1,188 research outputs found
Non-monotonic density dependence of the diffusion of DNA fragments in low-salt suspensions
The high linear charge density of 20-base-pair oligomers of DNA is shown to
lead to a striking non-monotonic dependence of the long-time self-diffusion on
the concentration of the DNA in low-salt conditions. This generic non-monotonic
behavior results from both the strong coupling between the electrostatic and
solvent-mediated hydrodynamic interactions, and from the renormalization of
these electrostatic interactions at large separations, and specifically from
the dominance of the far-field hydrodynamic interactions caused by the strong
repulsion between the DNA fragments.Comment: 4 pages, 2 figures. Physical Review E, accepted on November 24, 200
Nonlinear screening of charged macromolecules
We present several aspects of the screening of charged macromolecules in an
electrolyte. After a review of the basic mean field approach, based on the
linear Debye-Huckel theory, we consider the case of highly charged
macromolecules, where the linear approximation breaks down and the system is
described by full nonlinear Poisson-Boltzmann equation. Some analytical results
for this nonlinear equation give some interesting insight on physical phenomena
like the charge renormalization and the Manning counterion condensation
Non-linear screening of spherical and cylindrical colloids: the case of 1:2 and 2:1 electrolytes
From a multiple scale analysis, we find an analytic solution of spherical and
cylindrical Poisson-Boltzmann theory for both a 1:2 (monovalent co-ions,
divalent counter-ions) and a 2:1 (reversed situation) electrolyte. Our approach
consists in an expansion in powers of rescaled curvature , where
is the colloidal radius and the Debye length of the electrolytic
solution. A systematic comparison with the full numerical solution of the
problem shows that for cylinders and spheres, our results are accurate as soon
as . We also report an unusual overshooting effect where the
colloidal effective charge is larger than the bare one.Comment: 9 pages, 11 figure
Universal Elasticity and Fluctuations of Nematic Gels
We study elasticity of spontaneously orientationally-ordered amorphous
solids, characterized by a vanishing transverse shear modulus, as realized for
example by nematic elastomers and gels. We show that local heterogeneities and
elastic nonlinearities conspire to lead to anomalous nonlocal universal
elasticity controlled by a nontrivial infared fixed point. Namely, at long
scales, such solids are characterized by universal shear and bending moduli
that, respectively, vanish and diverge at long scales, are universally
incompressible and exhibit a universal negative Poisson ratio and a non-Hookean
elasticity down to arbitrarily low strains. Based on expansion about five
dimensions, we argue that the nematic order is stable to thermal fluctuation
and local hetergeneities down to d_lc < 3.Comment: 4 RevTeX pgs, submitted to PR
Pattern formation in binary fluid mixtures induced by short-range competing interactions
Molecular dynamics simulations and integral equation calculations of a simple
equimolar mixture of diatomic molecules and monomers interacting via attractive
and repulsive short-range potentials show the existence of pattern formation
(microheterogeneity), mostly due to depletion forces away from the demixing
region. Effective site-site potentials extracted from the pair correlation
functions using an inverse Monte Carlo approach and an integral equation
inversion procedure exhibit the features characteristic of a short-range
attractive and long-range repulsive potential. When charges are incorporated
into the model, this becomes a coarse grained representation of a room
temperature ionic liquid, and as expected, intermediate range order becomes
more pronounced and stable
Casimir-Polder interatomic potential between two atoms at finite temperature and in the presence of boundary conditions
We evaluate the Casimir-Polder potential between two atoms in the presence of
an infinite perfectly conducting plate and at nonzero temperature. In order to
calculate the potential, we use a method based on equal-time spatial
correlations of the electric field, already used to evaluate the effect of
boundary conditions on interatomic potentials. This method gives also a
transparent physical picture of the role of a finite temperature and boundary
conditions on the Casimir-Polder potential. We obtain an analytical expression
of the potential both in the near and far zones, and consider several limiting
cases of interest, according to the values of the parameters involved, such as
atom-atom distance, atoms-wall distance and temperature.Comment: 11 page
Mixtures of Charged Colloid and Neutral Polymer: Influence of Electrostatic Interactions on Demixing and Interfacial Tension
The equilibrium phase behavior of a binary mixture of charged colloids and
neutral, non-adsorbing polymers is studied within free-volume theory. A model
mixture of charged hard-sphere macroions and ideal, coarse-grained,
effective-sphere polymers is mapped first onto a binary hard-sphere mixture
with non-additive diameters and then onto an effective Asakura-Oosawa model [S.
Asakura and F. Oosawa, J. Chem. Phys. 22, 1255 (1954)]. The effective model is
defined by a single dimensionless parameter -- the ratio of the polymer
diameter to the effective colloid diameter. For high salt-to-counterion
concentration ratios, a free-volume approximation for the free energy is used
to compute the fluid phase diagram, which describes demixing into colloid-rich
(liquid) and colloid-poor (vapor) phases. Increasing the range of electrostatic
interactions shifts the demixing binodal toward higher polymer concentration,
stabilizing the mixture. The enhanced stability is attributed to a weakening of
polymer depletion-induced attraction between electrostatically repelling
macroions. Comparison with predictions of density-functional theory reveals a
corresponding increase in the liquid-vapor interfacial tension. The predicted
trends in phase stability are consistent with observed behavior of
protein-polysaccharide mixtures in food colloids.Comment: 16 pages, 5 figure
Zener double exchange from local valence fluctuations in magnetite
Magnetite (FeO) is a mixed valent system where electronic
conductivity occurs on the B-site (octahedral) iron sublattice of the spinel
structure. Below K, a metal-insulator transition occurs which is
argued to arise from the charge ordering of 2+ and 3+ iron valences on the
B-sites (Verwey transition). Inelastic neutron scattering measurements show
that optical spin waves propagating on the B-site sublattice (80 meV) are
shifted upwards in energy above due to the occurrence of B-B
ferromagnetic double exchange in the mixed valent metallic phase. The double
exchange interaction affects only spin waves of symmetry, not all
modes, indicating that valence fluctuations are slow and the double exchange is
constrained by electron correlations above .Comment: 4 pages, 5 figure
Electroneutrality and Phase Behavior of Colloidal Suspensions
Several statistical mechanical theories predict that colloidal suspensions of
highly charged macroions and monovalent microions can exhibit unusual
thermodynamic phase behavior when strongly deionized. Density-functional,
extended Debye-H\"uckel, and response theories, within mean-field and
linearization approximations, predict a spinodal phase instability of charged
colloids below a critical salt concentration. Poisson-Boltzmann cell model
studies of suspensions in Donnan equilibrium with a salt reservoir demonstrate
that effective interactions and osmotic pressures predicted by such theories
can be sensitive to the choice of reference system, e.g., whether the microion
density profiles are expanded about the average potential of the suspension or
about the reservoir potential. By unifying Poisson-Boltzmann and response
theories within a common perturbative framework, it is shown here that the
choice of reference system is dictated by the constraint of global
electroneutrality. On this basis, bulk suspensions are best modeled by
density-dependent effective interactions derived from a closed reference system
in which the counterions are confined to the same volume as the macroions.
Linearized theories then predict bulk phase separation of deionized suspensions
only when expanded about a physically consistent (closed) reference system.
Lower-dimensional systems (e.g., monolayers, small clusters), depending on the
strength of macroion-counterion correlations, may be governed instead by
density-independent effective interactions tied to an open reference system
with counterions dispersed throughout the reservoir, possibly explaining
observed structural crossover in colloidal monolayers and anomalous
metastability of colloidal crystallites.Comment: 12 pages, 5 figures. Discussion clarified, references adde
Fcc-bcc transition for Yukawa interactions determined by applied strain deformation
Calculations of the work required to transform between bcc and fcc phases
yield a high-precision bcc-fcc transition line for monodisperse point Yukawa
(screened-Couloumb) systems. Our results agree qualitatively but not
quantitatively with previously published simulations and phenomenological
criteria for the bcc-fcc transition. In particular, the bcc-fcc-fluid triple
point lies at a higher inverse screening length than previously reported.Comment: RevTex4, 9 pages, 6 figures. Discussion of phase coexistence
extended, a few other minor clarifications added, referencing improved.
Accepted for publication by Physical Review
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