1,113 research outputs found
Slipping friction of an optically and magnetically manipulated microsphere rolling at a glass-water interface
The motion of submerged magnetic microspheres rolling at a glass-water
interface has been studied using magnetic rotation and optical tweezers
combined with bright-field microscopy particle tracking techniques. Individual
microspheres of varying surface roughness were magnetically rotated both in and
out of an optical trap to induce rolling, along either plain glass cover slides
or glass cover slides functionalized with polyethylene glycol. It has been
observed that the manipulated microspheres exhibited nonlinear dynamic
rolling-while-slipping motion characterized by two motional regimes: At low
rotational frequencies, the speed of microspheres free-rolling along the
surface increased proportionately with magnetic rotation rate; however, a
further increase in the rotation frequency beyond a certain threshold revealed
a sharp transition to a motion in which the microspheres slipped with respect
to the external magnetic field resulting in decreased rolling speeds. The
effects of surface-microsphere interactions on the position of this threshold
frequency are posed and investigated. Similar experiments with microspheres
rolling while slipping in an optical trap showed congruent results.Comment: submitted to Journal of Applied Physics, 11 figure
Diffusion-Limited Coalescence with Finite Reaction Rates in One Dimension
We study the diffusion-limited process in one dimension, with
finite reaction rates. We develop an approximation scheme based on the method
of Inter-Particle Distribution Functions (IPDF), which was formerly used for
the exact solution of the same process with infinite reaction rate. The
approximation becomes exact in the very early time regime (or the
reaction-controlled limit) and in the long time (diffusion-controlled)
asymptotic limit. For the intermediate time regime, we obtain a simple
interpolative behavior between these two limits. We also study the coalescence
process (with finite reaction rates) with the back reaction , and in
the presence of particle input. In each of these cases the system reaches a
non-trivial steady state with a finite concentration of particles. Theoretical
predictions for the concentration time dependence and for the IPDF are compared
to computer simulations. P. A. C. S. Numbers: 82.20.Mj 02.50.+s 05.40.+j
05.70.LnComment: 13 pages (and 4 figures), plain TeX, SISSA-94-0
Concentration for One and Two Species One-Dimensional Reaction-Diffusion Systems
We look for similarity transformations which yield mappings between different
one-dimensional reaction-diffusion processes. In this way results obtained for
special systems can be generalized to equivalent reaction-diffusion models. The
coagulation (A + A -> A) or the annihilation (A + A -> 0) models can be mapped
onto systems in which both processes are allowed. With the help of the
coagulation-decoagulation model results for some death-decoagulation and
annihilation-creation systems are given. We also find a reaction-diffusion
system which is equivalent to the two species annihilation model (A + B ->0).
Besides we present numerical results of Monte Carlo simulations. An accurate
description of the effects of the reaction rates on the concentration in
one-species diffusion-annihilation model is made. The asymptotic behavior of
the concentration in the two species annihilation system (A + B -> 0) with
symmetric initial conditions is studied.Comment: 20 pages latex, uuencoded figures at the en
Stochastic Ballistic Annihilation and Coalescence
We study a class of stochastic ballistic annihilation and coalescence models
with a binary velocity distribution in one dimension. We obtain an exact
solution for the density which reveals a universal phase diagram for the
asymptotic density decay. By universal we mean that all models in the class are
described by a single phase diagram spanned by two reduced parameters. The
phase diagram reveals four regimes, two of which contain the previously studied
cases of ballistic annihilation. The two new phases are a direct consequence of
the stochasticity. The solution is obtained through a matrix product approach
and builds on properties of a q-deformed harmonic oscillator algebra.Comment: 4 pages RevTeX, 3 figures; revised version with some corrections,
additional discussion and in RevTeX forma
Disorder and Funneling Effects on Exciton Migration in Tree-Like Dendrimers
The center-bound excitonic diffusion on dendrimers subjected to several types
of non-homogeneous funneling potentials, is considered. We first study the
mean-first passage time (MFPT) for diffusion in a linear potential with
different types of correlated and uncorrelated random perturbations. Increasing
the funneling force, there is a transition from a phase in which the MFPT grows
exponentially with the number of generations , to one in which it does so
linearly. Overall the disorder slows down the diffusion, but the effect is much
more pronounced in the exponential compared to the linear phase. When the
disorder gives rise to uncorrelated random forces there is, in addition, a
transition as the temperature is lowered. This is a transition from a
high- regime in which all paths contribute to the MFPT to a low- regime
in which only a few of them do. We further explore the funneling within a
realistic non-linear potential for extended dendrimers in which the dependence
of the lowest excitonic energy level on the segment length was derived using
the Time-Dependent Hatree-Fock approximation. Under this potential the MFPT
grows initially linearly with but crosses-over, beyond a molecular-specific
and -dependent optimal size, to an exponential increase. Finally we consider
geometrical disorder in the form of a small concentration of long connections
as in the {\it small world} model. Beyond a critical concentration of
connections the MFPT decreases significantly and it changes to a power-law or
to a logarithmic scaling with , depending on the strength of the funneling
force.Comment: 13 pages, 9 figure
Soluble two-species diffusion-limited Models in arbitrary dimensions
A class of two-species ({\it three-states}) bimolecular diffusion-limited
models of classical particles with hard-core reacting and diffusing in a
hypercubic lattice of arbitrary dimension is investigated. The manifolds on
which the equations of motion of the correlation functions close, are
determined explicitly. This property allows to solve for the density and the
two-point (two-time) correlation functions in arbitrary dimension for both, a
translation invariant class and another one where translation invariance is
broken. Systems with correlated as well as uncorrelated, yet random initial
states can also be treated exactly by this approach. We discuss the asymptotic
behavior of density and correlation functions in the various cases. The
dynamics studied is very rich.Comment: 28 pages, 0 figure. To appear in Physical Review E (February 2001
Optochemical Nanosensors and Subcellular Applications in Living Cells
 What may be the smallest anthropogenic devices to date, spherical sensors (wireless and fiberless) with radii as small as 10 nm have been produced. This class of optochemical PEBBLE (Probe Encapsulated By Biologically Localized Embedding) sensors covers a wide range of analytes (pH, calcium, oxygen and potassium included here) with excellent spatial, temporal and chemical resolution. Examples of such sensors for the monitoring of intracellular analytes are given. Methods, such as pico-injection, liposomal delivery and gene gun bombardment, are used to inject PEBBLE sensors into single cells. These PEBBLEs have caused minimal perturbation when delivered and operated inside single mammalian cells, such as human neuroblastoma, mouse oocytes or rat alveolar macrophage.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42448/1/604-131-1-2-121_91310121.pd
A Feasibility Study of Quantifying Longitudinal Brain Changes in Herpes Simplex Virus (HSV) Encephalitis Using Magnetic Resonance Imaging (MRI) and Stereology.
OBJECTIVES: To assess whether it is feasible to quantify acute change in temporal lobe volume and total oedema volumes in herpes simplex virus (HSV) encephalitis as a preliminary to a trial of corticosteroid therapy. METHODS: The study analysed serially acquired magnetic resonance images (MRI), of patients with acute HSV encephalitis who had neuroimaging repeated within four weeks of the first scan. We performed volumetric measurements of the left and right temporal lobes and of cerebral oedema visible on T2 weighted Fluid Attenuated Inversion Recovery (FLAIR) images using stereology in conjunction with point counting. RESULTS: Temporal lobe volumes increased on average by 1.6% (standard deviation (SD 11%) in five patients who had not received corticosteroid therapy and decreased in two patients who had received corticosteroids by 8.5%. FLAIR hyperintensity volumes increased by 9% in patients not receiving treatment with corticosteroids and decreased by 29% in the two patients that had received corticosteroids. CONCLUSIONS: This study has shown it is feasible to quantify acute change in temporal lobe and total oedema volumes in HSV encephalitis and suggests a potential resolution of swelling in response to corticosteroid therapy. These techniques could be used as part of a randomized control trial to investigate the efficacy of corticosteroids for treating HSV encephalitis in conjunction with assessing clinical outcomes and could be of potential value in helping to predict the clinical outcomes of patients with HSV encephalitis
Influence of auto-organization and fluctuation effects on the kinetics of a monomer-monomer catalytic scheme
We study analytically kinetics of an elementary bimolecular reaction scheme
of the Langmuir-Hinshelwood type taking place on a d-dimensional catalytic
substrate. We propose a general approach which takes into account explicitly
the influence of spatial correlations on the time evolution of particles mean
densities and allows for the analytical analysis. In terms of this approach we
recover some of known results concerning the time evolution of particles mean
densities and establish several new ones.Comment: Latex, 25 pages, one figure, submitted to J. Chem. Phy
Coupled Maps on Trees
We study coupled maps on a Cayley tree, with local (nearest-neighbor)
interactions, and with a variety of boundary conditions. The homogeneous state
(where every lattice site has the same value) and the node-synchronized state
(where sites of a given generation have the same value) are both shown to occur
for particular values of the parameters and coupling constants. We study the
stability of these states and their domains of attraction. As the number of
sites that become synchronized is much higher compared to that on a regular
lattice, control is easier to effect. A general procedure is given to deduce
the eigenvalue spectrum for these states. Perturbations of the synchronized
state lead to different spatio-temporal structures. We find that a mean-field
like treatment is valid on this (effectively infinite dimensional) lattice.Comment: latex file (25 pages), 4 figures included. To be published in Phys.
Rev.
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