388 research outputs found
Aggregation of self-propelled colloidal rods near confining walls
Non-equilibrium collective behavior of self-propelled colloidal rods in a
confining channel is studied using Brownian dynamics simulations and dynamical
density functional theory. We observe an aggregation process in which rods
self-organize into transiently jammed clusters at the channel walls. In the
early stage of the process, fast-growing hedgehog-like clusters are formed
which are largely immobile. At later stages, most of these clusters dissolve
and mobilize into nematized aggregates sliding past the walls.Comment: 5 pages, 4 figure
Translational and rotational friction on a colloidal rod near a wall
We present particulate simulation results for translational and rotational
friction components of a shish-kebab model of a colloidal rod with aspect ratio
(length over diameter) in the presence of a planar hard wall.
Hydrodynamic interactions between rod and wall cause an overall enhancement of
the friction tensor components. We find that the friction enhancements to
reasonable approximation scale inversely linear with the closest distance
between the rod surface and the wall, for in the range between and
. The dependence of the wall-induced friction on the angle between
the long axis of the rod and the normal to the wall is studied and fitted with
simple polynomials in .Comment: 8 pages, 8 figure
The probability distribution of a trapped Brownian particle in plane shear flows
We investigate the statistical properties of an over-damped Brownian particle
that is trapped by a harmonic potential and simultaneously exposed to a linear
shear flow or to a plane Poiseuille flow. Its probability distribution is
determined via the corresponding Smoluchowski equation, which is solved
analytically for a linear shear flow. In the case of a plane Poiseuille flow,
analytical approximations for the distribution are obtained by a perturbation
analysis and they are substantiated by numerical results. There is a good
agreement between the two approaches for a wide range of parameters.Comment: 5 pages, 4 figur
Particles held by springs in a linear shear flow exhibit oscillatory motion
The dynamics of small spheres, which are held by linear springs in a low
Reynolds number shear flow at neighboring locations is investigated. The flow
elongates the beads and the interplay of the shear gradient with the nonlinear
behavior of the hydrodynamic interaction among the spheres causes in a large
range of parameters a bifurcation to a surprising oscillatory bead motion. The
parameter ranges, wherein this bifurcation is either super- or subcritical, are
determined.Comment: 4 pages, 5 figure
Hydrodynamic induced deformation and orientation of a microscopic elastic filament
We describe simulations of a microscopic elastic filament immersed in a fluid
and subject to a uniform external force. Our method accounts for the
hydrodynamic coupling between the flow generated by the filament and the
friction force it experiences. While models that neglect this coupling predict
a drift in a straight configuration, our findings are very different. Notably,
a force with a component perpendicular to the filament axis induces bending and
perpendicular alignment. Moreover, with increasing force we observe four shape
regimes, ranging from slight distortion to a state of tumbling motion that
lacks a steady state. We also identify the appearance of marginally stable
structures. Both the instability of these shapes and the observed alignment can
be explained by the combined action of induced bending and non-local
hydrodynamic interactions. Most of these effects should be experimentally
relevant for stiff micro-filaments, such as microtubules.Comment: three figures. To appear in Phys Rev Let
Stick boundary conditions and rotational velocity auto-correlation functions for colloidal particles in a coarse-grained representation of the solvent
We show how to implement stick boundary conditions for a spherical colloid in
a solvent that is coarse-grained by the method of stochastic rotation dynamics.
This allows us to measure colloidal rotational velocity auto-correlation
functions by direct computer simulation. We find quantitative agreement with
Enskog theory for short times and with hydrodynamic mode-coupling theory for
longer times. For aqueous colloidal suspensions, the Enskog contribution to the
rotational friction is larger than the hydrodynamic one when the colloidal
radius drops below 35nm.Comment: new version with some minor change
Reentrance effect in the lane formation of driven colloids
Recently it has been shown that a strongly interacting colloidal mixture
consisting of oppositely driven particles, undergoes a nonequilibrium
transition towards lane formation provided the driving strength exceeds a
threshold value. We predict here a reentrance effect in lane formation: for
fixed high driving force and increasing particle densities, there is first a
transition towards lane formation which is followed by another transition back
to a state with no lanes. Our result is obtained both by Brownian dynamics
computer simulations and by a phenomenological dynamical density functional
theory.Comment: 4 pages, 2 figure
Depletion forces in non-equilibrium
The concept of effective depletion forces between two fixed big colloidal
particles in a bath of small particles is generalized to a non-equilibrium
situation where the bath of small Brownian particles is flowing around the big
particles with a prescribed velocity. In striking contrast to the equilibrium
case, the non-equilibrium forces violate Newton's third law, are
non-conservative and strongly anisotropic, featuring both strong attractive and
repulsive domains.Comment: 4 pages, 3 figure
From Equilibrium to Steady-State Dynamics after Switch-On of Shear
A relation between equilibrium, steady-state, and waiting-time dependent
dynamical two-time correlation functions in dense glass-forming liquids subject
to homogeneous steady shear flow is discussed. The systems under study show
pronounced shear thinning, i.e., a significant speedup in their steady-state
slow relaxation as compared to equilibrium. An approximate relation that
recovers the exact limit for small waiting times is derived following the
integration through transients (ITT) approach for the nonequilibrium
Smoluchowski dynamics, and is exemplified within a schematic model in the
framework of the mode-coupling theory of the glass transition (MCT). Computer
simulation results for the tagged-particle density correlation functions
corresponding to wave vectors in the shear-gradient directions from both
event-driven stochastic dynamics of a two-dimensional hard-disk system and from
previously published Newtonian-dynamics simulations of a three-dimensional
soft-sphere mixture are analyzed and compared with the predictions of the
ITT-based approximation. Good qualitative and semi-quantitative agreement is
found. Furthermore, for short waiting times, the theoretical description of the
waiting time dependence shows excellent quantitative agreement to the
simulations. This confirms the accuracy of the central approximation used
earlier to derive fluctuation dissipation ratios (Phys. Rev. Lett. 102,
135701). For intermediate waiting times, the correlation functions decay faster
at long times than the stationary ones. This behavior is predicted by our
theory and observed in simulations.Comment: 16 pages, 12 figures, submitted to Phys Rev
A dynamic density functional theory for particles in a flowing solvent
We present a dynamic density functional theory (dDFT) which takes into accou
nt the advection of the particles by a flowing solvent. For potential flows we
can use the same closure as in the absence of solvent flow. The structure of
the resulting advected dDFT suggests that it could be used for non-potential
flows as well. We apply this dDFT to Brownian particles (e.g., polymer coils)
in a solvent flowing around a spherical obstacle (e.g., a colloid) and compare
the results with direct simulations of the underlying Brownian dynamics.
Although numerical limitations do not allow for an accurate quantitative
check of the advected dDFT both show the same qualitative features. In contrast
to previous works which neglected the deformation of the flow by the obstacle,
we find that the bow-wave in the density distribution of particles in front of
the obstacle as well as the wake behind it are reduced dramatically. As a
consequence the friction force exerted by the (polymer) particles on the
colloid can be reduced drastically.Comment: 7 pages, 5 figures, 2 tables, submitte
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