352 research outputs found
Bistability, oscillations and bidirectional motion of ensemble of hydrodynamically-coupled molecular motors
We analyze the collective behavior of hydrodynamically coupled molecular
motors. We show that the local fluxes induced by motors displacement can induce
the experimentally observed bidirectional motion of cargoes and vesicles. By
means of a mean--field approach we show that sustained oscillations as well as
bistable collective motor motion arise even for very large collection of
motors, when thermal noise is irrelevant. The analysis clarifies the physical
mechanisms responsible for such dynamics by identifying the relevant coupling
parameter and its dependence on the geometry of the hydrodynamic coupling as
well as on system size. We quantify the phase diagram for the different phases
that characterize the collective motion of hydrodynamically coupled motors and
show that sustained oscillations can be reached for biologically relevant
parameters, hence demonstrating the relevance of hydrodynamic interactions in
intracellular transport
Geometrically-tuned channel permeability
We characterize the motion of charged as well as neutral tracers, in an
electrolyte embedded in a varying section channel. We exploit a set of
systematic approximations that allows us to simplify the problem, yet capturing
the essential of the interplay between the geometrical confinement provided by
the corrugated channel walls and the electrolyte properties. Our simplified
approach allows us to characterize the transport properties of corrugated
channels when a net flux of tracers is obtained by keeping the extrema of the
channel at different chemical potentials. For highly diluted tracer
suspensions, we have characterized tracers currents and we have estimated the
net electric current which occurs when both positively and negatively charged
tracers are considered.Comment: Fixed reference
Local size segregation in polydisperse hard sphere fluids
The structure of polydisperse hard sphere fluids, in the presence of a wall,
is studied by the Rosenfeld density functional theory. Within this approach,
the local excess free energy depends on only four combinations of the full set
of density fields. The case of continuous polydispersity thereby becomes
tractable. We predict, generically, an oscillatory size segregation close to
the wall, and connect this, by a perturbation theory for narrow distributions,
with the reversible work for changing the size of one particle in a
monodisperse reference fluid.Comment: RevTeX, 4 pages, 3 figures, submitted to Phys. Rev. Let
A practical density functional for polydisperse polymers
The Flory Huggins equation of state for monodisperse polymers can be turned
into a density functional by adding a square gradient term, with a coefficient
fixed by appeal to RPA (random phase approximation). We present instead a model
nonlocal functional in which each polymer is replaced by a deterministic,
penetrable particle of known shape. This reproduces the RPA and square gradient
theories in the small deviation and/or weak gradient limits, and can readily be
extended to polydisperse chains. The utility of the new functional is shown for
the case of a polydisperse polymer solution at coexistence in a poor solvent.Comment: 9 pages, 3 figure
Driving an electrolyte through a corrugated nanopore
We characterize the dynamics of a electrolyte embedded in a
varying-section channel. In the linear response regime, by means of suitable
approximations, we derive the Onsager matrix associated to externally enforced
gradients in electrostatic potential, chemical potential, and pressure, for
both dielectric and conducting channel walls. We show here that the linear
transport coefficients are particularly sensitive to the geometry and the
conductive properties of the channel walls when the Debye length is comparable
to the channel width. In this regime, we found that one pair of off-diagonal
Onsager matrix elements increases with the corrugation of the channel
transport, in contrast to all other elements which are either unaffected by or
decrease with increasing corrugation. Our results have a possible impact on the
design of blue-energy devices as well as on the understanding of biological ion
channels through membrane
Colloidal Jamming at Interfaces: a Route to Fluid-bicontinuous Gels
Colloidal particles or nanoparticles, with equal affinity for two fluids, are
known to adsorb irreversibly to the fluid-fluid interface. We present
large-scale computer simulations of the demixing of a binary solvent containing
such particles. The newly formed interface sequesters the colloidal particles;
as the interface coarsens, the particles are forced into close contact by
interfacial tension. Coarsening is dramatically curtailed, and the jammed
colloidal layer seemingly enters a glassy state, creating a multiply connected,
solid-like film in three dimensions. The resulting gel contains percolating
domains of both fluids, with possible uses as, for example, a microreaction
medium
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