28,468 research outputs found
Enhanced diffusion by reciprocal swimming
Purcell's scallop theorem states that swimmers deforming their shapes in a
time-reversible manner ("reciprocal" motion) cannot swim. Using numerical
simulations and theoretical calculations we show here that in a fluctuating
environment, reciprocal swimmers undergo, on time scales larger than that of
their rotational diffusion, diffusive dynamics with enhanced diffusivities,
possibly by orders of magnitude, above normal translational diffusion.
Reciprocal actuation does therefore lead to a significant advantage over
non-motile behavior for small organisms such as marine bacteria
Efficiency of initiating cell adhesion in hydrodynamic flow
We theoretically investigate the efficiency of initial binding between a
receptor-coated sphere and a ligand-coated wall in linear shear flow. The mean
first passage time for binding decreases monotonically with increasing shear
rate. Above a saturation threshold of the order of a few 100 receptor patches,
the binding efficiency is enhanced only weakly by increasing their number and
size, but strongly by increasing their height. This explains why white blood
cells in the blood flow adhere through receptor patches localized to the tips
of microvilli, and why malaria-infected red blood cells form elevated receptor
patches (knobs).Comment: 4 pages, Revtex, 4 Postscript figures included, to appear in PR
Monotonicity and logarithmic convexity relating to the volume of the unit ball
Let stand for the volume of the unit ball in for
. In the present paper, we prove that the sequence
is logarithmically convex and that the sequence
is strictly
decreasing for . In addition, some monotonic and concave properties of
several functions relating to are extended and generalized.Comment: 12 page
Kinematics of the swimming of Spiroplasma
\emph{Spiroplasma} swimming is studied with a simple model based on
resistive-force theory. Specifically, we consider a bacterium shaped in the
form of a helix that propagates traveling-wave distortions which flip the
handedness of the helical cell body. We treat cell length, pitch angle, kink
velocity, and distance between kinks as parameters and calculate the swimming
velocity that arises due to the distortions. We find that, for a fixed pitch
angle, scaling collapses the swimming velocity (and the swimming efficiency) to
a universal curve that depends only on the ratio of the distance between kinks
to the cell length. Simultaneously optimizing the swimming efficiency with
respect to inter-kink length and pitch angle, we find that the optimal pitch
angle is 35.5 and the optimal inter-kink length ratio is 0.338, values
in good agreement with experimental observations.Comment: 4 pages, 5 figure
Individual differences in white matter microstructure reflect variation in functional connectivity during action choice.
The relation between brain structure and function is of fundamental importance in neuroscience. Comparisons between behavioral and brain imaging measures suggest that variation in brain structure correlates with the presence of specific skills[1-3]. Behavioral measures, however, reflect the integrated function of multiple brain regions. Rather than behavior, a physiological index of function could be a more sensitive and informative measure with which to compare structural measures. Here, we test for a relationship between a physiological measure of functional connectivity between two brain areas during a simple decision making task and a measure of structural connectivity. Paired-pulse transcranial magnetic stimulation indexed functional connectivity between two regions important for action choices: premotor and motor cortex. Fractional anisotropy (FA), a marker of microstructural integrity, indexed structural connectivity. Individual differences in functional connectivity during action selection show highly specific correlations with FA in localised regions of white matter interconnecting regions including the premotor and motor cortex. Probabilistic tractography[4, 5], a technique for identifying fibre pathways from diffusion-weighted imaging (DWI), reconstructed the anatomical networks linking the component brain regions involved in making decisions. These findings demonstrate a relationship between individual differences in functional and structural connectivity within human brain networks central to action choice
The smallest eigenvalue of Hankel matrices
Let H_N=(s_{n+m}),n,m\le N denote the Hankel matrix of moments of a positive
measure with moments of any order. We study the large N behaviour of the
smallest eigenvalue lambda_N of H_N. It is proved that lambda_N has exponential
decay to zero for any measure with compact support. For general determinate
moment problems the decay to 0 of lambda_N can be arbitrarily slow or
arbitrarily fast. In the indeterminate case, where lambda_N is known to be
bounded below by a positive constant, we prove that the limit of the n'th
smallest eigenvalue of H_N for N tending to infinity tends rapidly to infinity
with n. The special case of the Stieltjes-Wigert polynomials is discussed
Multicanonical Recursions
The problem of calculating multicanonical parameters recursively is
discussed. I describe in detail a computational implementation which has worked
reasonably well in practice.Comment: 23 pages, latex, 4 postscript figures included (uuencoded
Z-compressed .tar file created by uufiles), figure file corrected
Generalized-ensemble Monte carlo method for systems with rough energy landscape
We present a novel Monte Carlo algorithm which enhances equilibrization of
low-temperature simulations and allows sampling of configurations over a large
range of energies. The method is based on a non-Boltzmann probability weight
factor and is another version of the so-called generalized-ensemble techniques.
The effectiveness of the new approach is demonstrated for the system of a small
peptide, an example of the frustrated system with a rugged energy landscape.Comment: Latex; ps-files include
Collective chemotactic dynamics in the presence of self-generated fluid flows
In micro-swimmer suspensions locomotion necessarily generates fluid motion,
and it is known that such flows can lead to collective behavior from unbiased
swimming. We examine the complementary problem of how chemotaxis is affected by
self-generated flows. A kinetic theory coupling run-and-tumble chemotaxis to
the flows of collective swimming shows separate branches of chemotactic and
hydrodynamic instabilities for isotropic suspensions, the first driving
aggregation, the second producing increased orientational order in suspensions
of "pushers" and maximal disorder in suspensions of "pullers". Nonlinear
simulations show that hydrodynamic interactions can limit and modify
chemotactically-driven aggregation dynamics. In puller suspensions the dynamics
form aggregates that are mutually-repelling due to the non-trivial flows. In
pusher suspensions chemotactic aggregation can lead to destabilizing flows that
fragment the regions of aggregation.Comment: 4 page
Reaction-Diffusion Process Driven by a Localized Source: First Passage Properties
We study a reaction-diffusion process that involves two species of atoms,
immobile and diffusing. We assume that initially only immobile atoms, uniformly
distributed throughout the entire space, are present. Diffusing atoms are
injected at the origin by a source which is turned on at time t=0. When a
diffusing atom collides with an immobile atom, the two atoms form an immobile
stable molecule. The region occupied by molecules is asymptotically spherical
with radius growing as t^{1/d} in d>=2 dimensions. We investigate the survival
probability that a diffusing atom has not become a part of a molecule during
the time interval t after its injection and the probability density of such a
particle. We show that asymptotically the survival probability (i) saturates in
one dimension, (ii) vanishes algebraically with time in two dimensions (with
exponent being a function of the dimensionless flux and determined as a zero of
a confluent hypergeometric function), and (iii) exhibits a stretched
exponential decay in three dimensions.Comment: 7 pages; version 2: section IV is re-written, references added, 8
pages (final version
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