138 research outputs found
Dynamical Heterogeneities and Cooperative Motion in Smectic Liquid Crystals
Using simulations of hard rods in smectic-A states, we find non-gaussian
diffusion and heterogeneous dynamics due to the equilibrium periodic smectic
density profiles, which give rise to permanent barriers for layer-to-layer
diffusion. This relaxation behavior is surprisingly similar to that of
non-equilibrium supercooled liquids, although there the particles are trapped
in transient (instead of permanent) cages. Interestingly, we also find
stringlike clusters of up to 10 inter-layer rods exhibiting dynamic
cooperativity in this equilibrium state.Comment: 10 pages, 4 figure
Heterogeneous Dynamics in Columnar Liquid Crystals of Parallel Hard Rods
In the wake of previous studies on the rattling-and-jumping diffusion in
smectic liquid crystal phases of colloidal rods, we analyze here for the first
time the heterogeneous dynamics in columnar phases. More specifically, we
perform computer simulations to investigate the relaxation dynamics of a binary
mixture of perfectly aligned hard spherocylinders. We detect that the columnar
arrangement of the system produces free-energy barriers the particles should
overcome to jump from one column to another, thus determining a hopping-type
diffusion. This phenomenon accounts for the non-Gaussian inter-column diffusion
and shows a two-step structural relaxation which is remarkably analogous to
that of out-of-equilibrium glass-forming systems and gels. Surprisingly enough,
slight deviations from the behavior of simple liquids due to transient cages is
also observed in the direction perpendicular to this plane, where the system is
usually referred to as liquid-like.Comment: accepted by J Chem Phys; 10 pages, 10 figure
Phase diagrams of charged colloidal rods: can a uniaxial charge distribution break chiral symmetry?
We construct phase diagrams for charged rodlike colloids within the
second-virial approximation as a function of rod concentration, salt
concentration, and colloidal charge. Besides the expected isotropic-nematic
transition, we also find parameter regimes with a coexistence between a nematic
and a second, more highly aligned nematic phase including an
isotropic-nematic-nematic triple point and a nematic-nematic critical point,
which can all be explained in terms of the twisting effect. We compute the
Frank elastic constants to see if the twist elastic constant can become
negative, which would indicate the possibility of a cholesteric phase
spontaneously forming. Although the twisting effect reduces the twist elastic
constant, we find that it always remains positive. In addition, we find that
for finite aspect-ratio rods the twist elastic constant is also always
positive, such that there is no evidence of chiral symmetry breaking due to a
uniaxial charge distribution.Comment: Added a reference to Sec. 4 and extended discussions in Secs. 4 and
7, results unchange
Анализ и моделирование макроэкономических процессов в переходной период
Рассматривается построение моделей процессов инфляции и трансформирования отношений собственности. На основании разработанных моделей поставлены задачи оптимального управления данными процессами, для решения которых применяется метод динамического моделирования. Получены аналитические выражения для оптимальных управляющих воздействий и оптимальные траектории реализации процессов инфляции и трансформирования отношений собственности.У статті розглядається побудова моделей процесів інфляції і трансформування власності. На підставі розроблених моделей поставлені задачі оптимального керування даними процесами, для рішення яких застосовується метод динамічного моделювання. Отримані аналітичні співвідношення для оптимальних керуючих впливів і оптимальні траєкторії реалізації процесів інфляції і трансформування власності.The paper is devoted to construction of mathematical models for inflation and transforming the property form or privatization. The models constructed are used for the optimal control problem statement aiming to compute optimal trajectories for the processes implementation. To solve the optimal control problem dynamic programming method is applied. As a result of the problem solving analytical expressions were derived for optimal inputs, and respective optimal trajectories were computed for various initial conditions
Fluids of platelike particles near a hard wall
Fluids consisting of hard platelike particles near a hard wall are
investigated using density functional theory. The density and orientational
profiles as well as the surface tension and the excess coverage are determined
and compared with those of a fluid of rodlike particles. Even for low densities
slight orientational packing effects are found for the platelet fluid due to
larger intermolecular interactions between platelets as compared with those
between rods. A net depletion of platelets near the wall is exhibited by the
excess coverage, whereas a change of sign of the excess coverage of hard-rod
fluids is found upon increasing the bulk density.Comment: 6 pages, 9 figure
Интеграционные процессы в условиях международной глобализации экономики
We present a double-charge model for the interaction between parallel polarizable hard spherocylinders subject to an external electric field. Using Monte Carlo simulations and free-energy calculations, we predict the phase behaviour for this model as a function of the density and electric field strength, at a fixed length-to-diameter ratio L/D = 5. The resulting phase diagram contains, in addition to the well-known nematic, smectic A, ABC crystal, and columnar phases, a smectic C phase, and a low temperature crystal X phase. We also find a string fluid at low densities and field strengths, resembling results found for dipolar spheres
Demixing and orientational ordering in mixtures of rectangular particles
Using scaled-particle theory for binary mixtures of two-dimensional hard
particles with rotational freedom, we analyse the stability of nematic phases
and the demixing phase behaviour of a variety of mixtures, focussing on cases
where at least one of the components consists of hard rectangles or hard
squares. A pure fluid of hard rectangles may exhibit, aside from the usual
uniaxial nematic phase, an additional (tetratic) oriented phase, possessing two
directors, which is the analogue of the biaxial or cubatic phases in three-
dimensional fluids. There is computer simulation evidence that the tetratic
phase might be stable with respect to phases with spatial order for rectangles
with low aspect ratios. As hard rectangles are mixed with other particles not
possessing stable tetratic order by themselves, the tetratic phase is
destabilised, via a first- or second-order phase transition, to uniaxial
nematic or isotropic phases; for hard rectangles of low aspect ratio tetratic
order persists in a relatively large range of volume fractions. The order of
these transitions depends on the particle geometry, dimensions and
thermodynamic conditions of the mixture. The second component of the mixture
has been chosen to be hard discs or disco-rectangles, the geometry of which is
different from that of rectangles, leading to packing frustration and demixing
behaviour, or simply rectangles of different aspect ratio. These mixtures may
be good candidates for observing thermodynamically stable tetratic phases in
monolayers of hard particles. Finally, demixing between fluid
(isotropic--tetratic or tetratic--tetratic) phases is seen to occur in mixtures
of hard squares of different sizes when the size ratio is sufficiently large.Comment: 27 pages, 9 figure
"Blue energy" from ion adsorption and electrode charging in sea- and river water
A huge amount of entropy is produced at places where fresh water and seawater
mix, for example at river mouths. This mixing process is a potentially enormous
source of sustainable energy, provided it is harnessed properly, for instance
by a cyclic charging and discharging process of porous electrodes immersed in
salt and fresh water, respectively [D. Brogioli, Phys. Rev. Lett. 103, 058501
(2009)]. Here we employ a modified Poisson-Boltzmann free-energy density
functional to calculate the ionic adsorption and desorption onto and from the
charged electrodes, from which the electric work of a cycle is deduced. We
propose optimal (most efficient) cycles for two given salt baths involving two
canonical and two grand-canonical (dis)charging paths, in analogy to the
well-known Carnot cycle for heat-to-work conversion from two heat baths
involving two isothermal and two adiabatic paths. We also suggest a slightly
modified cycle which can be applied in cases that the stream of fresh water is
limited.Comment: 7 Figure
Orientational order in dipolar fluids consisting of nonspherical hard particles
We investigate fluids of dipolar hard particles by a certain variant of
density-functional theory. The proper treatment of the long range of the
dipolar interactions yields a contribution to the free energy which favors
ferromagnetic order. This corrects previous theoretical analyses. We determine
phase diagrams for dipolar ellipsoids and spherocylinders as a function of the
aspect ratio of the particles and their dipole moment. In the nonpolar limit
the results for the phase boundary between the isotropic and nematic phase
agree well with simulation data. Adding a longitudinal dipole moment favors the
nematic phase. For oblate or slightly elongated particles we find a
ferromagnetic liquid phase, which has also been detected in computer
simulations of fluids consisting of spherical dipolar particles. The detailed
structure of the phase diagram and its evolution upon changing the aspect ratio
are discussed in detail.Comment: 35 pages LaTeX with epsf style, 11 figures in eps format, submitted
to Phys. Rev.
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