2,049 research outputs found
Electrophoresis of a polyelectrolyte through a nanopore
A hydrodynamic model for determining the electrophoretic speed of a
polyelectrolyte through a nanopore is presented. It is assumed that the speed
is determined by a balance of electrical and viscous forces arising from within
the pore and that classical continuum electrostatics and hydrodynamics may be
considered applicable. An explicit formula for the translocation speed as a
function of the pore geometry and other physical parameters is obtained and is
shown to be consistent with experimental measurements on DNA translocation
through nanopores in silicon membranes. Experiments also show a weak dependence
of the translocation speed on polymer length that is not accounted for by the
present model. It is hypothesized that this is due to secondary effects that
are neglected here.Comment: 5 pages, 2 column, 2 figure
Colloidal Electrostatic Interactions Near a Conducting Surface
Charge-stabilized colloidal spheres dispersed in deionized water are supposed
to repel each other. Instead, artifact-corrected video microscopy measurements
reveal an anomalous long-ranged like-charge attraction in the interparticle
pair potential when the spheres are confined to a layer by even a single
charged glass surface. These attractions can be masked by electrostatic
repulsions at low ionic strengths. Coating the bounding surfaces with a
conducting gold layer suppresses the attraction. These observations suggest a
possible mechanism for confinement-induced attractions.Comment: 4 pages, 2 figure
Integration through transients for Brownian particles under steady shear
Starting from the microscopic Smoluchowski equation for interacting Brownian
particles under stationary shearing, exact expressions for shear-dependent
steady-state averages, correlation and structure functions, and
susceptibilities are obtained, which take the form of generalized Green-Kubo
relations. They require integration of transient dynamics. Equations of motion
with memory effects for transient density fluctuation functions are derived
from the same microscopic starting point. We argue that the derived formal
expressions provide useful starting points for approximations in order to
describe the stationary non-equilibrium state of steadily sheared dense
colloidal dispersions.Comment: 17 pages, Submitted to J. Phys.: Condens. Matter; revised version
with minor correction
Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga
We present high resolution optical spectra obtained with the HIRES
spectrograph on the Keck I telescope of low mass T Tauri stars and brown dwarfs
(LMTTs) in Taurus-Auriga. Of particular interest is the previously classified
"continuum T Tauri star" GM Tau, which has a spectral type of M6.5 and a mass
just below the stellar/substellar boundary. None of the LMTTs in Taurus are
rapidly rotating (vsini < 30 km/s), unlike low mass objects in Orion. Many of
the slowly rotating, non-accreting stars and brown dwarfs exhibit prominent
H-alpha emission (EWs of 3 - 36 A), indicative of active chromospheres. We
demonstrate empirically that the full-width at 10% of the H-alpha emission
profile peak is a more practical and possibly more accurate indicator of
accretion than either the equivalent width of H-alpha or optical veiling:
10%-widths > 270 km/s are classical T Tauri stars (i.e. accreting), independent
of stellar spectral type. Although LMTTs can have accretion rates comparable to
that of more typical, higher-mass T Tauri stars (e.g. K7-M0), the average mass
accretion rate appears to decrease with decreasing mass. The diminished
frequency of accretion disks for LMTTs, in conjunction with their lower, on
average, mass accretion rates, implies that they are formed with less massive
disks than higher-mass T Tauri stars. The radial velocities, circumstellar
properties and known binaries do not support the suggestion that many of the
lowest mass members of Taurus have been ejected from higher stellar density
regions within the cloud. Instead, LMTTs appear to have formed and are evolving
in the same way as higher-mass T Tauri stars, but with smaller disks and
shorter disk lifetimes.Comment: 27 pages, plus 8 figures, accepted for publication in Ap
Entropic phase separation of linked beads
We study theoretically a model system of a transient network of microemulsion
droplets connected by telechelic polymers and explain recent experimental
findings. Despite the absence of any specific interactions between either the
droplets or polymer chains, we predict that as the number of polymers per drop
is increased, the system undergoes a first order phase separation into a dense,
highly connected phase, in equilibrium with dilute droplets, decorated by
polymer loops. The phase transition is purely entropic and is driven by the
interplay between the translational entropy of the drops and the
configurational entropy of the polymer connections between them. Because it is
dominated by entropic effects, the phase separation mechanism of the system is
extremely robust and does not depend on the particlular physical realization of
the network. The discussed model applies as well to other polymer linked
particle aggregates, such as nano-particles connected with short DNA linkers
Interaction of sulphonylurea derivatives with vascular ATP-sensitive potassium channels in humans
Simulation of Claylike Colloids
We investigate properties of dense suspensions and sediments of small
spherical silt particles by means of a combined Molecular Dynamics (MD) and
Stochastic Rotation Dynamics (SRD) simulation. We include van der Waals and
effective electrostatic interactions between the colloidal particles, as well
as Brownian motion and hydrodynamic interactions which are calculated in the
SRD-part. We present the simulation technique and first results. We have
measured velocity distributions, diffusion coefficients, sedimentation
velocity, spatial correlation functions and we have explored the phase diagram
depending on the parameters of the potentials and on the volume fraction.Comment: 20 pages, 14 figure
QCD Corrections to Production of Higgs Pseudoscalars
Models of electroweak symmetry breaking with more than a single doublet of
Higgs scalars contain a neutral pseudoscalar boson. The production of such a
pseudoscalar in hadron collisions proceeds primarily via gluon fusion through a
top-quark loop (except for those models in which the pseudoscalar coupling to
bottom quarks is strongly enhanced). We compute the QCD corrections to this
process in the heavy-quark limit, using an effective Lagrangian derived from
the axial anomaly.Comment: 9 pages, (BNL number added, 1 typo corrected, PHYZZX format, 4
figures not included, available on request), BNL-4906
Self diffusion in a system of interacting Langevin particles
The behavior of the self diffusion constant of Langevin particles interacting
via a pairwise interaction is considered. The diffusion constant is calculated
approximately within a perturbation theory in the potential strength about the
bare diffusion constant. It is shown how this expansion leads to a systematic
double expansion in the inverse temperature and the particle density
. The one-loop diagrams in this expansion can be summed exactly and we
show that this result is exact in the limit of small and
constant. The one-loop result can also be re-summed using a
semi-phenomenological renormalization group method which has proved useful in
the study of diffusion in random media. In certain cases the renormalization
group calculation predicts the existence of a diverging relaxation time
signalled by the vanishing of the diffusion constant -- possible forms of
divergence coming from this approximation are discussed. Finally, at a more
quantitative level, the results are compared with numerical simulations, in
two-dimensions, of particles interacting via a soft potential recently used to
model the interaction between coiled polymers.Comment: 12 pages, 8 figures .ep
Phase behaviour of charged colloidal sphere dispersions with added polymer chains
We study the stability of mixtures of highly screened repulsive charged
spheres and non-adsorbing ideal polymer chains in a common solvent using free
volume theory. The effective interaction between charged colloids in an aqueous
salt solution is described by a screened-Coulomb pair potential, which
supplements the pure hard-sphere interaction. The ideal polymer chains are
treated as spheres that are excluded from the colloids by a hard-core
interaction, whereas the interaction between two ideal chains is set to zero.
In addition, we investigate the phase behaviour of charged colloid-polymer
mixtures in computer simulations, using the two-body (Asakura-Oosawa pair
potential) approximation to the effective one-component Hamiltonian of the
charged colloids. Both our results obtained from simulations and from free
volume theory show similar trends. We find that the screened-Coulomb repulsion
counteracts the effect of the effective polymer-mediated attraction. For
mixtures of small polymers and relatively large charged colloidal spheres, the
fluid-crystal transition shifts to significantly larger polymer concentrations
with increasing range of the screened-Coulomb repulsion. For relatively large
polymers, the effect of the screened-Coulomb repulsion is weaker. The resulting
fluid-fluid binodal is only slightly shifted towards larger polymer
concentrations upon increasing the range of the screened-Coulomb repulsion. In
conclusion, our results show that the miscibility of dispersions containing
charged colloids and neutral non-adsorbing polymers increases, upon increasing
the range of the screened-Coulomb repulsion, or upon lowering the salt
concentration, especially when the polymers are small compared to the colloids.Comment: 25 pages,13 figures, accepted for publication on J.Phys.:Condens.
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