19,105 research outputs found
Linear response of a grafted semiflexible polymer to a uniform force field
We use the worm-like chain model to analytically calculate the linear
response of a grafted semiflexible polymer to a uniform force field. The result
is a function of the bending stiffness, the temperature, the total contour
length, and the orientation of the field with respect to that of the grafted
end. We also study the linear response of a worm-like chain with a periodic
alternating sequence of positive and negative charges. This can be considered
as a model for a polyampholyte with intrinsic bending siffness and negligible
intramolecular interactions. We show how the finite intrinsic persistence
length affects the linear response to the external field.Comment: 6 pages, 3 figure
Step Position Distributions and the Pairwise Einstein Model for Steps on Crystal Surfaces
The Pairwise Einstein Model (PEM) of steps not only justifies the use of the
Generalized Wigner Distribution (GWD) for Terrace Width Distributions (TWDs),
it also predicts a specific form for the Step Position Distribution (SPD),
i.e., the probability density function for the fluctuations of a step about its
average position. The predicted form of the SPD is well approximated by a
Gaussian with a finite variance. However, the variance of the SPD measured from
either real surfaces or Monte Carlo simulations depends on , the
length of step over which it is calculated, with the measured variance
diverging in the limit . As a result, a length scale
can be defined as the value of at which the measured and
theoretical SPDs agree. Monte Carlo simulations of the terrace-step-kink model
indicate that , where is the correlation
length in the direction parallel to the steps, independent of the strength of
the step-step repulsion. can also be understood as the length over
which a {\em single} terrace must be sampled for the TWD to bear a "reasonable"
resemblence to the GWD.Comment: 4 pages, 3 figure
A two-state model for helicase translocation and unwinding of nucleic acids
Helicases are molecular motors that unwind double-stranded nucleic acids
(dsNA), such as DNA and RNA). Typically a helicase translocates along one of
the NA single strands while unwinding and uses adenosine triphosphate (ATP)
hydrolysis as an energy source. Here we model of a helicase motor that can
switch between two states, which could represent two different points in the
ATP hydrolysis cycle. Our model is an extension of the earlier
Betterton-J\"ulicher model of helicases to incorporate switching between two
states. The main predictions of the model are the speed of unwinding of the
dsNA and fluctuations around the average unwinding velocity. Motivated by a
recent claim that the NS3 helicase of Hepatitis C virus follows a flashing
ratchet mechanism, we have compared the experimental results for the NS3
helicase with a special limit of our model which corresponds to the flashing
ratchet scenario. Our model accounts for one key feature of the experimental
data on NS3 helicase. However, contradictory observations in experiments
carried out under different conditions limit the ability to compare the model
to experiments.Comment: minor modification
Inertial sensor-based knee flexion/extension angle estimation
A new method for estimating knee joint flexion/extension angles from segment acceleration and angular velocity data is described. The approach uses a combination of Kalman filters and biomechanical constraints based on anatomical knowledge. In contrast to many recently published methods, the proposed approach does not make use of the earthâs magnetic field and hence is insensitive to the complex field distortions commonly found in modern buildings. The method was validated experimentally by calculating knee angle from measurements taken from two IMUs placed on adjacent body segments. In contrast to many previous studies which have validated their approach during relatively slow activities or over short durations, the performance of the algorithm was evaluated during both walking and running over 5 minute periods. Seven healthy subjects were tested at various speeds from 1 to 5 miles/hour. Errors were estimated by comparing the results against data obtained simultaneously from a 10 camera motion tracking system (Qualysis). The average measurement error ranged from 0.7 degrees for slow walking (1 mph) to 3.4 degrees for running (5mph). The joint constraint used in the IMU analysis was derived from the Qualysis data. Limitations of the method, its clinical application and its possible extension are discussed
Complementarity and Scientific Rationality
Bohr's interpretation of quantum mechanics has been criticized as incoherent
and opportunistic, and based on doubtful philosophical premises. If so Bohr's
influence, in the pre-war period of 1927-1939, is the harder to explain, and
the acceptance of his approach to quantum mechanics over de Broglie's had no
reasonable foundation. But Bohr's interpretation changed little from the time
of its first appearance, and stood independent of any philosophical
presuppositions. The principle of complementarity is itself best read as a
conjecture of unusually wide scope, on the nature and future course of
explanations in the sciences (and not only the physical sciences). If it must
be judged a failure today, it is not because of any internal inconsistency.Comment: 29 page
Friction and wear of plasma-deposited diamond films
Reciprocating sliding friction experiments in humid air and in dry nitrogen and unidirectional sliding friction experiments in ultrahigh vacuum were conducted with a natural diamond pin in contact with microwave-plasma-deposited diamond films. Diamond films with a surface roughness (R rms) ranging from 15 to 160 nm were produced by microwave-plasma-assisted chemical vapor deposition. In humid air and in dry nitrogen, abrasion occurred when the diamond pin made grooves in the surfaces of diamond films, and thus the initial coefficients of friction increased with increasing initial surface roughness. The equilibrium coefficients of friction were independent of the initial surface roughness of the diamond films. In vacuum the friction for diamond films contacting a diamond pin arose primarily from adhesion between the sliding surfaces. In these cases, the initial and equilibrium coefficients of friction were independent of the initial surface roughness of the diamond films. The equilibrium coefficients of friction were 0.02 to 0.04 in humid air and in dry nitrogen, but 1.5 to 1.8 in vacuum. The wear factor of the diamond films depended on the initial surface roughness, regardless of environment; it increased with increasing initial surface roughness. The wear factors were considerably higher in vacuum than in humid air and in dry nitrogen
Light hadron spectra and wave functions in quenched QCD with overlap quarks on a large lattice
A simulation of quenched QCD with the overlap Dirac operator has been
completed using 100 Wilson gauge configurations at beta=6 on an 18^3x64
lattice. We present results for meson and baryon masses, meson final state
"wave functions'' and other observables.Comment: 5 LaTeX pages (espcrc2.sty), 13 figures. Combined contributions by
J.H., L.L. and C.R. at Lattice2004(spectrum), Fermilab, June 21-26, 200
Spreading of thin films assisted by thermal fluctuations
We study the spreading of viscous drops on a solid substrate, taking into
account the effects of thermal fluctuations in the fluid momentum. A nonlinear
stochastic lubrication equation is derived, and studied using numerical
simulations and scaling analysis. We show that asymptotically spreading drops
admit self-similar shapes, whose average radii can increase at rates much
faster than these predicted by Tanner's law. We discuss the physical
realizability of our results for thin molecular and complex fluid films, and
predict that such phenomenon can in principal be observed in various flow
geometries.Comment: 5 pages, 3 figure
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