1,894 research outputs found
Budding and Domain Shape Transformations in Mixed Lipid Films and Bilayer Membranes
We study the stability and shapes of domains with spontaneous curvature in
fluid films and membranes, embedded in a surrounding membrane with zero
spontaneous curvature. These domains can result from the inclusion of an
impurity in a fluid membrane, or from phase separation within the membrane. We
show that for small but finite line and surface tensions and for finite
spontaneous curvatures, an equilibrium phase of protruding circular domains is
obtained at low impurity concentrations. At higher concentrations, we predict a
transition from circular domains, or "caplets", to stripes. In both cases, we
calculate the shapes of these domains within the Monge representation for the
membrane shape. With increasing line tension, we show numerically that there is
a budding transformation from stable protruding circular domains to spherical
buds. We calculate the full phase diagram, and demonstrate a two triple points,
of respectively bud-flat-caplet and flat-stripe-caplet coexistence.Comment: 14 pages, to appear in Phys Rev
A symmetrical method to obtain shear moduli from microrheology
Passive microrheology typically deduces shear elastic loss and storage moduli
from displacement time series or mean-squared displacement (MSD) of thermally
fluctuating probe particles in equilibrium materials. Common data analysis
methods use either Kramers-Kronig (KK) transformations or functional fitting to
calculate frequency-dependent loss and storage moduli. We propose a new
analysis method for passive microrheology that avoids the limitations of both
of these approaches. In this method, we determine both real and imaginary
components of the complex, frequency-dependent response function as direct integral
transforms of the MSD of thermal particle motion. This procedure significantly
improves the high-frequency fidelity of relative to the use of
KK transformation, which has been shown to lead to artifacts in
. We test our method on both model data and experimental
data. Experiments were performed on solutions of worm-like micelles and dilute
collagen solutions. While the present method agrees well with established
KK-based methods at low frequencies, we demonstrate significant improvement at
high frequencies using our symmetric analysis method, up to almost the
fundamental Nyquist limit.Comment: 8 pages, 4 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
Stress relaxation in F-actin solutions by severing
Networks of filamentous actin (F-actin) are important for the mechanics of
most animal cells. These cytoskeletal networks are highly dynamic, with a
variety of actin-associated proteins that control cross-linking, polymerization
and force generation in the cytoskeleton. Inspired by recent rheological
experiments on reconstituted solutions of dynamic actin filaments, we report a
theoretical model that describes stress relaxation behavior of these solutions
in the presence of severing proteins. We show that depending on the kinetic
rates of assembly, disassembly, and severing, one can observe both
length-dependent and length-independent relaxation behavior
Inspiration for the Future: The Role of Inspiratory Muscle Training in Cystic Fibrosis.
Cystic fibrosis (CF) is an inherited, multi-system, life-limiting disease characterized by a progressive decline in lung function, which accounts for the majority of CF-related morbidity and mortality. Inspiratory muscle training (IMT) has been proposed as a rehabilitative strategy to treat respiratory impairments associated with CF. However, despite evidence of therapeutic benefits in healthy and other clinical populations, the routine application of IMT in CF can neither be supported nor refuted due to the paucity of methodologically rigorous research. Specifically, the interpretation of available studies regarding the efficacy of IMT in CF is hampered by methodological threats to internal and external validity. As such, it is important to highlight the inherent risk of bias that differences in patient characteristics, IMT protocols, and outcome measurements present when synthesizing this literature prior to making final clinical judgments. Future studies are required to identify the characteristics of individuals who may respond to IMT and determine whether the controlled application of IMT can elicit meaningful improvements in physiological and patient-centered clinical outcomes. Given the equivocal evidence regarding its efficacy, IMT should be utilized on a case-by-case basis with sound clinical reasoning, rather than simply dismissed, until a rigorous evidence-based consensus has been reached
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Structural features and nonlinear rheology of self-assembled networks of crosslinked semiflexible polymers
Random networks of semiflexible filaments are common support structures in
biology. Familiar examples include the fibrous matrices in blood clots,
bacterial biofilms, and essential components of the cells and tissues of
plants, animals, and fungi. Despite the ubiquity of these networks in
biomaterials, we have only a limited understanding of the relationship between
their structural features and highly strain-sensitive mechanical properties. In
this work, we perform simulations of three-dimensional networks produced by the
irreversible formation of crosslinks between linker-decorated semiflexible
filaments. We characterize the structure of networks formed by a simple
diffusion-dependent assembly process and measure their associated steady-state
rheological features at finite temperature over a range of applied prestrains
encompassing the strain-stiffening transition. We quantify the dependence of
network connectivity on crosslinker availability and detail the associated
connectivity dependence of both linear elasticity and nonlinear strain
stiffening behavior, drawing comparisons with prior experimental measurements
of the crosslinker concentration-dependent elasticity of actin gels
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
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