1,934 research outputs found
Determining Microscopic Viscoelasticity in Flexible and Semiflexible Polymer Networks from Thermal Fluctuations
We have developed a new technique to measure viscoelasticity in soft
materials such as polymer solutions, by monitoring thermal fluctuations of
embedded probe particles using laser interferometry in a microscope.
Interferometry allows us to obtain power spectra of fluctuating beads from 0.1
Hz to 20 kHz, and with sub-nanometer spatial resolution. Using linear response
theory, we determined the frequency-dependent loss and storage shear moduli up
to frequencies on the order of a kHz. Our technique measures local values of
the viscoelastic response, without actively straining the system, and is
especially suited to soft biopolymer networks. We studied semiflexible F-actin
solutions and, as a control, flexible polyacrylamide (PAAm) gels, the latter
close to their gelation threshold. With small particles, we could probe the
transition from macroscopic viscoelasticity to more complex microscopic
dynamics. In the macroscopic limit we find shear moduli at 0.1 Hz of G'=0.11
+/- 0.03 Pa and 0.17 +/- 0.07 Pa for 1 and 2 mg/ml actin solutions, close to
the onset of the elastic plateau, and scaling behavior consistent with G(omega)
as omega^(3/4) at higher frequencies. For polyacrylamide we measured plateau
moduli of 2.0, 24, 100 and 280 Pa for crosslinked gels of 2, 2.5, 3 and 5%
concentration (weight/volume) respectively, in agreement to within a factor of
two with values obtained from conventional rheology. We also found evidence for
scaling of G(omega) as \omega^(1/2), consistent with the predictions of the
Rouse model for flexible polymers.Comment: 16 pages, with 15 PostScript figures (to be published in
Macromolecules
Non-equilibrium mechanics and dynamics of motor activated gels
The mechanics of cells is strongly affected by molecular motors that generate
forces in the cellular cytoskeleton. We develop a model for cytoskeletal
networks driven out of equilibrium by molecular motors exerting transient
contractile stresses. Using this model we show how motor activity can
dramatically increase the network's bulk elastic moduli. We also show how motor
binding kinetics naturally leads to enhanced low-frequency stress fluctuations
that result in non-equilibrium diffusive motion within an elastic network, as
seen in recent \emph{in vitro} and \emph{in vivo} experiments.Comment: 21 pages, 8 figure
Poisson's ratio in composite elastic media with rigid rods
We study the elastic response of composites of rods embedded in elastic
media. We calculate the micro-mechanical response functions, and bulk elastic
constants as functions of rod density. We find two fixed points for Poisson's
ratio with respect to the addition of rods in 3D composites: there is an
unstable fixed point for Poisson's ratio=1/2 (an incompressible system) and a
stable fixed point for Poisson's ratio=1/4 (a compressible system). We also
derive an approximate expression for the elastic constants for arbitrary rod
density that yields exact results for both low and high density. These results
may help to explain recent experiments [Physical Review Letters 102, 188303
(2009)] that reported compressibility for composites of microtubules in F-actin
networks.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Mechanics and force transmission in soft composites of rods in elastic gels
We report detailed theoretical investigations of the micro-mechanics and bulk
elastic properties of composites consisting of randomly distributed stiff
fibers embedded in an elastic matrix in two and three dimensions. Recent
experiments published in Physical Review Letters [102, 188303 (2009)] have
suggested that the inclusion of stiff microtubules in a softer, nearly
incompressible biopolymer matrix can lead to emergent compressibility. This can
be understood in terms of the enhancement of the compressibility of the
composite relative to its shear compliance as a result of the addition of stiff
rod-like inclusions. We show that the Poisson's ratio of such a composite
evolves with increasing rod density towards a particular value, or {\em fixed
point}, independent of the material properties of the matrix, so long as it has
a finite initial compressibility. This fixed point is in three
dimensions and in two dimensions. Our results suggest an important
role for stiff filaments such as microtubules and stress fibers in cell
mechanics. At the same time, our work has a wider elasticity context, with
potential applications to composite elastic media with a wide separation of
scales in stiffness of its constituents such as carbon nanotube-polymer
composites, which have been shown to have highly tunable mechanics.Comment: 10 pages, 8 figure
On-site residence time in a driven diffusive system: violation and recovery of mean-field
We investigate simple one-dimensional driven diffusive systems with open
boundaries. We are interested in the average on-site residence time defined as
the time a particle spends on a given site before moving on to the next site.
Using mean-field theory, we obtain an analytical expression for the on-site
residence times. By comparing the analytic predictions with numerics, we
demonstrate that the mean-field significantly underestimates the residence time
due to the neglect of time correlations in the local density of particles. The
temporal correlations are particularly long-lived near the average shock
position, where the density changes abruptly from low to high. By using Domain
wall theory (DWT), we obtain highly accurate estimates of the residence time
for different boundary conditions. We apply our analytical approach to
residence times in a totally asymmetric exclusion process (TASEP), TASEP
coupled to Langmuir kinetics (TASEP + LK), and TASEP coupled to mutually
interactive LK (TASEP + MILK). The high accuracy of our predictions is verified
by comparing these with detailed Monte Carlo simulations
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Cognitive Bias Modification Training & Exercise: For Alleviating Depression, Anxiety & Stress Related Disorders
Spin 1 inversion: a Majorana tensor force for deuteron alpha scattering
We demonstrate, for the first time, successful S-matrix to potential
inversion for spin one projectiles with non-diagonal yielding a
interaction. The method is a generalization of the
iterative-perturbative, IP, method. We present a test case indicating the
degree of uniqueness of the potential. The method is adapted, using established
procedures, into direct observable to potential inversion, fitting ,
, , and for d + alpha scattering over
a range of energies near 10 MeV. The interaction which we find is
very different from that proposed elsewhere, both real and imaginary parts
being very different for odd and even parity channels.Comment: 7 pages Revtex, 4 ps figure
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Two Session Cognitive Bias Modification Training; Exercise Interpretation Bias
Cognitive bias modification is the direct manipulation of a target bias by extensive exposure to task contingencies that encourage predetermined patterns of processing selectivity1,2. Forty-eight participants from a general population sample recruited for a two-part study investigating the effect of two sessions of exercise orientated Positive CBM-I Training, in comparison to a Neutral Pseudo Training condition. Baseline, post session one and post session two self-report measures of Anxiety, Depression and Stress were collected. Measures of interpretation bias were collected at baseline and post session two, both with and without cognitive load. Results suggest that for individuals undergoing two sessions of Positive CBM-I Training over a fortnight had significantly decreased Trait Anxiety scores, relative to the Pseudo Neutral Training condition. The findings hold promise for the Cognitive Bias Modification paradigm for decreasing symptoms of Anxiety
Cognitive Interpretation Bias: The Effect of a Single Session Moderate Exercise Protocol on Anxiety and Depression
Research conducted within the Cognitive Bias Modification (CBM) paradigm has revealed that cognitive biases such as negative cognitive interpretation biases contribute to mental health disorders such as anxiety (Beard, 2011). It has been shown that exercise reduces anxiety (Ensari, Greenlee, Motl, & Petruzzello, 2015). Exercise has also been found to reduce negative cognitive attention biases (Tian & Smith, 2011), however no research to date has investigated the effect of exercise on cognitive interpretation bias. The key aims of the current project is to investigate whether moderate exercise reduces self-reported symptoms of depression and stress. Additionally, to establish which intensity of exercise is required to achieve anxiety reduction and reduce an individual’s negative cognitive interpretation biases. Study one recruited a healthy sample of adult participants who were randomly assigned to one of two conditions: a walking exercise protocol or a control condition (n=2x12). Participants completed anxiety and cognitive interpretation bias measures before and after the walking exercise or control condition. Those in the walking exercise condition presented less symptoms of trait anxiety on a measure of state and trait anxiety inventory (STAI), compared to controls relative to baseline measures following the intervention. Study two recruited frequent exercisers who were assigned to an exercise or control group (n=2x24). Participants completed anxiety, depression, psychological stress and cognitive interpretation bias measures before and after the exercise or control condition. Following the intervention, negative interpretation biases decreased in the exercise group and stayed stable in the control group. The exercise group also had significantly decreased anxiety, depression and stress measures after the exercise condition, whilst controls did not. The research concludes that CBM holds promise for the management of mood disorders and exercise is an effective accompaniment to psychotherapy
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