63 research outputs found
Counterion-Mediated Weak and Strong Coupling Electrostatic Interaction between Like-Charged Cylindrical Dielectrics
We examine the effective counterion-mediated electrostatic interaction
between two like-charged dielectric cylinders immersed in a continuous
dielectric medium containing neutralizing mobile counterions. We focus on the
effects of image charges induced as a result of the dielectric mismatch between
the cylindrical cores and the surrounding dielectric medium and investigate the
counterion-mediated electrostatic interaction between the cylinders in both
limits of weak and strong electrostatic couplings (corresponding, e.g., to
systems with monovalent and multivalent counterions, respectively). The results
are compared with extensive Monte-Carlo simulations exhibiting good agreement
with the limiting weak and strong coupling results in their respective regime
of validity.Comment: 19 pages, 10 figure
Overscreening in 1D lattice Coulomb gas model of ionic liquids
Overscreening in the charge distribution of ionic liquids at electrified
interfaces is shown to proceed from purely electrostatic and steric
interactions in an exactly soluble one dimensional lattice Coulomb gas model.
Being not a mean-field effect, our results suggest that even in higher
dimensional systems the overscreening could be accounted for by a more accurate
treatment of the basic lattice Coulomb gas model, that goes beyond the mean
field level of approximation, without any additional interactions.Comment: 4 pages 5 .eps figure
Cubature rules based on bivariate spline quasi-interpolation for weakly singular integrals
In this paper we present a new class of cubature rules with the aim of
accurately integrating weakly singular double integrals. In particular we focus
on those integrals coming from the discretization of Boundary Integral
Equations for 3D Laplace boundary value problems, using a collocation method
within the Isogeometric Analysis paradigm. In such setting the regular part of
the integrand can be defined as the product of a tensor product B-spline and a
general function. The rules are derived by using first the spline
quasi-interpolation approach to approximate such function and then the
extension of a well known algorithm for spline product to the bivariate
setting. In this way efficiency is ensured, since the locality of any spline
quasi-interpolation scheme is combined with the capability of an ad--hoc
treatment of the B-spline factor. The numerical integration is performed on the
whole support of the B-spline factor by exploiting inter-element continuity of
the integrand
Electrostatic image effects for counter-ions between charged planar walls
We study the effect of dielectric inhomogeneities on the interaction between
two planparallel charged surfaces with oppositely charged mobile charges in
between. The dielectric constant between the surfaces is assumed to be
different from the dielectric constant of the two semiinfinite regions bounded
by the surfaces, giving rise to electrostatic image interactions. We show that
on the weak coupling level the image charge effects are generally small, making
their mark only in the second order fluctuation term. However, in the strong
coupling limit, the image effects are large and fundamental. They modify the
interactions between the two surfaces in an essential way. Our calculations are
particularly useful in the regime of parameters where computer simulations
would be difficult and extremely time consuming due to the complicated nature
of the long range image potentials.Comment: 21 pages, 8 figure
Role of Multipoles in Counterion-Mediated Interactions between Charged Surfaces: Strong and Weak Coupling
We present general arguments for the importance, or lack thereof, of the
structure in the charge distribution of counterions for counterion-mediated
interactions between bounding symmetrically charged surfaces. We show that on
the mean field or weak coupling level, the charge quadrupole contributes the
lowest order modification to the contact value theorem and thus to the
intersurface electrostatic interactions. The image effects are non-existent on
the mean-field level even with multipoles. On the strong coupling level the
quadrupoles and higher order multipoles contribute additional terms to the
interaction free energy only in the presence of dielectric inhomogeneities.
Without them, the monopole is the only multipole that contributes to the strong
coupling electrostatics. We explore the consequences of these statements in all
their generality.Comment: 12 pages, 3 figure
Strong Coupling Electrostatics in the Presence of Dielectric Inhomogeneities
We study the strong-coupling (SC) interaction between two like-charged
membranes of finite thickness embedded in a medium of higher dielectric
constant. A generalized SC theory is applied along with extensive Monte-Carlo
simulations to study the image charge effects induced by multiple dielectric
discontinuities in this system. These effects lead to strong counterion
crowding in the central region of the inter-surface space upon increasing the
solvent/membrane dielectric mismatch and change the membrane interactions from
attractive to repulsive at small separations. These features agree
quantitatively with the SC theory at elevated couplings or dielectric mismatch
where the correlation hole around counterions is larger than the thickness of
the central counterion layer.Comment: 4 pages, 3 figure
Field theoretic description of charge regulation interaction
In order to find the exact form of the electrostatic interaction between two
proteins with dissociable charge groups in aqueous solution, we have studied a
model system composed of two macroscopic surfaces with charge dissociation
sites immersed in a counterion-only ionic solution. Field-theoretic
representation of the grand canonical partition function is derived and
evaluated within the mean-field approximation, giving the Poisson-Boltzmann
theory with the Ninham-Parsegian boundary condition. Gaussian fluctuations
around the mean-field are then analyzed in the lowest order correction that we
calculate analytically and exactly, using the path integral representation for
the partition function of a harmonic oscillator with time-dependent frequency.
The first order (one loop) free energy correction gives the interaction free
energy that reduces to the zero-frequency van der Waals form in the appropriate
limit but in general gives rise to a mono-polar fluctuation term due to charge
fluctuation at the dissociation sites. Our formulation opens up the possibility
to investigate the Kirkwood-Shumaker interaction in more general contexts where
their original derivation fails.Comment: 12 pages, 9 figures, submitted to EPJ
Counterion-mediated Electrostatic Interactions between Helical Molecules
We study the interaction of two cylinders with helical charge distribution
mediated by neutralizing counterions, by analyzing the separation as well as
the azimuthal angle dependence of the interaction force in the weak and strong
coupling limit. While the azimuthal dependence of the interaction in the weak
coupling limit is overall small and mostly negligible, the strong coupling
limit leads to qualitatively new features of the interaction, among others also
to an orientationally dependent optimal configuration that is driven by angular
dependence of the correlation attraction. We investigate the properties of this
azimuthal ordering in detail and compare it to existing results.Comment: 11 pages, 12 figure
Investigating the process of ethical approval in citizen science research. The case of public health
Undertaking citizen science research in Public Health involving human subjects poses significant challenges concerning the traditional process of ethical approval. It requires an extension of the ethics of protection of research subjects in order to include the empowerment of citizens as citizen scientists. This paper investigates these challenges and illustrates the ethical framework and the strategies developed within the CitieS-Health project. It also proposes first recommendations generated from the experiences of five citizen science pilot studies in environmental epidemiology within this project
Diffusion and interaction effects on molecular release from collapsed microgels
The transport of biomolecules, drugs, or reactants in stimuli-responsive
polymer networks in aqueous media is fundamental for many material and
environmental science applications, including drug delivery, biosensing,
catalysis, nanofiltration, water purification, and desalination. The transport
is particularly complex in dense polymer media, such as collapsed hydrogels,
where the molecules strongly interact with the polymer network and diffuse via
the hopping mechanism. In this study, we employ the Dynamical Density
Functional Theory (DDFT) to investigate the non-equilibrium release kinetics of
non-ionic subnanometer-sized molecules from collapsed microgel particles. The
theory is consistent with previous molecular dynamics simulations of collapsed
poly(-isopropylacrylamide) (PNIPAM) polymer matrices, accommodating
molecules of varying shapes and sizes. We found that, despite the intricate
physico-chemical properties involved in the released process, the kinetics is
predominantly dictated by two material parameters: the diffusion coefficient of
the molecules inside the microgel () and the interaction free energy of
the molecules with the microgel (). Our results reveal two distinct
limiting regimes. For large, slowly diffusing molecules weakly attracted to the
polymer network, the release is primarily driven by diffusion, with a release
time scaling as . Conversely, for small molecules
strongly attracted to the polymer network, the release time is dominated by the
interaction, scaling as . Our
DDFT calculations are directly compared with an analytical equation for the
half-release time, demonstrating excellent quantitative agreement. This
equation represents a valuable tool for predicting release kinetics from
collapsed microgels of non-ionic molecules.Comment: 15 pages, 6 figure
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