27 research outputs found
Hydrodynamics of Micro-swimmers in Films
One of the principal mechanisms by which surfaces and interfaces affect
microbial life is by perturbing the hydrodynamic flows generated by swimming.
By summing a recursive series of image systems we derive a numerically
tractable approximation to the three-dimensional flow fields of a Stokeslet
(point force) within a viscous film between a parallel no-slip surface and
no-shear interface and, from this Green's function, we compute the flows
produced by a force- and torque-free micro-swimmer. We also extend the exact
solution of Liron & Mochon (1976) to the film geometry, which demonstrates that
the image series gives a satisfactory approximation to the swimmer flow fields
if the film is sufficiently thick compared to the swimmer size, and we derive
the swimmer flows in the thin-film limit. Concentrating on the thick film case,
we find that the dipole moment induces a bias towards swimmer accumulation at
the no-slip wall rather than the water-air interface, but that higher-order
multipole moments can oppose this. Based on the analytic predictions we propose
an experimental method to find the multipole coefficient that induces circular
swimming trajectories, allowing one to analytically determine the swimmer's
three-dimensional position under a microscope.Comment: 35 pages, 11 figures, 5 table
Upstream Swimming in Microbiological Flows
Interactions between microorganisms and their complex flowing environments are essential in many biological systems. We develop a model for microswimmer dynamics in non-Newtonian Poiseuille flows. We predict that swimmers in shear-thickening (-thinning) fluids migrate upstream more (less) quickly than in Newtonian fluids and demonstrate that viscoelastic normal stress differences reorient swimmers causing them to migrate upstream at the centreline, in contrast to well-known boundary accumulation in quiescent Newtonian fluids. Based on these observations, we suggest a sorting mechanism to select microbes by swimming speed
Magnetic behavior of nanocrystalline ErCo2
We have investigated the magnetic behavior of the nanocrystalline form of a
well-known Laves phase compound, ErCo2 - the bulk form of which has been known
to undergo an interesting first-order ferrimagnetic ordering near 32 K -
synthesized by high-energy ball-milling. It is found that, in these
nanocrystallites, Co exhibits ferromagnetic order at room temperature as
inferred from the magnetization data. However, the magnetic transition
temperature for Er sublattice remains essentially unaffected as though the
(Er)4f-Co(3d) coupling is weak on Er magnetism. The net magnetic moment as
measured at high fields, sat at 120 kOe, is significantly reduced with respect
to that for the bulk in the ferrimagnetically ordered state and possible
reasons are outlined. We have also compared the magnetocaloric behavior for the
bulk and the nano particles.Comment: JPCM, in pres
Hotspots of boundary accumulation: dynamics and statistics of micro-swimmers in flowing films
Biological flows over surfaces and interfaces can result in accumulation hotspots or depleted voids of microorganisms in natural environments. Apprehending the mechanisms that lead to such distributions is essential for understanding biofilm initiation. Using a systematic framework we resolve the dynamics and statistics of swimming microbes within flowing films, considering the impact of confinement through steric and hydrodynamic interactions, flow, and motility, along with Brownian and run-tumble fluctuations. Micro-swimmers can be peeled o↵ the solid wall above a critical flow strength. However, the interplay of flow and fluctuations causes organisms to migrate back towards the wall above a secondary critical value. Hence, faster flows may not always be the most e"cacious strategy to discourage biofilm initiation. Moreover, we find run-tumble dynamics commonly used by flagellated microbes to be an intrinsically more successful strategy to escape from boundaries than equivalent levels of enhanced Brownian noise in ciliated organisms
Lattice-Boltzmann hydrodynamics of anisotropic active matter
A plethora of active matter models exist that describe the behavior of
self-propelled particles (or swimmers), both with and without hydrodynamics.
However, there are few studies that consider shape-anisotropic swimmers and
include hydrodynamic interactions. Here, we introduce a simple method to
simulate self-propelled colloids interacting hydrodynamically in a viscous
medium using the lattice-Boltzmann technique. Our model is based on
raspberry-type viscous coupling and a force/counter-force formalism which
ensures that the system is force free. We consider several anisotropic shapes
and characterize their hydrodynamic multipolar flow field. We demonstrate that
shape-anisotropy can lead to the presence of a strong quadrupole and octupole
moments, in addition to the principle dipole moment. The ability to simulate
and characterize these higher-order moments will prove crucial for
understanding the behavior of model swimmers in confining geometries.Comment: 11 pages, 3 figures, 3 table
Micromagnetic simulations of interacting dipoles on a fcc lattice: Application to nanoparticle assemblies
Micromagnetic simulations are used to examine the effects of cubic and axial
anisotropy, magnetostatic interactions and temperature on M-H loops for a
collection of magnetic dipoles on fcc and sc lattices. We employ a simple model
of interacting dipoles that represent single-domain particles in an attempt to
explain recent experimental data on ordered arrays of magnetoferritin
nanoparticles that demonstrate the crucial role of interactions between
particles in a fcc lattice. Significant agreement between the simulation and
experimental results is achieved, and the impact of intra-particle degrees of
freedom and surface effects on thermal fluctuations are investigated.Comment: 10 pages, 9 figure
Exchange bias effect in alloys and compounds
The phenomenology of exchange bias effects observed in structurally
single-phase alloys and compounds but composed of a variety of coexisting
magnetic phases such as ferromagnetic, antiferromagnetic, ferrimagnetic,
spin-glass, cluster-glass and disordered magnetic states are reviewed. The
investigations on exchange bias effects are discussed in diverse types of
alloys and compounds where qualitative and quantitative aspects of magnetism
are focused based on macroscopic experimental tools such as magnetization and
magnetoresistance measurements. Here, we focus on improvement of fundamental
issues of the exchange bias effects rather than on their technological
importance