13,111 research outputs found
Polyelectrolyte-colloid complexes: polarizability and effective interaction
We theoretically study the polarizability and the interactions of neutral
complexes consisting of a semi-flexible polyelectrolyte adsorbed onto an
oppositely charged spherical colloid. In the systems we studied, the bending
energy of the chain is small compared to the Coulomb energy and the chains are
always adsorbed on the colloid. We observe that the polarizability is large for
short chains and small electrical fields and shows a non-monotonic behavior
with the chain length at fixed charge density. The polarizability has a maximum
for a chain length equal to half of the circumference of the colloid. For long
chains we recover the polarizability of a classical conducting sphere. For
short chains, the existence of a permanent dipole moment of the complexes leads
to a van der Waal's-type long-range attraction between them. This attractive
interaction vanishes for long chains (i.e., larger than the colloidal size),
where the permanent dipole moment is negligible. For short distances the
complexes interact with a deep short-ranged attraction which is due to
energetic bridging for short chains and entropic bridging for long chains.
Exceeding a critical chain length eventually leads to a pure repulsion. This
shows that the stabilization of colloidal suspensions by polyelectrolyte
adsorption is strongly dependent on the chain size relative to the colloidal
size: for long chains the suspensions are always stable (only repulsive forces
between the particles), while for mid-sized and short chains there is
attraction between the complexes and a salting-out can occur.Comment: 13 pages, 14 figure
Analysis of the velocity field of granular hopper flow
We report the analysis of radial characteristics of the flow of granular
material through a conical hopper. The discharge is simulated for various
orifice sizes and hopper opening angles. Velocity profiles are measured along
two radial lines from the hopper cone vertex: along the main axis of the cone
and along its wall. An approximate power law dependence on the distance from
the orifice is observed for both profiles, although differences between them
can be noted. In order to quantify these differences, we propose a Local Mass
Flow index that is a promising tool in the direction of a more reliable
classification of the flow regimes in hoppers
Counterions at charge-modulated substrates
We consider counterions in the presence of a single planar surface with a
spatially inhomogeneous charge distribution using Monte-Carlo simulations and
strong-coupling theory. For high surface charges, multivalent counterions, or
pronounced substrate charge modulation the counterions are laterally correlated
with the surface charges and their density profile deviates strongly from the
limit of a smeared-out substrate charge distribution, in particular exhibiting
a much increased laterally averaged density at the surface.Comment: 7 page
Counterions at Charged Cylinders: Criticality and universality beyond mean-field
The counterion-condensation transition at charged cylinders is studied using
Monte-Carlo simulation methods. Employing logarithmically rescaled radial
coordinates, large system sizes are tractable and the critical behavior is
determined by a combined finite-size and finite-ion-number analysis. Critical
counterion localization exponents are introduced and found to be in accord with
mean-field theory both in 2 and 3 dimensions. In 3D the heat capacity shows a
universal jump at the transition, while in 2D, it consists of discrete peaks
where single counterions successively condense.Comment: 4 pages, 3 figures; submitted to Phys. Rev. Lett. (2005
Timed Consistent Network Updates
Network updates such as policy and routing changes occur frequently in
Software Defined Networks (SDN). Updates should be performed consistently,
preventing temporary disruptions, and should require as little overhead as
possible. Scalability is increasingly becoming an essential requirement in SDN.
In this paper we propose to use time-triggered network updates to achieve
consistent updates. Our proposed solution requires lower overhead than existing
update approaches, without compromising the consistency during the update. We
demonstrate that accurate time enables far more scalable consistent updates in
SDN than previously available. In addition, it provides the SDN programmer with
fine-grained control over the tradeoff between consistency and scalability.Comment: This technical report is an extended version of the paper "Timed
Consistent Network Updates", which was accepted to the ACM SIGCOMM Symposium
on SDN Research (SOSR) '15, Santa Clara, CA, US, June 201
Effects of rotation on the evolution and asteroseismic properties of red giants
The influence of rotation on the properties of red giants is studied in the
context of the asteroseismic modelling of these stars. While red giants exhibit
low surface rotational velocities, we find that the rotational history of the
star has a large impact on its properties during the red giant phase. In
particular, for stars massive enough to ignite He burning in non-degenerate
conditions, rotational mixing induces a significant increase of the stellar
luminosity and shifts the location of the core helium burning phase to a higher
luminosity in the HR diagram. This of course results in a change of the seismic
properties of red giants at the same evolutionary state. As a consequence the
inclusion of rotation significantly changes the fundamental parameters of a red
giant star as determined by performing an asteroseismic calibration. In
particular rotation decreases the derived stellar mass and increases the age.
Depending on the rotation law assumed in the convective envelope and on the
initial velocity of the star, non-negligible values of rotational splitting can
be reached, which may complicate the observation and identification of
non-radial oscillation modes for red giants exhibiting moderate surface
rotational velocities. By comparing the effects of rotation and overshooting,
we find that the main-sequence widening and the increase of the H-burning
lifetime induced by rotation (Vini=150 km/s) are well reproduced by
non-rotating models with an overshooting parameter of 0.1, while the increase
of luminosity during the post-main sequence evolution is better reproduced by
non-rotating models with overshooting parameters twice as large. This is due to
the fact that rotation not only increases the size of the convective core but
also changes the chemical composition of the radiative zone.Comment: 9 pages, 13 figures, accepted for publication in A&
Monovalent counterion distributions at highly charged water interfaces: Proton-transfer and Poisson-Boltzmann theory
Surface sensitive synchrotron-X-ray scattering studies reveal the
distributions of monovalent ions next to highly charged interfaces. A lipid
phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the
air-water interface, containing CsI at various concentrations. Using anomalous
reflectivity off and at the Cs resonance, we provide, for the first
time, spatial counterion distributions (Cs) next to the negatively charged
interface over a wide range of ionic concentrations. We argue that at low salt
concentrations and for pure water the enhanced concentration of hydroniums
HO at the interface leads to proton-transfer back to the phosphate
group by a high contact-potential, whereas high salt concentrations lower the
contact-potential resulting in proton-release and increased surface
charge-density. The experimental ionic distributions are in excellent agreement
with a renormalized-surface-charge Poisson-Boltzmann theory without fitting
parameters or additional assumptions
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