323 research outputs found
Generalized van der Waals theory for the twist elastic modulus and helical pitch of cholesterics
We present a generalized van der Waals theory for a lyotropic cholesteric
system of chiral spherocylinders based on the classical Onsager theory for hard
anisometric bodies. The rods consist of a hard spherocylindrical backbone
surrounded with a square-well potential to account for attractive (or soft
repulsive) interactions. Long-ranged chiral interactions are described by means
of a simple pseudo-scalar potential which is appropriate for weak chiral forces
of a predominant electrostatic origin. Based on the formalism proposed by
Straley [Phys. Rev. A {\bf 14}, 1835 (1976)] we derive explicit algebraic
expressions for the twist elastic modulus and the cholesteric pitch for rods as
a function of density and temperature. The pitch varies non-monotonically with
density, with a sharp decrease at low packing fractions and a marked increase
at higher packing fractions. A similar trend is found for the temperature
dependence. The unwinding of the helical pitch at high densities (or low
temperatures) originates from a marked increase in the local nematic order and
a steep increase of the twist elastic resistance associated with near-parallel
local rod configurations. This contrasts with the commonly held view that the
increase in pitch with decreasing temperature as often observed in cholesterics
is due to layer formation resulting from pre-smectic fluctuations. The increase
in pitch with increasing temperature is consistent with an entropic unwinding
as the chiral interaction becomes less and less significant than the thermal
energy. The variation of the pitch with density, temperature and contour length
is in qualitative agreement with recent experimental results on colloidal {\em
fd} rods.Comment: 17 pages, 6 figures, to appear in J. Chem. Phy
Aggregation of self-propelled colloidal rods near confining walls
Non-equilibrium collective behavior of self-propelled colloidal rods in a
confining channel is studied using Brownian dynamics simulations and dynamical
density functional theory. We observe an aggregation process in which rods
self-organize into transiently jammed clusters at the channel walls. In the
early stage of the process, fast-growing hedgehog-like clusters are formed
which are largely immobile. At later stages, most of these clusters dissolve
and mobilize into nematized aggregates sliding past the walls.Comment: 5 pages, 4 figure
Nanorings in planar confinement: the role of repulsive surfaces on the formation of lacuna smectics
We study the structure and liquid-crystalline phase behaviour of a model of nonconvex circular soft-repulsive nanorings con ned in a planar slit geometry using molecular-dynamics simulation. The separation distance between the structureless parallel soft-repulsive walls is large enough to allow for the formation of a distinct bulk phase in the central region of the box which is in coexistence with the adsorbed uid thus allowing the analysis of single wall e ects. As the concentration of the particles is increased, the uid adsorbs (wets) onto the planar surfaces leading to the formation of well-de ned smectic-A layers with a spacing proportional to the diameter of the rings. An analysis of the nematic order parameter at distances perpendicular to the surface reveals that the particles in each layer exhibit antinematic behaviour and planar (edge-on) anchoring relative to the short symmetry axis of the rings. This behaviour is in stark contrast to the behaviour observed in convex disc-like particles that have the tendency to form nematic (discotic) structures with hometropic (face-on) anchoring. The smectic phases formed by nanorings in the bulk and under con nement are characterized by the formation of low-density layered liquid-crystalline states with large voids, referred to here as lacuna smectic phases. In contrast to what is typically found for con ned liquid-crystalline systems involving convex particles, no apparent biaxiality is found for the nanorings in planar con nement. We argue that formation of the low-density lacuna smectic layers with planar anchoring is a consequence of the non-convex shape of the circular rings that allow for interpenetration between the particles as observed for nanorings under bulk conditions [Avenda~no et al., Proc. Natl. Acad. Sci. U. S. A. 113, 9699 (2016); H. H. Wensink and C. Avenda~no, Phys. Rev. E 94 062704 (2016)]
Supersaturated dispersions of rod-like viruses with added attraction
The kinetics of isotropic-nematic (I-N) and nematic-isotropic (N-I) phase
transitions in dispersions of rod-like {\it fd}-viruses are studied.
Concentration quenches were applied using pressure jumps in combination with
polarization microscopy, birefringence and turbidity measurements. The full
biphasic region could be accessed, resulting in the construction of a first
experimental analogue of the bifurcation diagram. The N-I spinodal points for
dispersions of rods with varying concentrations of depletion agents (dextran)
were obtained from orientation quenches, using cessation of shear flow in
combination with small angle light scattering. We found that the location of
the N-I spinodal point is independent of the attraction, which was confirmed by
theoretical calculations. Surprisingly, the experiments showed that also the
absolute induction time, the critical nucleus and the growth rate are
insensitive of the attraction, when the concentration is scaled to the distance
to the phase boundaries.Comment: 13 pages, 14 figures. accepted in Phsical Review
Nematic order of model goethite nanorods in a magnetic field
We explore the nematic order of model goethite nanorods in an external
magnetic field within Onsager-Parsons density functional theory. The goethite
rods are represented by monodisperse, charged spherocylinders with a permanent
magnetic moment along the rod main axis, forcing the particles to align
parallel to the magnetic field at low field strength. The intrinsic diamagnetic
susceptibility anisometry of the rods is negative which leads to a preferred
perpendicular orientation at higher field strength. It is shown that these
counteracting effects may give rise to intricate phase behavior, including a
pronounced stability of biaxial nematic order and the presence of reentrant
phase transitions and demixing phenomena. The effect of the applied field on
the nematic-to-smectic transition will also be addressed.Comment: 12 pages, 5 figures, submitted to Phys. Rev.
The Use of Radar Imagery to Assess the Bottom Topography of Shallow Seas
Under favourable conditions features of the bottom topography of shallow seas are visible in radar images, which are nowadays obtained from satellites on a routine basis. A Bathymetry Assessment System (BAS) was developed to use these images in order to produce depth maps. This paper describes the principles behind the system, indicates for what type of applications it might be useful, notes on the accuracy and gives an example of an application
Mapping of sea bottom topography
Under suitable conditions the bottom topography of shallow seas is visible in remote sensing radar imagery. Two experiments were performed to establish which remote sensing technique or combination yields optimal imaging of bottom topography and which hydro-meteorological conditions are favorable. A further goal is to gain experience with these techniques. Two experiments were performed over an area in the North Sea near the measuring platform Meetpost Noordwijk (MPN). The bottom topography in the test area is dominated by sand waves. The crests of the sand waves are perpendicular to the coast line and the dominating (tidal-)current direction. A 4x4 sq km wide section of the test area was studied in more detail. The first experiment was undertaken on 16 Aug. 1989. During the experiment the following remote sensing instruments were used: Landsat-Thematic Mapper, and NASA/JPL Airborne Imaging Radar (AIR). The hydro-meteorological conditions; current, wind, wave, and air and water temperature were monitored by MPN, a ship of Rijkswaterstaat (the OCTANS), and a pitch-and-roll WAVEC-buoy. The second experiment took place on 12 July 1992. During this experiment data were collected with the NASA/JPL polarimetric synthetic aperture radar (SAR), and a five-band helicopter-borne scatterometer. Again the hydro-meteorological conditions were monitored at MPN and the OCTANS. Furthermore, interferometric radar data were collected
On the equation of state of a dense columnar liquid crystal
An accurate description of a columnar liquid crystal of hard disks at high
packing fractions is presented using an improved free-volume theory. It is
shown that the orientational entropy of the disks in the one-dimensional fluid
direction leads to a different high-density scaling pressure compared to the
prediction from traditional cell theory. Excellent quantitative agreement is
found with recent Monte-Carlo simulation results for various thermodynamic and
structural properties of the columnar state.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
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