848 research outputs found
Lucile Elliott Scholarship Announcement, 1989
An announcement for the 1989 Lucile Elliott Scholarship
Microrheology probes length scale dependent rheology
We exploit the power of microrheology to measure the viscoelasticity of entangled F-actin solutions at different length scales from 1 to 100 mu m over a wide frequency range. We compare the behavior of single probe-particle motion to that of the correlated motion of two particles. By varying the average length of the filaments, we identify fluctuations that dissipate diffusively over the filament length. These provide an important relaxation mechanism of the elasticity between 0.1 and 30 rad/sec
Fragmentation is crucial for the steady-state dynamics of actin filaments
Despite the recognition that actin filaments are important for numerous cellular processes, and decades of investigation, the dynamics of in vitro actin filaments are still not completely understood. Here, we follow the time evolution of the length distribution of labeled actin reporter filaments in an unlabeled F-actin solution via fluorescence microscopy. Whereas treadmilling and diffusive length fluctuations cannot account for the observed dynamics, our results suggest that at low salt conditions, spontaneous fragmentation is crucial
Precise Particle Tracking Against a Complicated Background: Polynomial Fitting with Gaussian Weight
We present a new particle tracking software algorithm designed to accurately
track the motion of low-contrast particles against a background with large
variations in light levels. The method is based on a polynomial fit of the
intensity around each feature point, weighted by a Gaussian function of the
distance from the centre, and is especially suitable for tracking endogeneous
particles in the cell, imaged with bright field, phase contrast or fluorescence
optical microscopy. Furthermore, the method can simultaneously track particles
of all different sizes, and allows significant freedom in their shape. The
algorithm is evaluated using the quantitative measures of accuracy and
precision of previous authors, using simulated images at variable
signal-to-noise ratios. To these we add a new test of the error due to a
non-uniform background. Finally the tracking of particles in real cell images
is demonstrated. The method is made freely available for non-commencial use as
a software package with a graphical user-inferface, which can be run within the
Matlab programming environment
Generalized Dynamic Scaling for Critical Magnetic Systems
The short-time behaviour of the critical dynamics for magnetic systems is
investigated with Monte Carlo methods. Without losing the generality, we
consider the relaxation process for the two dimensional Ising and Potts model
starting from an initial state with very high temperature and arbitrary
magnetization. We confirm the generalized scaling form and observe that the
critical characteristic functions of the initial magnetization for the Ising
and the Potts model are quite different.Comment: 32 pages with15 eps-figure
One-Bead Microrheology with Rotating Particles
We lay the theoretical basis for one-bead microrheology with rotating
particles, i.e, a method where colloids are used to probe the mechanical
properties of viscoelastic media. Based on a two-fluid model, we calculate the
compliance and discuss it for two cases. We first assume that the elastic and
fluid component exhibit both stick boundary conditions at the particle surface.
Then, the compliance fulfills a generalized Stokes law with a complex shear
modulus whose validity is only limited by inertial effects, in contrast to
translational motion. Secondly, we find that the validity of the Stokes regime
is reduced when the elastic network is not coupled to the particleComment: 7 pages, 5 figures, submitted to Europhys. Let
Clinical Manifestations and Case Management of Ebola Haemorrhagic Fever caused by a newly identified virus strain, Bundibugyo, Uganda, 2007-2008
A confirmed Ebola haemorrhagic fever (EHF) outbreak in Bundibugyo, Uganda, November 2007-February 2008, was caused by a putative new species (Bundibugyo ebolavirus). It included 93 putative cases, 56 laboratory-confirmed cases, and 37 deaths (CFR = 25%). Study objectives are to describe clinical manifestations and case management for 26 hospitalised laboratory-confirmed EHF patients. Clinical findings are congruous with previously reported EHF infections. The most frequently experienced symptoms were non-bloody diarrhoea (81%), severe headache (81%), and asthenia (77%). Seven patients reported or were observed with haemorrhagic symptoms, six of whom died. Ebola care remains difficult due to the resource-poor setting of outbreaks and the infection-control procedures required. However, quality data collection is essential to evaluate case definitions and therapeutic interventions, and needs improvement in future epidemics. Organizations usually involved in EHF case management have a particular responsibility in this respect
Dynamic Monte Carlo Measurement of Critical Exponents
Based on the scaling relation for the dynamics at the early time, a new
method is proposed to measure both the static and dynamic critical exponents.
The method is applied to the two dimensional Ising model. The results are in
good agreement with the existing results. Since the measurement is carried out
in the initial stage of the relaxation process starting from independent
initial configurations, our method is efficient.Comment: (5 pages, 1 figure) Siegen Si-94-1
On Critical Exponents and the Renormalization of the Coupling Constant in Growth Models with Surface Diffusion
It is shown by the method of renormalized field theory that in contrast to a
statement based on a mathematically ill-defined invariance transformation and
found in most of the recent publications on growth models with surface
diffusion, the coupling constant of these models renormalizes nontrivially.
This implies that the widely accepted supposedly exact scaling exponents are to
be corrected. A two-loop calculation shows that the corrections are small and
these exponents seem to be very good approximations.Comment: 4 pages, revtex, 2 postscript figures, to appear in Phys.Rev.Let
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