuration, with the interface always
attached to the substrate, even for very large surface fields. Both transitions
are only observed for a same substrate in a narrow range of amplitudes
KrP laser CVD of chromium oxide by photodissociation of Cr(CO)(6)
This work reports on the synthesis of chromium oxide thin films prepared by photodissociation of Cr(CO)(6) in an oxidizing atmosphere, using a pulsed UV laser (KrF, lambda = 248 nm). The experimental conditions, which should enable the synthesis of CrO2, are discussed and results on the deposition of CrxOy films on Al2O3 (0001) substrates are presented
Production and detection of doubly charmed tetraquarks
The feasibility of tetraquark detection is studied. For the cc\bar{u}\bar{d}
tetraquark we show that in present (SELEX, Tevatron, RHIC) and future
facilities (LHCb, ALICE) the production rate is promising and we propose some
detectable decay channels.Comment: 6 pages, 5 figure
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