463 research outputs found
Cooperative molecular motors moving back and forth
We use a two-state ratchet model to study the cooperative bidirectional
motion of molecular motors on cytoskeletal tracks with randomly alternating
polarities. Our model is based on a previously proposed model [Badoual et al.,
{\em Proc. Natl. Acad. Sci. USA} {\bf 99}, 6696 (2002)] for collective motor
dynamics and, in addition, takes into account the cooperativity effect arising
from the elastic tension that develops in the cytoskeletal track due to the
joint action of the walking motors. We show, both computationally and
analytically, that this additional cooperativity effect leads to a dramatic
reduction in the characteristic reversal time of the bidirectional motion,
especially in systems with a large number of motors. We also find that
bidirectional motion takes place only on (almost) a-polar tracks, while on even
slightly polar tracks the motion is unidirectional. We argue that the origin of
these observations is the sensitive dependence of the cooperative dynamics on
the difference between the number of motors typically working in and against
the instantaneous direction of motion.Comment: Accepted for publication in Phys. Rev.
The Developmental Course of Hyperactive-Impulsive and Inattentive (HIA) Behavior in a Community Sample of Girls: A Group-Based Model of Development from Kindergarten to Fifth Grade
The goal of the present study was to extend our current understanding of the female-specific manifestations, developmental course, and outcomes of hyperactive-impulsive and inattentive (HIA) behaviors. This study modeled the developmental trajectories of hyperactive-impulsive and inattentive behavior in a population-based study of girls followed from kindergarten to fifth grade. Four trajectories of both hyperactive-impulsive and inattentive behavior were identified. Joint trajectory analyses revealed an association between the highest trajectories of hyperactive-impulsive behavior and inattentive behavior. Similarly, the lowest level trajectories of both hyperactive-impulsive and inattentive behaviors showed a high degree of association. Contrary to expectations and descriptions of ADHD in girls, the number of girls assigned to both the highest trajectories of inattentive behavior and the lowest trajectories of hyperactive-impulsive behavior was extremely low. Teacher- and self-reported measures of child functioning varied by trajectory group membership across all models, suggesting that sub-clinical levels of HIA behavior are associated with adjustment difficulties in girls
Bidirectional cooperative motion of myosin-II motors on actin tracks with randomly alternating polarities
The cooperative action of many molecular motors is essential for dynamic
processes such as cell motility and mitosis. This action can be studied by
using motility assays in which the motion of cytoskeletal filaments over a
surface coated with motor proteins is tracked. In previous studies of
actin-myosin II systems, fast directional motion was observed, reflecting the
tendency of myosin II motors to propagate unidirectionally along actin
filaments. Here, we present a motility assay with actin bundles consisting of
short filamentous segments with randomly alternating polarities. These actin
tracks exhibit bidirectional motion with macroscopically large time intervals
(of the order of several seconds) between direction reversals. Analysis of this
bidirectional motion reveals that the characteristic reversal time,
, does not depend on the size of the moving bundle or on the number
of motors, . This observation contradicts previous theoretical calculations
based on a two-state ratchet model [Badoual et al., Proc. Natl. Acad. Sci. USA,
vol. 99, p. 6696 (2002)], predicting an exponential increase of
with . We present a modified version of this model that takes into account
the elastic energy due to the stretching of the actin track by the myosin II
motors. The new model yields a very good quantitative agreement with the
experimental results.Comment: A slightly revised version. Figures 2 and 7 were modified. Accepted
for publication in "Soft Matter
Uniformity of thickness of metal sheets by patchwork blanks: potential of adhesive bonding
The sheet metal forming operations generally involve the production of parts characterized by a non-uniform thickness distribution. However, in some cases, a product characterized by a distribution of thicknesses that is as uniform as possible may be desirable. This result can be obtained by using multiphase processes or by subtraction or addition of material from the blank. In this work, which deals with the method for adding material, an innovative methodology has been proposed as an alternative to the welding process. Specifically, the methodology is based on the bonding of a patch (before the deformation process), on the base plate with a constant thickness, in the area that most suffers from the thinning caused by the forming process. In this way, it was possible to influence the deformation of the patchwork blank and its thicknesses distribution. Through finite element analysis, it was possible to study the formability of a patchwork blank by varying the thickness and size of the patch, in order to produce an axially symmetric component by stretching through a hemispherical punch. Preliminary experimental tests demonstrated the reliability of the bonding and the potential of this method to uniform the final thickness of the sheet
Friction influence on the AA6060 aluminium alloy formability
Using a finite element calculation code, this work analyses the influence of friction during a stamping test conducted on the AA6060 aluminium-based alloy. The study focuses on phenomena happening when the sheet necking appears. This condition, based on the Hill's localized necking theory and the Swift's diffuse necking theory, is dependent on the material hardening index. This work shows that the punch stroke at the necking condition point is maximum when the main strain measured on the sheet surface are unbalanced and close to a balanced biaxial tension condition
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High-Speed Curtain Recoating for Stereolithography
The University of Leuven uses a liquid curtain recoating system for resin deposition in
stereolithography. This system deposits new liquid layers of photo-polymer by means of a liquid
curtain travelling over the build vat. Experiments have been carried out to increase the speed of the
liquid curtain while depositing a layer. Speeds up to 1.2 m/s, and accelerations up to 1 g have been
tested successfully, meaning that it is possible to coat high-quality layers of 75 µm thickness with
this recoating technique. However, the curtain restores too slowly after acceleration. This paper
discusses possible reasons and tries to formulate adequate solutions. Possible solutions consist in
controlling small pressure differences in the curtain’s neighbourhood. A solution to this problem
is necessary, as to make the travelling length of the curtain, and so the machine length acceptable
with respect to the dimensions of the build vat.Mechanical Engineerin
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