242 research outputs found
Modeling the dynamics of a tracer particle in an elastic active gel
The internal dynamics of active gels, both in artificial (in-vitro) model
systems and inside the cytoskeleton of living cells, has been extensively
studied by experiments of recent years. These dynamics are probed using tracer
particles embedded in the network of biopolymers together with molecular
motors, and distinct non-thermal behavior is observed. We present a theoretical
model of the dynamics of a trapped active particle, which allows us to quantify
the deviations from equilibrium behavior, using both analytic and numerical
calculations. We map the different regimes of dynamics in this system, and
highlight the different manifestations of activity: breakdown of the virial
theorem and equipartition, different elasticity-dependent "effective
temperatures" and distinct non-Gaussian distributions. Our results shed light
on puzzling observations in active gel experiments, and provide physical
interpretation of existing observations, as well as predictions for future
studies.Comment: 11 pages, 6 figure
Activity driven fluctuations in living cells
We propose a model for the dynamics of a probe embedded in a living cell,
where both thermal fluctuations and nonequilibrium activity coexist. The model
is based on a confining harmonic potential describing the elastic cytoskeletal
matrix, which undergoes random active hops as a result of the nonequilibrium
rearrangements within the cell. We describe the probe's statistics and we bring
forth quantities affected by the nonequilibrium activity. We find an excellent
agreement between the predictions of our model and experimental results for
tracers inside living cells. Finally, we exploit our model to arrive at
quantitative predictions for the parameters characterizing nonequilibrium
activity, such as the typical time scale of the activity and the amplitude of
the active fluctuations.Comment: 6 pages, 4 figure
Spatial fluctuations at vertices of epithelial layers: quantification of regulation by Rho pathway
In living matter, shape fluctuations induced by acto-myosin are usually
studied in vitro via reconstituted gels, whose properties are controlled by
changing the concentrations of actin, myosin and cross-linkers. Such an
approach deliberately avoids to consider the complexity of biochemical
signaling inherent to living systems. Acto-myosin activity inside living cells
is mainly regulated by the Rho signaling pathway which is composed of multiple
layers of coupled activators and inhibitors. We investigate how such a pathway
controls the dynamics of confluent epithelial tissues by tracking the
displacements of the junction points between cells. Using a phenomenological
model to analyze the vertex fluctuations, we rationalize the effects of
different Rho signaling targets on the emergent tissue activity by quantifying
the effective diffusion coefficient, the persistence time and persistence
length of the fluctuations. Our results reveal an unanticipated correlation
between layers of activation/inhibition and spatial fluctuations within
tissues. Overall, this work connects the regulation via biochemical signaling
with mesoscopic spatial fluctuations, with potential application to the study
of structural rearrangements in epithelial tissues.Comment: 8 pages, 3 figure
Bcc He as a Coherent Quantum Solid
In this work we investigate implications of the quantum nature of bcc %
He. We show that it is a unique solid phase with both a lattice structure and
an Off-Diagonal Long Range Order of coherently oscillating local electric
dipole moments. These dipoles arise from the local motion of the atoms in the
crystal potential well, and oscillate in synchrony to reduce the dipolar
interaction energy. The dipolar ground-state is therefore found to be a
coherent state with a well defined global phase and a three-component complex
order parameter. The condensation energy of the dipoles in the bcc phase
stabilizes it over the hcp phase at finite temperatures. We further show that
there can be fermionic excitations of this ground-state and predict that they
form an optical-like branch in the (110) direction. A comparison with
'super-solid' models is also discussed.Comment: 12 pages, 8 figure
Force balance and membrane shedding at the Red Blood Cell surface
During the aging of the red-blood cell, or under conditions of extreme
echinocytosis, membrane is shed from the cell plasma membrane in the form of
nano-vesicles. We propose that this process is the result of the
self-adaptation of the membrane surface area to the elastic stress imposed by
the spectrin cytoskeleton, via the local buckling of membrane under increasing
cytoskeleton stiffness. This model introduces the concept of force balance as a
regulatory process at the cell membrane, and quantitatively reproduces the rate
of area loss in aging red-blood cells.Comment: 4 pages, 3 figure
Similarities between Insect Swarms and Isothermal Globular Clusters
Previous work has suggested that disordered swarms of flying insects can be well modeled as selfgravitating systems, as long as the “gravitational” interaction is adaptive. Motivated by this work we compare the predictions of the classic, mean-field King model for isothermal globular clusters to observations of insect swarms. Detailed numerical simulations of regular and adaptive gravity allow us to expose the features of the swarms’ density and velocity profiles that are due to longrange interactions, and are captured by the King model phenomenology, and those that are due to adaptivity and short-range repulsion. Our results provide further support for adaptive gravity as a model for swarms
Local modes, phonons, and mass transport in solid He
We propose a model to treat the local motion of atoms in solid He as a
local mode. In this model, the solid is assumed to be described by the Self
Consistent Harmonic approximation, combined with an array of local modes. We
show that in the bcc phase the atomic local motion is highly directional and
correlated, while in the hcp phase there is no such correlation. The correlated
motion in the bcc phase leads to a strong hybridization of the local modes with
the T phonon branch, which becomes much softer than that obtained
through a Self Consistent Harmonic calculation, in agreement with experiment.
In addition we predict a high energy excitation branch which is important for
self-diffusion. Both the hybridization and the presence of a high energy branch
are a consequence of the correlation, and appear only in the bcc phase. We
suggest that the local modes can play the role in mass transport usually
attributed to point defects (vacancies). Our approach offers a more overall
consistent picture than obtained using vacancies as the predominant point
defect. In particular, we show that our approach resolves the long standing
controversy regarding the contribution of point defects to the specific heat of
solid He.Comment: 10 pages, 10 figure
Performance of kevlar fibre-reinforced rubber composite armour against shaped-charge jet penetration
Ferromagnetism of He Films in the Low Field Limit
We provide evidence for a finite temperature ferromagnetic transition in
2-dimensions as in thin films of He on graphite, a model system
for the study of two-dimensional magnetism. We perform pulsed and CW NMR
experiments at fields of 0.03 - 0.48 mT on He at areal densities of 20.5 -
24.2 atoms/nm. At these densities, the second layer of He has a
strongly ferromagnetic tendency. With decreasing temperature, we find a rapid
onset of magnetization that becomes independent of the applied field at
temperatures in the vicinity of 1 mK. Both the dipolar field and the NMR
linewidth grow rapidly as well, which is consistent with a large (order unity)
polarization of the He spins.Comment: 4 figure
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