1,680 research outputs found
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
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
The chemical composition and nutritive value of Australian pulses
Grain legumes are the harvested seed of leguminous crops, typically peas, beans and their close relatives within the Fabaceae. Another term for many of these crops is pulses. In Australia, generally grain legumes are referred to as pulses. The term pulse is derived from the Latin puls meaning that the seed or grain can be made into a thick soup or pottage. The term pulse is most commonly associated with the food legumes whereas grain legumes are mostly associated with the feed industry. The terms are interchangeable, but with few exceptions (notably chickpeas, lentils and mung beans), the majority of the crops are used for animal feed. Both soybeans and peanuts are leguminous plants; however they are traditionally regarded as oilseed crops, and will not be discussed here.https://researchlibrary.agric.wa.gov.au/books/1020/thumbnail.jp
Progress report : 1996 / Cattle Industry Compensation Act Research Advisory Committee.
The Cattle Industry Compensation Fund was established under the Cattle Industry Compensation Aet 1965. Monies received on cattle sales by the Commissioner for State Taxation are paid into the Fund and matched by State Government appropriation.
Funds are held to compensate producers for losses incurred in disease eradication programs, to promote and encourage scientific research for improvement of cattle health and promotion and for purposes that, in the opinion of the Minister, will promote and encourage the cattle industry
An Advisory Committee, established by the Hon. Minister for Primary Industry makes recommendations for funding on a project basis. The Advisory Committee published progress reports in 1984 and 1989. This report covers the period 1990 to 1995
The mechanical response of semiflexible networks to localized perturbations
Previous research on semiflexible polymers including cytoskeletal networks in
cells has suggested the existence of distinct regimes of elastic response, in
which the strain field is either uniform (affine) or non-uniform (non-affine)
under external stress. Associated with these regimes, it has been further
suggested that a new fundamental length scale emerges, which characterizes the
scale for the crossover from non-affine to affine deformations. Here, we extend
these studies by probing the response to localized forces and force dipoles. We
show that the previously identified nonaffinity length [D.A. Head et al. PRE
68, 061907 (2003).] controls the mesoscopic response to point forces and the
crossover to continuum elastic behavior at large distances.Comment: 16 pages, 18 figures; substantial changes to text and figures to
clarify the crossover to continuum elasticity and the role of finite-size
effect
Actively Contracting Bundles of Polar Filaments
We introduce a phenomenological model to study the properties of bundles of
polar filaments which interact via active elements. The stability of the
homogeneous state, the attractors of the dynamics in the unstable regime and
the tensile stress generated in the bundle are discussed. We find that the
interaction of parallel filaments can induce unstable behavior and is
responsible for active contraction and tension in the bundle. Interaction
between antiparallel filaments leads to filament sorting. Our model could apply
to simple contractile structures in cells such as stress fibers.Comment: 4 pages, 4 figures, RevTex, to appear in Phys. Rev. Let
Bi-defects of Nematic Surfactant Bilayers
We consider the effects of the coupling between the orientational order of
the two monolayers in flat nematic bilayers. We show that the presence of a
topological defect on one bilayer generates a nontrivial orientational texture
on both monolayers. Therefore, one cannot consider isolated defects on one
monolayer, but rather associated pairs of defects on either monolayer, which we
call bi-defects. Bi-defects generally produce walls, such that the textures of
the two monolayers are identical outside the walls, and different in their
interior. We suggest some experimental conditions in which these structures
could be observed.Comment: RevTeX, 4 pages, 3 figure
Dynamics of viscoelastic membranes
We determine both the in-plane and out-of-plane dynamics of viscoelastic
membranes separating two viscous fluids in order to understand microrheological
studies of such membranes. We demonstrate the general viscoelastic signatures
in the dynamics of shear, bending, and compression modes. We also find a
screening of the otherwise two-dimensional character of the response to point
forces due to the presence of solvent. Finally, we show that there is a linear,
hydrodynamic coupling between the in-plane compression modes of the membrane
and the out-of-plane bending modes in the case where the membrane separates two
different fluids or environments
Two-point microrheology and the electrostatic analogy
The recent experiments of Crocker et al. suggest that microrheological
measurements obtained from the correlated fluctuations of widely-separatedprobe
particles determine the rheological properties of soft, complex materials more
accurately than do the more traditional particle autocorrelations. This
presents an interesting problem in viscoelastic dynamics. We develop an
important, simplifing analogy between the present viscoelastic problem and
classical electrostatics. Using this analogy and direct calculation we analyze
both the one and two particle correlations in a viscoelastic medium in order to
explain this observation
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