382 research outputs found
Migration of semiflexible polymers in microcapillary flow
The non-equilibrium structural and dynamical properties of a semiflexible
polymer confined in a cylindrical microchannel and exposed to a Poiseuille flow
is studied by mesoscale hydrodynamic simulations. For a polymer with a length
half of its persistence length, large variations in orientation and
conformations are found as a function of radial distance and flow strength. In
particular, the polymer exhibits U-shaped conformations near the channel
center. Hydrodynamic interactions lead to strong cross-streamline migration.
Outward migration is governed by the polymer orientation and the corresponding
anisotropy in its diffusivity. Strong tumbling motion is observed, with a
tumbling time which exhibits the same dependence on Peclet number as a polymer
in shear flow.Comment: 6 pages, 7 figures, accepted by EP
Flow-Induced Helical Coiling of Semiflexible Polymers in Structured Microchannels
The conformations of semiflexible (bio)polymers are studied in flow through
geometrically structured microchannels. Using mesoscale hydrodynamics
simulations, we show that the polymer undergoes a rod-to-helix transition as it
moves from the narrow high-velocity region into the wide low-velocity region of
the channel. The transient helix formation is the result of a non-equilibrium
and non-stationary buckling transition of the semiflexible polymer, which is
subjected to a compressive force originating from the fluid-velocity variation
in the channel. The helix properties depend on the diameter ratio of the
channel, the polymer bending rigidity, and the flow strength.Comment: Accepted in Phys. Rev. Let
Physics of Microswimmers - Single Particle Motion and Collective Behavior
Locomotion and transport of microorganisms in fluids is an essential aspect
of life. Search for food, orientation toward light, spreading of off-spring,
and the formation of colonies are only possible due to locomotion. Swimming at
the microscale occurs at low Reynolds numbers, where fluid friction and
viscosity dominates over inertia. Here, evolution achieved propulsion
mechanisms, which overcome and even exploit drag. Prominent propulsion
mechanisms are rotating helical flagella, exploited by many bacteria, and
snake-like or whip-like motion of eukaryotic flagella, utilized by sperm and
algae. For artificial microswimmers, alternative concepts to convert chemical
energy or heat into directed motion can be employed, which are potentially more
efficient. The dynamics of microswimmers comprises many facets, which are all
required to achieve locomotion. In this article, we review the physics of
locomotion of biological and synthetic microswimmers, and the collective
behavior of their assemblies. Starting from individual microswimmers, we
describe the various propulsion mechanism of biological and synthetic systems
and address the hydrodynamic aspects of swimming. This comprises
synchronization and the concerted beating of flagella and cilia. In addition,
the swimming behavior next to surfaces is examined. Finally, collective and
cooperate phenomena of various types of isotropic and anisotropic swimmers with
and without hydrodynamic interactions are discussed.Comment: 54 pages, 59 figures, review article, Reports of Progress in Physics
(to appear
Cooperative Motion of Active Brownian Spheres in Three-Dimensional Dense Suspensions
The structural and dynamical properties of suspensions of self-propelled
Brownian particles of spherical shape are investigated in three spatial
dimensions. Our simulations reveal a phase separation into a dilute and a dense
phase, above a certain density and strength of self-propulsion. The packing
fraction of the dense phase approaches random close packing at high activity,
yet the system remains fluid. Although no alignment mechanism exists in this
model, we find long-lived cooperative motion of the particles in the dense
regime. This behavior is probably due to an interface-induced sorting process.
Spatial displacement correlation functions are nearly scale-free for systems
with densities close to or above the glass transition density of passive
systems.Comment: 6 pages, 7 figure
Bacterial swarmer cells in confinement: A mesoscale hydrodynamic simulation study
A wide spectrum of Peritrichous bacteria undergo considerable physiological
changes when they are inoculated onto nutrition-rich surfaces and exhibit a
rapid and collective migration denoted as swarming. Thereby, the length of such
swarmer cells and their number of flagella increases substantially. In this
article, we investigated the properties of individual E. coli-type swarmer
cells confined between two parallel walls via mesoscale hydrodynamic
simulations, combining molecular dynamics simulations of the swarmer cell with
the multiparticle particle collision dynamics approach for the embedding fluid.
E. coli-type swarmer cells are three-times longer than their planktonic counter
parts, but their flagella density is comparable. By varying the wall
separation, we analyze the confinement effect on the flagella arrangement, on
the distribution of cells in the gap between the walls, and on the cell
dynamics. We find only a weak dependence of confinement on the bundle structure
and dynamics. The distribution of cells in the gap changes from a
geometry-dominated behavior for very narrow to fluid-dominated behavior for
wider gaps, where cells are preferentially located in the gap center for
narrower gaps and stay preferentially next to one of the walls for wider gaps.
Dynamically, the cells exhibit a wide spectrum of migration behaviors,
depending on their flagella bundle arrangement, and ranges from straight
swimming to wall rolling
Steady state sedimentation of ultrasoft colloids
The structural and dynamical properties of ultra-soft colloids - star
polymers - exposed to a uniform external force field are analyzed applying the
multiparticle collision dynamics approach, a hybrid coarse-grain mesoscale
simulation approach, which captures thermal fluctuations and long-range
hydrodynamic interactions. In the weak field limit, the structure of the star
polymer is nearly unchanged, however in an intermediate regime, the radius of
gyration decreases, in particular transverse to the sedimentation direction. In
the limit of a strong field, the radius of gyration increases with field
strength. Correspondingly, the sedimentation coefficient increases with
increasing field strength, passes through a maximum and decreases again at high
field strengths. The maximum value depends on the functionality of the star
polymer. High field strengths lead to symmetry breaking with trailing, strongly
stretched polymer arms and a compact star polymer body. In the weak field
linear response regime, the sedimentation coefficient follows the scaling
relation of a star polymer in terms of functionality and arm length
Modelling the Mechanics and Hydrodynamics of Swimming E. coli
The swimming properties of an E. coli-type model bacterium are investigated
by mesoscale hy- drodynamic simulations, combining molecular dynamics
simulations of the bacterium with the multiparticle particle collision dynamics
method for the embedding fluid. The bacterium is com- posed of a
spherocylindrical body with attached helical flagella, built up from discrete
particles for an efficient coupling with the fluid. We measure the hydrodynamic
friction coefficients of the bacterium and find quantitative agreement with
experimental results of swimming E. coli. The flow field of the bacterium shows
a force-dipole-like pattern in the swimming plane and two vor- tices
perpendicular to its swimming direction arising from counterrotation of the
cell body and the flagella. By comparison with the flow field of a force dipole
and rotlet dipole, we extract the force- dipole and rotlet-dipole strengths for
the bacterium and find that counterrotation of the cell body and the flagella
is essential for describing the near-field hydrodynamics of the bacterium
Hydrodynamics of Binary Fluid Mixtures - An Augmented Multiparticle Collison Dynamics Approach
The Multiparticle Collision Dynamics technique (MPC) for hydrodynamics
simulations is generalized to binary fluid mixtures and multiphase flows, by
coupling the particle-based fluid dynamics to a Ginzburg-Landau free-energy
functional for phase-separating binary fluids. To describe fluids with a
non-ideal equation of state, an additional density-dependent term is
introduced. The new approach is verified by applying it to thermodynamics near
the critical demixing point, and interface fluctuations of droplets. The
interfacial tension obtained from the analysis of the capillary wave spectrum
agrees well with the results based on the Laplace-Young equation.
Phase-separation dynamics follows the Lifshitz-Slyozov law
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