607 research outputs found
Nonlinear elasticity of composite networks of stiff biopolymers with flexible linkers
Motivated by recent experiments showing nonlinear elasticity of in vitro
networks of the biopolymer actin cross-linked with filamin, we present an
effective medium theory of flexibly cross-linked stiff polymer networks. We
model such networks by randomly oriented elastic rods connected by flexible
connectors to a surrounding elastic continuum, which self-consistently
represents the behavior of the rest of the network. This model yields a
crossover from a linear elastic regime to a highly nonlinear elastic regime
that stiffens in a way quantitatively consistent with experiment.Comment: 4 pages, 3 figure
Semiflexible polymers under external fields confined to two dimensions
The non-equilibrium structural and dynamical properties of semiflexible
polymers confined to two dimensions are investigated by molecular dynamics
simulations. Three different scenarios are considered: The force-extension
relation of tethered polymers, the relaxation of an initially stretched
semiflexible polymer, and semiflexible polymers under shear flow. We find
quantitative agreement with theoretical predictions for the force-extension
relation and the time dependence of the entropically contracting polymer. The
semiflexible polymers under shear flow exhibit significant conformational
changes at large shear rates, where less stiff polymers are extended by the
flow, whereas rather stiff polymers are contracted. In addition, the polymers
are aligned by the flow, thereby the two-dimensional semiflexible polymers
behave similarly to flexible polymers in three dimensions. The tumbling times
display a power-law dependence at high shear rate rates with an exponent
comparable to the one of flexible polymers in three-dimensional systems.Comment: Accepted for publication in J. Chem. Phy
Cooperativity and Frustration in Protein-Mediated Parallel Actin Bundles
We examine the mechanism of bundling of cytoskeletal actin filaments by two
representative bundling proteins, fascin and espin. Small-angle X-ray studies
show that increased binding from linkers drives a systematic \textit{overtwist}
of actin filaments from their native state, which occurs in a linker-dependent
fashion. Fascin bundles actin into a continuous spectrum of intermediate twist
states, while espin only allows for untwisted actin filaments and
fully-overtwisted bundles. Based on a coarse-grained, statistical model of
protein binding, we show that the interplay between binding geometry and the
intrinsic \textit{flexibility} of linkers mediates cooperative binding in the
bundle. We attribute the respective continuous/discontinous bundling mechanisms
of fascin/espin to differences in the stiffness of linker bonds themselves.Comment: 5 pages, 3 figures, figure file has been corrected in v
Force-Extension Relation and Plateau Modulus for Wormlike Chains
We derive the linear force-extension relation for a wormlike chain of
arbitrary stiffness including entropy elasticity, bending and thermodynamic
buckling. From this we infer the plateau modulus of an isotropic
entangled solution of wormlike chains. The entanglement length is
expressed in terms of the characteristic network parameters for three different
scaling regimes in the entangled phase. The entanglement transition and the
concentration dependence of are analyzed. Finally we compare our findings
with experimental data.Comment: 5 pages, 1 eps-figure, to appear in PR
Microrheology probes length scale dependent rheology
We exploit the power of microrheology to measure the viscoelasticity of entangled F-actin solutions at different length scales from 1 to 100 mu m over a wide frequency range. We compare the behavior of single probe-particle motion to that of the correlated motion of two particles. By varying the average length of the filaments, we identify fluctuations that dissipate diffusively over the filament length. These provide an important relaxation mechanism of the elasticity between 0.1 and 30 rad/sec
Microrheology, stress fluctuations and active behavior of living cells
We report the first measurements of the intrinsic strain fluctuations of
living cells using a recently-developed tracer correlation technique along with
a theoretical framework for interpreting such data in heterogeneous media with
non-thermal driving. The fluctuations' spatial and temporal correlations
indicate that the cytoskeleton can be treated as a course-grained continuum
with power-law rheology, driven by a spatially random stress tensor field.
Combined with recent cell rheology results, our data imply that intracellular
stress fluctuations have a nearly power spectrum, as expected for
a continuum with a slowly evolving internal prestress.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
Entanglement, elasticity and viscous relaxation of actin solutions
We have investigated the viscosity and the plateau modulus of actin solutions
with a magnetically driven rotating disc rheometer. For entangled solutions we
observed a scaling of the plateau modulus versus concentration with a power of
7/5. The measured terminal relaxation time increases with a power 3/2 as a
function of polymer length. We interpret the entanglement transition and the
scaling of the plateau modulus in terms of the tube model for semiflexible
polymers.Comment: 5 pages, 4 figures, published versio
Visualizing the strain field in semiflexible polymer networks: strain fluctuations and nonlinear rheology of F-actin gels
We image semi-flexible polymer networks under shear at the micrometer scale.
By tracking embedded probe particles, we determine the local strain field, and
directly measure its uniformity, or degree of affineness, on scales of 2-100
micron. The degree of nonaffine strain depends on polymer length and crosslink
density, consistent with theoretical predictions. We also find a direct
correspondence between the uniformity of the microscale strain and the
nonlinear elasticity of the networks in the bulk.Comment: 9 pages (double-spaced) of text, 4 figures + 1 supplementary figur
Dynamics of folding in Semiflexible filaments
We investigate the dynamics of a single semiflexible filament, under the
action of a compressing force, using numerical simulations and scaling
arguments. The force is applied along the end to end vector at one extremity of
the filament, while the other end is held fixed. We find that, unlike in
elastic rods the filament folds asymmetrically with a folding length which
depends only on the bending stiffness and the applied force. It is shown that
this behavior can be attributed to the exponentially falling tension profile in
the filament. While the folding time depends on the initial configuration, at
late time, the distance moved by the terminal point of the filament and the
length of the fold shows a power law dependence on time with an exponent 1/2.Comment: 13 pages, Late
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