20,385 research outputs found
Hydrodynamics of flagellated microswimmers near free-slip interfaces
The hydrodynamics of a flagellated microorganism is investigated when
swimming close to a planar free-slip surface by means of numerical solu- tions
of the Stokes equations obtained via a Boundary Element Method. Depending on
the initial condition, the swimmer can either escape from the free-slip surface
or collide with the boundary. Interestingly, the mi- croorganism does not
exhibit a stable orbit. Independently of escape or attraction to the interface,
close to a free-slip surface, the swimmer fol- lows a counter-clockwise
trajectory, in agreement with experimental find- ings, [15]. The hydrodynamics
is indeed modified by the free-surface. In fact, when the same swimmer moves
close to a no-slip wall, a set of initial conditions exists which result in
stable orbits. Moreover when moving close to a free-slip or a no-slip boundary
the swimmer assumes a different orientation with respect to its trajectory.
Taken together, these results contribute to shed light on the hydrodynamical
behaviour of microorgan- isms close to liquid-air interfaces which are relevant
for the formation of interfacial biofilms of aerobic bacteria
Human sperm accumulation near surfaces: a simulation study
A hybrid boundary integral/slender body algorithm for modelling flagellar cell motility is presented. The algorithm uses the boundary element method to represent the ‘wedge-shaped’ head of the human sperm cell and a slender body theory representation of the flagellum. The head morphology is specified carefully due to its significant effect on the force and torque balance and hence movement of the free-swimming cell. The technique is used to investigate the mechanisms for the accumulation of human spermatozoa near surfaces. Sperm swimming in an infinite fluid, and near a plane boundary, with prescribed planar and three-dimensional flagellar waveforms are simulated. Both planar and ‘elliptical helicoid’ beating cells are predicted to accumulate at distances of approximately 8.5–22 μm from surfaces, for flagellar beating with angular wavenumber of 3π to 4π. Planar beating cells with wavenumber of approximately 2.4π or greater are predicted to accumulate at a finite distance, while cells with wavenumber of approximately 2π or less are predicted to escape from the surface, likely due to the breakdown of the stable swimming configuration. In the stable swimming trajectory the cell has a small angle of inclination away from the surface, no greater than approximately 0.5°. The trapping effect need not depend on specialized non-planar components of the flagellar beat but rather is a consequence of force and torque balance and the physical effect of the image systems in a no-slip plane boundary. The effect is relatively weak, so that a cell initially one body length from the surface and inclined at an angle of 4°–6° towards the surface will not be trapped but will rather be deflected from the surface. Cells performing rolling motility, where the flagellum sweeps out a ‘conical envelope’, are predicted to align with the surface provided that they approach with sufficiently steep angle. However simulation of cells swimming against a surface in such a configuration is not possible in the present framework. Simulated human sperm cells performing a planar beat with inclination between the beat plane and the plane-of-flattening of the head were not predicted to glide along surfaces, as has been observed in mouse sperm. Instead, cells initially with the head approximately 1.5–3 μm from the surface were predicted to turn away and escape. The simulation model was also used to examine rolling motility due to elliptical helicoid flagellar beating. The head was found to rotate by approximately 240° over one beat cycle and due to the time-varying torques associated with the flagellar beat was found to exhibit ‘looping’ as has been observed in cells swimming against coverslips
Intrinsic Inference on the Mean Geodesic of Planar Shapes and Tree Discrimination by Leaf Growth
For planar landmark based shapes, taking into account the non-Euclidean
geometry of the shape space, a statistical test for a common mean first
geodesic principal component (GPC) is devised. It rests on one of two
asymptotic scenarios, both of which are identical in a Euclidean geometry. For
both scenarios, strong consistency and central limit theorems are established,
along with an algorithm for the computation of a Ziezold mean geodesic. In
application, this allows to verify the geodesic hypothesis for leaf growth of
Canadian black poplars and to discriminate genetically different trees by
observations of leaf shape growth over brief time intervals. With a test based
on Procrustes tangent space coordinates, not involving the shape space's
curvature, neither can be achieved.Comment: 28 pages, 4 figure
Impulsive cylindrical gravitational wave: one possible radiative form emitted from cosmic strings and corresponding electromagnetic response
The cosmic strings(CSs) may be one important source of gravitational
waves(GWs), and it has been intensively studied due to its special properties
such as the cylindrical symmetry. The CSs would generate not only usual
continuous GW, but also impulsive GW that brings more concentrated energy and
consists of different GW components broadly covering low-, intermediate- and
high-frequency bands simultaneously. These features might underlie interesting
electromagnetic(EM) response to these GWs generated by the CSs. In this paper,
with novel results and effects, we firstly calculate the analytical solutions
of perturbed EM fields caused by interaction between impulsive cylindrical GWs
(would be one of possible forms emitted from CSs) and background celestial high
magnetic fields or widespread cosmological background magnetic fields, by using
rigorous Einstein - Rosen metric. Results show: perturbed EM fields are also in
the impulsive form accordant to the GW pulse, and asymptotic behaviors of the
perturbed EM fields are fully consistent with the asymptotic behaviors of the
energy density, energy flux density and Riemann curvature tensor of
corresponding impulsive cylindrical GWs. The analytical solutions naturally
give rise to the accumulation effect which is proportional to the term of
distance^1/2, and based on it, we for the first time predict potentially
observable effects in region of the Earth caused by the EM response to GWs from
the CSs.Comment: 34 pages, 12 figure
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
Novel metallic and insulating states at a bent quantum Hall junction
A non-planar geometry for the quantum Hall (QH) effect is studied, whereby
two quantum Hall (QH) systems are joined at a sharp right angle. When both
facets are at equal filling factor nu the junction hosts a channel with
non-quantized conductance, dependent on nu. The state is metallic at nu = 1/3,
with conductance along the junction increasing as the temperature T drops. At
nu = 1, 2 it is strongly insulating, and at nu = 3, 4 shows only weak T
dependence. Upon applying a dc voltage bias along the junction, the
differential conductance again shows three different behaviors. Hartree
calculations of the dispersion at the junction illustrate possible
explanations, and differences from planar QH structures are highlighted.Comment: 5 pages, 4 figures, text + figs revised for clarit
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