897 research outputs found
Surface-charge-induced freezing of colloidal suspensions
Using grand-canonical Monte Carlo simulations we investigate the impact of
charged walls on the crystallization properties of charged colloidal
suspensions confined between these walls. The investigations are based on an
effective model focussing on the colloids alone. Our results demonstrate that
the fluid-wall interaction stemming from charged walls has a crucial impact on
the fluid's high-density behavior as compared to the case of uncharged walls.
In particular, based on an analysis of in-plane bond order parameters we find
surface-charge-induced freezing and melting transitions
Crystal structures and freezing of dipolar fluids
We investigate the crystal structure of classical systems of spherical
particles with an embedded point dipole at T=0. The ferroelectric ground state
energy is calculated using generalizations of the Ewald summation technique.
Due to the reduced symmetry compared to the nonpolar case the crystals are
never strictly cubic. For the Stockmayer (i.e., Lennard-Jones plus dipolar)
interaction three phases are found upon increasing the dipole moment:
hexagonal, body-centered orthorhombic, and body-centered tetragonal. An even
richer phase diagram arises for dipolar soft spheres with a purely repulsive
inverse power law potential . A crossover between qualitatively
different sequences of phases occurs near the exponent . The results are
applicable to electro- and magnetorheological fluids. In addition to the exact
ground state analysis we study freezing of the Stockmayer fluid by
density-functional theory.Comment: submitted to Phys. Rev.
Self-assembly of amphiphilic Janus particles at planar walls: A density functional study
We investigate the structure formation of amphiphilic molecules at planar
walls using density functional theory. The molecules are modeled as (hard)
spheres composed of a hydrophilic and hydrophobic part. The orientation of the
resulting Janus-particles is described as a vector representing an internal
degree of freedom. Our density functional approach involves Fundamental Measure
Theory combined with a mean-field approximation for the anisotropic
interaction. Considering neutral, hydrophilic and hydrophobic walls, we study
the adsorption of the particles, focussing on the competition between the
surface field and interact ion-induced ordering phenomena. Finally, we consider
systems confined between two planar walls. It is shown that the anisotropic
Janus interaction yields pronounced frustration effects at low temperatures.Comment: 11 page
Density functional formalism in the canonical ensemble
Density functional theory, when applied to systems with , is based
on the grand canonical extension of the Hohenberg-Kohn-Sham theorem due to
Mermin (HKSM theorem). While a straightforward canonical ensemble
generalization fails, work in nanopore systems could certainly benefit from
such extension. We show that, if the asymptotic behaviour of the canonical
distribution functions is taken into account, the HKSM theorem can be extended
to the canonical ensemble. We generate -modified correlation and
distribution functions hierarchies and prove that, if they are employed, either
a modified external field or the density profiles can be indistinctly used as
independent variables. We also write down the % -modified free energy
functional and prove that its minimum is reached when the equilibrium values of
the new hierarchy are used. This completes the extension of the HKSM theorem.Comment: revtex, to be submitted to Phys. Rev. Let
Inhomogeneous magnetization in dipolar ferromagnetic liquids
At high densities fluids of strongly dipolar spherical particles exhibit
spontaneous long-ranged orientational order. Typically, due to demagnetization
effects induced by the long range of the dipolar interactions, the
magnetization structure is spatially inhomogeneous and depends on the shape of
the sample. We determine this structure for a cubic sample by the free
minimization of an appropriate microscopic density functional using simulated
annealing. We find a vortex structure resembling four domains separated by four
domain walls whose thickness increases proportional to the system size L. There
are indications that for large L the whole configuration scales with the system
size. Near the axis of the mainly planar vortex structure the direction of the
magnetization escapes into the third dimension or, at higher temperatures, the
absolute value of the magnetization is strongly reduced. Thus the orientational
order is characterized by two point defects at the top and the bottom of the
sample, respectively. The equilibrium structure in an external field and the
transition to a homogeneous magnetization for strong fields are analyzed, too.Comment: 17 postscript figures included, submitted to Phys. Rev.
Cosmological Effects of the First Stars: Evolving Spectra of Population III
The first stars hold intrinsic interest for their uniqueness and for their
potential importance to galaxy formation, chemical enrichment, and feedback on
the intergalactic medium (IGM). The metal-free composition of the first stars
restricts the stellar energy source to proton-proton burning rather than the
more efficient CNO cycle. Consequently they are hotter, smaller, and have
harder spectra than their present-day counterparts of finite metallicity. We
present new results from a continuing study of metal-free stars from a
cosmological point of view. We have calculated evolving spectra of Pop III
clusters, derived from a grid of zero-metallicity stellar evolutionary tracks.
We find that H-ionizing photon production from metal-free stellar clusters
takes twice as long as that of Pop II to decline to 1/10 its peak value. In
addition, metal-free stars produce substantially more photons than Pop II in
the HeII ( Ryd) continuum. We suggest that large Ly equivalent
widths ( Angstroms) may provide a means of detecting
metal-free stellar populations at high redshift, and that HeII recombination
lines (1640, 4686) may confirm identifications of Population
III. While Pop III clusters are intrinsically bluer than their Pop II
counterparts, nebular continuum emission makes up this difference and may
confuse attempts to discern Pop III stars with broadband colors. In a companion
paper, we explore the consequences of evolving spectra of Pop III for the
reionization of the IGM in both H and He. (ABRIDGED)Comment: Accepted by ApJ; 32 pages, 10 figures, with an updated discussion of
the effects of mass loss on Pop III and Pop II comparisons. For
full-resolution color figures, see http://casa.colorado.edu/~tumlinso/fs.htm
Double-Mode Stellar Pulsations
The status of the hydrodynamical modelling of nonlinear multi-mode stellar
pulsations is discussed. The hydrodynamical modelling of steady double-mode
(DM) pulsations has been a long-standing quest that is finally being concluded.
Recent progress has been made thanks to the introduction of turbulent
convection in the numerical hydrodynamical codes which provide detailed results
for individual models. An overview of the modal selection problem in the HR
diagram can be obtained in the form of bifurcation diagrams with the help of
simple nonresonant amplitude equations that capture the DM phenomenon.Comment: 34 pages, to appear as a chapter in Nonlinear Stellar Pulsation in
the Astrophysics and Space Science Library (ASSL), Editors: M. Takeuti & D.
Sasselov (prints double column with pstops
'2:[email protected](22.0cm,-2cm)[email protected](22.0cm,11.0cm)' in.ps out.ps
Smart Containers With Bidding Capacity: A Policy Gradient Algorithm for Semi-Cooperative Learning
Smart modular freight containers -- as propagated in the Physical Internet
paradigm -- are equipped with sensors, data storage capability and intelligence
that enable them to route themselves from origin to destination without manual
intervention or central governance. In this self-organizing setting, containers
can autonomously place bids on transport services in a spot market setting.
However, for individual containers it may be difficult to learn good bidding
policies due to limited observations. By sharing information and costs between
one another, smart containers can jointly learn bidding policies, even though
simultaneously competing for the same transport capacity. We replicate this
behavior by learning stochastic bidding policies in a semi-cooperative multi
agent setting. To this end, we develop a reinforcement learning algorithm based
on the policy gradient framework. Numerical experiments show that sharing
solely bids and acceptance decisions leads to stable bidding policies.
Additional system information only marginally improves performance; individual
job properties suffice to place appropriate bids. Furthermore, we find that
carriers may have incentives not to share information with the smart
containers. The experiments give rise to several directions for follow-up
research, in particular the interaction between smart containers and transport
services in self-organizing logistics.Comment: 15 page
Does Being Bored Make Us More Creative?
Boredom has traditionally been associated with a range of negative outcomes, both within the workplace and outside it. More recently, however, it has been suggested that boredom can have positive outcomes, one of which might be increased creativity. This study addressed this proposition by examining the relationship between boredom and creative potential on a range of tasks. Two studies were carried out; the first involved 80 participants taking part in either a boring writing activity or not (control group) followed by a creative task. The second study involved a further 90 participants who varied in the type of boring activity they undertook (either a boring written activity, a boring reading activity or control) and the type of creative task that followed. Results suggested that boring activities resulted in increased creativity and that boring reading activities lead to more creativity in some circumstances (such as convergent tasks) than boring written activities. The role of daydreaming as a mediator between boredom and creativity is discussed and implications are outline
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