103,521 research outputs found
Bulk inhomogeneous phases of anisotropic particles: A fundamental measure functional study of the restricted orientations model
The phase diagram of prolate and oblate particles in the restricted
orientations approximation (Zwanzig model) is calculated. Transitions to
different inhomogeneous phases (smectic, columnar, oriented, or plastic solid)
are studied through minimization of the fundamental measure functional (FMF) of
hard parallelepipeds. The study of parallel hard cubes (PHC's) as a particular
case is also included motivated by recent simulations of this system. As a
result a rich phase behavior is obtained which include, apart from the usual
liquid crystal phases, a very peculiar phase (called here discotic smectic)
which was already found in the only existing simulation of the model, and which
turns out to be stable because of the restrictions imposed on the orientations.
The phase diagram is compared at a qualitative level with simulation results of
other anisotropic particle systems.Comment: 11 pages, 10 figure
Monte Carlo simulation of binary mixtures of hard colloidal cuboids
We perform extensive Monte Carlo simulations to investigate the phase
behaviour of colloidal suspensions of hard board-like particles (HBPs). While
theories restricting particle orientation or ignoring higher ordered phases
suggest the existence of a stable biaxial nematic phase, our recent simulation
results on monodisperse systems indicate that this is not necessarily the case,
even for particle shapes exactly in between prolate and oblate geometries,
usually referred to as self-dual shape. Motivated by the potentially striking
impact of incorporating biaxial ordering into display applications, we extend
our investigation to bidisperse mixtures of short and long HBPs and analyse
whether size dispersity can further enrich the phase behaviour of HBPs,
eventually destabilise positionally ordered phases and thus favour the
formation of the biaxial nematic phase. Not only do our results indicate that
bidisperse mixtures of self-dual shaped HBPs cannot self-assemble into biaxial
nematic phases, but they also show that these particles are not able to form
uniaxial nematic phases either. This surprising behaviour is also observed in
monodisperse systems. Additionally, bidisperse HBPs tend to phase separate in
coexisting isotropic and smectic phases or, at relatively large pressures, in a
smectic phase of mostly short HBPs and a smectic phase of mostly long HBPs. We
conclude that limiting the particle orientational degrees of freedom or
neglecting the presence of positionally ordered (smectic, columnar and crystal)
phases can dramatically alter the phase behaviour of HBPs and unrealistically
enlarge the region of stability of the biaxial nematic phase.Comment: 12 pages, 7 figure
Timescales of emulsion formation caused by anisotropic particles
Particle stabilized emulsions have received an enormous interest in the
recent past, but our understanding of the dynamics of emulsion formation is
still limited. For simple spherical particles, the time dependent growth of
fluid domains is dominated by the formation of droplets, particle adsorption
and coalescence of droplets (Ostwald ripening), which eventually can be almost
fully blocked due to the presence of the particles. Ellipsoidal particles are
known to be more efficient stabilizers of fluid interfaces than spherical
particles and their anisotropic shape and the related additional rotational
degrees of freedom have an impact on the dynamics of emulsion formation. In
this paper, we investigate this point by means of simple model systems
consisting of a single ellipsoidal particle or a particle ensemble at a flat
interface as well as a particle ensemble at a spherical interface. By applying
combined multicomponent lattice Boltzmann and molecular dynamics simulations we
demonstrate that the anisotropic shape of ellipsoidal particles causes two
additional timescales to be of relevance in the dynamics of emulsion formation:
a relatively short timescale can be attributed to the adsorption of single
particles and the involved rotation of particles towards the interface. As soon
as the interface is jammed, however, capillary interactions between the
particles cause a local reordering on very long timescales leading to a
continuous change in the interface configuration and increase of interfacial
area. This effect can be utilized to counteract the thermodynamic instability
of particle stabilized emulsions and thus offers the possibility to produce
emulsions with exceptional stability.Comment: 14 pages, 14 figure
Recoiling Massive Black Holes in Gas-Rich Galaxy Mergers
The asymmetric emission of gravitational waves produced during the
coalescence of a massive black hole (MBH) binary imparts a velocity "kick" to
the system that can displace the hole from the center of its host. Here we
study the trajectories and observability of MBHs recoiling in three (one major,
two minor) gas-rich galaxy merger remnants that were previously simulated at
high resolution, and in which the pairing of the MBHs had been shown to be
successful. We run new simulations of MBHs recoiling in the major merger
remnant with Mach numbers in the range 1<M<6, and use simulation data to
construct a semi-analytical model for the orbital evolution of MBHs in gas-rich
systems. We show that: 1) in major merger remnants the energy deposited by the
moving hole into the rotationally supported, turbulent medium makes a
negligible contribution to the thermodynamics of the gas. This contribution
becomes significant in minor merger remnants, potentially allowing for an
electromagnetic signature of MBH recoil; 2) in major merger remnants, the
combination of both deeper central potential well and drag from high-density
gas confines even MBHs with kick velocities as high as 1200 km/s within 1 kpc
from the host's center; 3) kinematically offset nuclei may be observable for
timescales of a few Myr in major merger remnants in the case of recoil
velocities in the range 700-1,000 km/s; 4) in minor mergers remnants the effect
of gas drag is weaker, and MBHs with recoil speeds in the range 300-600 km/s
will wander through the host halo for longer timescales. When accounting for
the probability distribution of kick velocities, however, we find that the
likelihood of observing recoiling MBHs in gas-rich galaxy mergers is very low,
typically below 10^-5 - 10^-6.Comment: Revised version, accepted for publication in the Astrophysical
Journa
Adsorption of Self-Assembled Rigid Rods on Two-Dimensional Lattices
Monte Carlo (MC) simulations have been carried out to study the adsorption on
square and triangular lattices of particles with two bonding sites that, by
decreasing temperature or increasing density, polymerize reversibly into chains
with a discrete number of allowed directions and, at the same time, undergo a
continuous isotropic-nematic (IN) transition. The process has been monitored by
following the behavior of the adsorption isotherms for different values of
lateral interaction energy/temperature. The numerical data were compared with
mean-field analytical predictions and exact functions for noninteracting and 1D
systems. The obtained results revealed the existence of three adsorption
regimes in temperature. (1) At high temperatures, above the critical one
characterizing the IN transition at full coverage Tc(\theta=1), the particles
are distributed at random on the surface and the adlayer behaves as a
noninteracting 2D system. (2) At very low temperatures, the asymmetric monomers
adsorb forming chains over almost the entire range of coverage, and the
adsorption process behaves as a 1D problem. (3) In the intermediate regime, the
system exhibits a mixed regime and the filling of the lattice proceeds
according to two different processes. In the first stage, the monomers adsorb
isotropically on the lattice until the IN transition occurs in the system and,
from this point, particles adsorb forming chains so that the adlayer behaves as
a 1D fluid. The two adsorption processes are present in the adsorption
isotherms, and a marked singularity can be observed that separates both
regimes. Thus, the adsorption isotherms appear as sensitive quantities with
respect to the IN phase transition, allowing us (i) to reproduce the phase
diagram of the system for square lattices and (ii) to obtain an accurate
determination of the phase diagram for triangular lattices.Comment: Langmuir, 201
Simulation of Ultra-Relativistic Electrons and Positrons Channeling in Crystals with MBN Explorer
A newly developed code, implemented as a part of the \MBNExplorer package
\cite{MBN_ExplorerPaper,MBN_ExplorerSite} to simulate trajectories of an
ultra-relativistic projectile in a crystalline medium, is presented. The motion
of a projectile is treated classically by integrating the relativistic
equations of motion with account for the interaction between the projectile and
crystal atoms. The probabilistic element is introduced by a random choice of
transverse coordinates and velocities of the projectile at the crystal entrance
as well as by accounting for the random positions of the atoms due to thermal
vibrations. The simulated trajectories are used for numerical analysis of the
emitted radiation. Initial approbation and verification of the code have been
carried out by simulating the trajectories and calculating the radiation
emitted by \E=6.7 GeV and \E=855 MeV electrons and positrons in oriented
Si(110) crystal and in amorphous silicon. The calculated spectra are compared
with the experimental data and with predictions of the Bethe-Heitler theory for
the amorphous environment.Comment: 41 pages, 11 figures. Initially submitted on Dec 29, 2012 to Phys.
Rev.
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