2,593 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
Theory of nonlinear rheology and yielding of dense colloidal suspensions
A first principles approach to the nonlinear flow of dense suspensions is
presented which captures shear thinning of colloidal fluids and dynamical
yielding of colloidal glasses. The advection of density fluctuations plays a
central role, suppressing the caging of particles and speeding up structural
relaxation. A mode coupling approach is developed to explore these effects.Comment: 4 pages, 2 figures; slightly corrected version; Phys. Rev. Lett., to
be published (2002
Fermions on half-quantum vortex
The spectrum of the fermion zero modes in the vicinity of the vortex with
fractional winding number is discussed. This is inspired by the observation of
the 1/2 vortex in high-temperature superconductors (Kirtley, et al, Phys. Rev.
Lett. 76 (1996) 1336). The fractional value of the winding number leads to the
fractional value of the invariant, which describes the topology of the energy
spectrum of fermions. This results in the phenomenon of the "half-crossing":
the spectrum approaches zero but does not cross it, being captured at the zero
energy level. The similarity with the phenomenon of the fermion condensation is
discussed.Comment: In revised version the discussion is extended and 4 references are
added. The paper is accepted for publication in JETP Letters. 10 pages, LaTeX
file, 3 figures are available at
ftp://boojum.hut.fi/pub/publications/lowtemp/LTL-96004.p
Shear-induced reaction-limited aggregation kinetics of Brownian particles at arbitrary concentrations
The aggregation of interacting Brownian particles in sheared concentrated
suspensions is an important issue in colloid and soft matter science per se.
Also, it serves as a model to understand biochemical reactions occurring in
vivo where both crowding and shear play an important role. We present an
effective medium approach within the Smoluchowski equation with shear which
allows one to calculate the encounter kinetics through a potential barrier
under shear at arbitrary colloid concentrations. Experiments on a model
colloidal system in simple shear flow support the validity of the model in the
range considered. By generalizing Kramers' rate theory to the presence of
collective hydrodynamics, our model explains the significant increase in the
shear-induced reaction-limited aggregation kinetics upon increasing the colloid
concentration
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
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
How to create Alice string (half-quantum vortex) in a vector Bose-Einstein condensate
We suggest a procedure how to prepare the vortex with N=1/2 winding number --
the counterpart of the Alice string -- in a Bose--Einstein condensate with
hyperfine spin F=1. Other possible vortices in Bose-condensates are also
discussed.Comment: RevTex file, 3 pages, no figures, extended version submitted to JETP
Letter
Correlated particle dynamics in concentrated quasi-two-dimensional suspensions
We investigate theoretically and experimentally how the hydrodynamically
correlated lateral motion of particles in a suspension confined between two
surfaces is affected by the suspension concentration. Despite the long range of
the correlations (decaying as 1/r^2 with the inter-particle distance r), the
concentration effect is present only at short inter-particle distances for
which the static pair correlation is nonuniform. This is in sharp contrast with
the effect of hydrodynamic screening present in unconfined suspensions, where
increasing the concentration changes the prefactor of the large-distance
correlation.Comment: 13 page
Non--Newtonian viscosity of interacting Brownian particles: comparison of theory and data
A recent first-principles approach to the non-linear rheology of dense
colloidal suspensions is evaluated and compared to simulation results of
sheared systems close to their glass transitions. The predicted scenario of a
universal transition of the structural dynamics between yielding of glasses and
non-Newtonian (shear-thinning) fluid flow appears well obeyed, and calculations
within simplified models rationalize the data over variations in shear rate and
viscosity of up to 3 decades.Comment: 6 pages, 2 figures; J. Phys. Condens. Matter to be published (Jan.
2003
Influence of polydispersity on the critical parameters of an effective potential model for asymmetric hard sphere mixtures
We report a Monte Carlo simulation study of the properties of highly
asymmetric binary hard sphere mixtures. This system is treated within an
effective fluid approximation in which the large particles interact through a
depletion potential (R. Roth {\em et al}, Phys. Rev. E{\bf 62} 5360 (2000))
designed to capture the effects of a virtual sea of small particles. We
generalize this depletion potential to include the effects of explicit size
dispersity in the large particles and consider the case in which the particle
diameters are distributed according to a Schulz form having degree of
polydispersity 14%. The resulting alteration (with respect to the monodisperse
limit) of the metastable fluid-fluid critical point parameters is determined
for two values of the ratio of the diameters of the small and large particles:
and . We find that inclusion of
polydispersity moves the critical point to lower reservoir volume fractions of
the small particles and high volume fractions of the large ones. The estimated
critical point parameters are found to be in good agreement with those
predicted by a generalized corresponding states argument which provides a link
to the known critical adhesion parameter of the adhesive hard sphere model.
Finite-size scaling estimates of the cluster percolation line in the one phase
fluid region indicate that inclusion of polydispersity moves the critical point
deeper into the percolating regime. This suggests that phase separation is more
likely to be preempted by dynamical arrest in polydisperse systems.Comment: 11 pages, 10 figure
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