796 research outputs found
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
The Role of the Gouy Phase in the Coherent Phase Control of the Photoionization and Photodissociation of Vinyl Chloride
We demonstrate theoretically and experimentally that the Gouy phase of a
focused laser beam may be used to control the photo-induced reactions of a
polyatomic molecule. Quantum mechanical interference between one- and
three-photon excitation of vinyl chloride produces a small phase lag between
the dissociation and ionization channels on the axis of the molecular beam.
Away from the axis, the Gouy phase introduces a much larger phase lag that
agrees quantitatively with theory without any adjustable parameters.Comment: 4 pages, 4 figure
Description beyond the mean field approximation of an electrolyte confined between two planar metallic electrodes
We study an electrolyte confined in a slab of width composed of two
grounded metallic parallel electrodes. We develop a description of this system
in a low coupling regime beyond the mean field (Poisson--Boltzmann)
approximation. There are two ways to model the metallic boundaries: as ideal
conductors in which the electric potential is zero and it does not fluctuate,
or as good conductors in which the average electric potential is zero but the
thermal fluctuations of the potential are not zero. This latter model is more
realistic. For the ideal conductor model we find that the disjoining pressure
is positive behaves as for large separations with a prefactor that is
universal, i.e. independent of the microscopic constitution of the system. For
the good conductor boundaries the disjoining pressure is negative and it has an
exponential decay for large . We also compute the density and electric
potential profiles inside the electrolyte. These are the same in both models.
If the electrolyte is charge asymmetric we find that the system is not locally
neutral and that a non-zero potential difference builds up between any
electrode and the interior of the system although both electrodes are grounded.Comment: 16 pages, 5 figures, added a new appendix B and a discussion on ideal
conductors vs. good conductor
Ion-Size Effect at the Surface of a Silica Hydrosol
The author used synchrotron x-ray reflectivity to study the ion-size effect
for alkali ions (Na, K, Rb, and Cs), with densities as high as
m, suspended above the surface of a
colloidal solution of silica nanoparticles in the field generated by the
surface electric-double layer. According to the data, large alkali ions
preferentially accumulate at the sol's surface replacing smaller ions, a
finding that qualitatively agrees with the dependence of the Kharkats-Ulstrup
single-ion electrostatic free energy on the ion's radius.Comment: 17 pages, 4 figure
Radius of a Photon Beam with Orbital Angular Momentum
We analyze the transverse structure of the Gouy phase shift in light beams
carrying orbital angular momentum and show that the Gouy radius
characterizing the transverse structure grows as with the
nodal number and photon angular momentum number . The Gouy radius is
shown to be closely related to the root-mean-square radius of the beam, and the
divergence of the radius away from the focal plane is determined. Finally, we
analyze the rotation of the Poynting vector in the context of the Gouy radius.Comment: 11 page
The interplay between screening properties and colloid anisotropy: towards a reliable pair potential for disc-like charged particles
The electrostatic potential of a highly charged disc (clay platelet) in an
electrolyte is investigated in detail. The corresponding non-linear
Poisson-Boltzmann (PB) equation is solved numerically, and we show that the
far-field behaviour (relevant for colloidal interactions in dilute suspensions)
is exactly that obtained within linearized PB theory, with the surface boundary
condition of a uniform potential. The latter linear problem is solved by a new
semi-analytical procedure and both the potential amplitude (quantified by an
effective charge) and potential anisotropy coincide closely within PB and
linearized PB, provided the disc bare charge is high enough. This anisotropy
remains at all scales; it is encoded in a function that may vary over several
orders of magnitude depending on the azimuthal angle under which the disc is
seen. The results allow to construct a pair potential for discs interaction,
that is strongly orientation dependent.Comment: 13 pages, 19 figure
Non-mean-field theory of anomalously large double-layer capacitance
Mean-field theories claim that the capacitance of the double-layer formed at
a metal/ionic conductor interface cannot be larger than that of the Helmholtz
capacitor, whose width is equal to the radius of an ion. However, in some
experiments the apparent width of the double-layer capacitor is substantially
smaller. We propose an alternate, non-mean-field theory of the ionic
double-layer to explain such large capacitance values. Our theory allows for
the binding of discrete ions to their image charges in the metal, which results
in the formation of interface dipoles. We focus primarily on the case where
only small cations are mobile and other ions form an oppositely-charged
background. In this case, at small temperature and zero applied voltage dipoles
form a correlated liquid on both contacts. We show that at small voltages the
capacitance of the double-layer is determined by the transfer of dipoles from
one electrode to the other and is therefore limited only by the weak
dipole-dipole repulsion between bound ions, so that the capacitance is very
large. At large voltages the depletion of bound ions from one of the capacitor
electrodes triggers a collapse of the capacitance to the much smaller
mean-field value, as seen in experimental data. We test our analytical
predictions with a Monte Carlo simulation and find good agreement. We further
argue that our ``one-component plasma" model should work well for strongly
asymmetric ion liquids. We believe that this work also suggests an improved
theory of pseudo-capacitance.Comment: 19 pages, 14 figures; some Monte Carlo results and a section about
aqueous solutions adde
Screening of Spherical Colloids beyond Mean Field -- A Local Density Functional Approach
We study the counterion distribution around a spherical macroion and its
osmotic pressure in the framework of the recently developed
Debye-H"uckel-Hole-Cavity (DHHC) theory. This is a local density functional
approach which incorporates correlations into Poisson-Boltzmann theory by
adding a free energy correction based on the One Component Plasma. We compare
the predictions for ion distribution and osmotic pressure obtained by the full
theory and by its zero temperature limit with Monte Carlo simulations. They
agree excellently for weakly developed correlations and give the correct trend
for stronger ones. In all investigated cases the DHHC theory and its
computationally simpler zero temperature limit yield better results than the
Poisson-Boltzmann theory.Comment: 10 pages, 4 figures, 2 tables, RevTeX4-styl
Heterodyne non-demolition measurements on cold atomic samples: towards the preparation of non-classical states for atom interferometry
We report on a novel experiment to generate non-classical atomic states via
quantum non-demolition (QND) measurements on cold atomic samples prepared in a
high finesse ring cavity. The heterodyne technique developed for the QND
detection exhibits an optical shot-noise limited behavior for local oscillator
optical power of a few hundred \muW, and a detection bandwidth of several GHz.
This detection tool is used in single pass to follow non destructively the
internal state evolution of an atomic sample when subjected to Rabi
oscillations or a spin-echo interferometric sequence.Comment: 23 page
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