205 research outputs found
Local pressure for confined systems
We derive a general closed expression for the local pressure exerted onto the
corrugated walls of a channel confining a fluid medium. When the fluid medium
is at equilibrium the local pressure is a functional of the shape of the walls.
It is shown that, due to the intrinsic non-local character of the interactions
among the particles forming the fluid, the applicability of approximate schemes
such as the concept of a surface of tension or morphometric thermodynamics is
limited to wall curvatures small compared to the range of particle-particle
interactions.Comment: corrections of typos of the previous version and reorganization of
part of the material in an additional appendi
Model microswimmers in channels with varying cross section
We study different types of microswimmers moving in channels with varying
cross section and thereby interacting hydrodynamically with the channel walls.
Starting from the Smoluchowski equation for a dilute suspension, for which
interactions among swimmers can be neglected, we derive analytic expressions
for the lateral probability distribution between plane channel walls. For
weakly corrugated channels we extend the Fick--Jacobs approach to microswimmers
and thereby derive an effective equation for the probability distribution along
the channel axis. Two regimes arise dominated either by entropic forces due to
the geometrical confinement or by the active motion. In particular, our results
show that the accumulation of microswimmers at channel walls is sensitive to
both, the underlying swimming mechanism and the geometry of the channels.
Finally, for asymmetric channel corrugation our model predicts a rectification
of microswimmers along the channel, the strength and direction of which
strongly depends on the swimmer type.Comment: Added reference #4
Mechanical stability of bipolar spindle assembly
Assembly and stability of mitotic spindle is governed by the interplay of
various intra-cellular forces, e.g. the forces generated by motor proteins by
sliding overlapping anti-parallel microtubules (MTs) polymerized from the
opposite centrosomes, the interaction of kinetochores with MTs, and the
interaction of MTs with the chromosomes arms. We study the mechanical behavior
and stability of spindle assembly within the framework of a minimal model which
includes all these effects. For this model, we derive a closed--form analytical
expression for the force acting between the centrosomes as a function of their
separation distance and we show that an effective potential can be associated
with the interactions at play. We obtain the stability diagram of spindle
formation in terms of parameters characterizing the strength of motor sliding,
repulsive forces generated by polymerizing MTs, and the forces arising out of
interaction of MTs with kinetochores. The stability diagram helps in
quantifying the relative effects of the different interactions and elucidates
the role of motor proteins in formation and inhibition of spindle structures
during mitotic cell division. We also predict a regime of bistability for
certain parameter range, wherein the spindle structure can be stable for two
different finite separation distances between centrosomes. This occurrence of
bistability also suggests mechanical versatility of such self-assembled spindle
structures.Comment: 7 pages, 6 figures, under review in EP
Charge polarization, local electroneutrality breakdown and eddy formation due to electroosmosis in varying-section channels
We characterize the dynamics of an electrolyte embedded in a varying-section
channel under the action of a constant external electrostatic field. By means
of molecular dynamics simulations we determine the stationary density, charge
and velocity profiles of the electrolyte. Our results show that when the Debye
length is comparable to the width of the channel bottlenecks a concentration
polarization along with two eddies sets inside the channel. Interestingly, upon
increasing the external field, local electroneutrality breaks down and charge
polarization sets leading to the onset of net dipolar field. This novel
scenario, that cannot be captured by the standard approaches based on local
electroneutrality, opens the route for the realization of novel micro and
nano-fluidic devices
Globule-like conformation and enhanced diffusion of active polymers
We study the dynamics and conformation of polymers composed by active
monomers. By means of Brownian dynamics simulations we show that when the
direction of the self-propulsion of each monomer is aligned with the backbone,
the polymer undergoes a coil-to-globule-like transition, highlighted by a
marked change of the scaling exponent of the gyration radius. Concurrently, the
diffusion coefficient of the center of mass of the polymer becomes essentially
independent of the polymer size for sufficiently long polymers or large
magnitudes of the self-propulsion. These effects are reduced when the
self-propulsion of the monomers is not bound to be tangent to the backbone of
the polymer. Our results, rationalized by a minimal stochastic model, open new
routes for activity-controlled polymer and, possibly, for a new generation of
polymer-based drug carriers.Comment: 5 pages, 5 figures, Supplementary Materials 7 page
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
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
Active colloids at fluid interfaces
If an active Janus particle is trapped at the interface between a liquid and
a fluid, its self-propelled motion along the interface is affected by a net
torque on the particle due to the viscosity contrast between the two adjacent
fluid phases. For a simple model of an active, spherical Janus colloid we
analyze the conditions under which translation occurs along the interface and
we provide estimates of the corresponding persistence length. We show that
under certain conditions the persistence length of such a particle is
significantly larger than the corresponding one in the bulk liquid, which is in
line with the trends observed in recent experimental studies
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
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