12,786 research outputs found
Virial expansion for charged colloids and electrolytes
Using a field-theoretic approach, we derive the first few coefficients of the
exact low-density (``virial'') expansion of a binary mixture of positively and
negatively charged hard spheres (two-component hard-core plasma, TCPHC). Our
calculations are nonperturbative with respect to the diameters and
and charge valences and of positive and negative ions.
Consequently, our closed-form expressions for the coefficients of the free
energy and activity can be used to treat dilute salt solutions, where typically
and , as well as colloidal suspensions, where the
difference in size and valence between macroions and counterions can be very
large. We show how to map the TCPHC on a one-component hard-core plasma (OCPHC)
in the colloidal limit of large size and valence ratio, in which case the
counterions effectively form a neutralizing background. A sizable discrepancy
with the standard OCPHC with uniform, rigid background is detected, which can
be traced back to the fact that the counterions cannot penetrate the colloids.
For the case of electrolyte solutions, we show how to obtain the cationic and
anionic radii as independent parameters from experimental data for the activity
coefficient.Comment: 15 page
Phase behavior of three-component ionic fluids
We study the phase behavior of solutions consisting of positive and negative
ions of valence z to which a third ionic species of valence Z>z is added. Using
a discretized Debye-Hueckel theory, we analyze the phase behavior of such
systems for different values of the ratio Z/z. We find, for Z/z>1.934, a
three-phase coexistence region and, for Z/z>2, a closed (reentrant) coexistence
loop at high temperatures. We characterize the behavior of these ternary ionic
mixtures as function of charge asymmetry and temperature, and show the complete
phase diagrams for the experimentally relevant cases of Z/z=2 and Z/z=3,
corresponding to addition of divalent and trivalent ions to monovalent ionic
fluids, respectively.Comment: 6 pages, 4 figures; to appear in the European Physical Journal
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
Electrostatic Interactions in Strongly-Coupled Soft Matter
Charged soft-matter systems--such as colloidal dispersions and charged
polymers--are dominated by attractive forces between constituent like-charged
particles when neutralizing counterions of high charge valency are introduced.
Such counter-intuitive effects indicate strong electrostatic coupling between
like-charged particles, which essentially results from electrostatic
correlations among counterions residing near particle surfaces. In this paper,
the attraction mechanism and the structure of counterionic correlations are
discussed in the limit of strong coupling based on recent numerical and
analytical investigations and for various geometries (planar, spherical and
cylindrical) of charged objects.Comment: 26 pages, 13 figure
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
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
Counterion density profiles at charged flexible membranes
Counterion distributions at charged soft membranes are studied using
perturbative analytical and simulation methods in both weak coupling
(mean-field or Poisson-Boltzmann) and strong coupling limits. The softer the
membrane, the more smeared out the counterion density profile becomes and
counterions pentrate through the mean-membrane surface location, in agreement
with anomalous scattering results. Membrane-charge repulsion leads to a
short-scale roughening of the membrane.Comment: 4 pages, 4 figure
Strong-Coupling Theory for Counter-Ion Distributions
The Poisson-Boltzmann approach gives asymptotically exact counter-ion density
profiles around charged objects in the weak-coupling limit of low valency and
high temperature. In this paper we derive, using field-theoretic methods, a
theory which becomes exact in the opposite limit of strong coupling. Formally,
it corresponds to a standard virial expansion. Long-range divergences, which
render the virial expansion intractable for homogeneous bulk systems, are shown
to be renormalizable for the case of inhomogeneous distribution functions by a
systematic expansion in inverse powers of the coupling parameter. For a planar
charged wall, our analytical results compare quantitatively with extensive
Monte-Carlo simulations.Comment: 7 pages, 3 figures; to appear in Europhys. Let
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
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