579 research outputs found
Director Field Configurations around a Spherical Particle in a Nematic Liquid Crystal
We study the director field around a spherical particle immersed in a
uniformly aligned nematic liquid crystal and assume that the molecules prefer a
homeotropic orientation at the surface of the particle. Three structures are
possible: a dipole, a Saturn-ring, and a surface-ring configuration, which we
investigate by numerically minimizing the Frank free energy supplemented by a
magnetic-field and a surface term. In the dipole configuration, which is the
absolutely stable structure for micron-size particles and sufficiently strong
surface anchoring, a twist transition is found and analyzed. We show that a
transition from the dipole to the Saturn ring configuration is induced by
either decreasing the particle size or by applying a magnetic field. The effect
of metastability and the occurence of hysteresis in connection with a magnetic
field are discussed. The surface-ring configuration appears when the
surface-anchoring strength W is reduced. It is also favored by a large
saddle-splay constant K_24. A comparison with recent experiments by Poulin et
al. gives a lower bound for W, i.e., W > 0.6 erg/cm^2 for the interface of
water and pentylcyanobiphenyl (5CB) in the presence of the surfactant sodium
dodecyl sulfate.Comment: 11 pages, Revtex, 16 eps figures, submitted to Eur. Phys. J.
Feedback control of inertial microfluidics using axial control forces
Inertial microfluidics is a promising tool for many lab-on-a-chip
applications. Particles in channel flows with Reynolds numbers above one
undergo cross-streamline migration to a discrete set of equilibrium positions
in square and rectangular channel cross sections. This effect has been used
extensively for particle sorting and the analysis of particle properties. Using
the lattice Boltzmann method, we determine equilibrium positions in square and
rectangular cross sections and classify their types of stability for different
Reynolds numbers, particle sizes, and channel aspect ratios. Our findings
thereby help to design microfluidic channels for particle sorting. Furthermore,
we demonstrate how an axial control force, which slows down the particles,
shifts the stable equilibrium position towards the channel center. Ultimately,
the particles then stay on the centerline for forces exceeding a threshold
value. This effect is sensitive to particle size and channel Reynolds number
and therefore suggests an efficient method for particle separation. In
combination with a hysteretic feedback scheme, we can even increase particle
throughput
Emergent behavior in active colloids
Active colloids are microscopic particles, which self-propel through viscous
fluids by converting energy extracted from their environment into directed
motion. We first explain how articial microswimmers move forward by generating
near-surface flow fields via self-phoresis or the self-induced Marangoni
effect. We then discuss generic features of the dynamics of single active
colloids in bulk and in confinement, as well as in the presence of gravity,
field gradients, and fluid flow. In the third part, we review the emergent
collective behavior of active colloidal suspensions focussing on their
structural and dynamic properties. After summarizing experimental observations,
we give an overview on the progress in modeling collectively moving active
colloids. While active Brownian particles are heavily used to study collective
dynamics on large scales, more advanced methods are necessary to explore the
importance of hydrodynamic and phoretic particle interactions. Finally, the
relevant physical approaches to quantify the emergent collective behavior are
presented.Comment: 31 pages, 14 figure
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
Modelling bacterial flagellar growth
The growth of bacterial flagellar filaments is a self-assembly process where
flagellin molecules are transported through the narrow core of the flagellum
and are added at the distal end. To model this situation, we generalize a
growth process based on the TASEP model by allowing particles to move both
forward and backward on the lattice. The bias in the forward and backward jump
rates determines the lattice tip speed, which we analyze and also compare to
simulations. For positive bias, the system is in a non-equilibrium steady state
and exhibits boundary-induced phase transitions. The tip speed is constant. In
the no-bias case we find that the length of the lattice grows as
, whereas for negative drift . The
latter result agrees with experimental data of bacterial flagellar growth.Comment: 6 pages, 7 figure
Metachronal waves in a chain of rowers with hydrodynamic interactions
Filaments on the surface of a microorganism such as Paramecium or Ophalina
beat highly synchronized and form so-called metachronal waves that travel along
the surfaces. In order to study under what principal conditions these waves
form, we introduce a chain of beads, called rowers, each periodically driven by
an external force on a straight line segment. To implement hydrodynamic
interactions between the beads, they are considered point-like. Two beads
synchronize in antiphase or in phase depending on the positive or negative
curvature of their driving-force potential. Concentrating on in-phase
synchronizing rowers, we find that they display only transient synchronization
in a bulk fluid. On the other hand, metachronal waves with wavelengths of 7-10
rower distances emerge, when we restrict the range of hydrodynamic interactions
either artificially to nearest neighbors or by the presence of a bounding
surface as in any relevant biological system.Comment: 9 pages, 10 figure
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