196 research outputs found
Linear response of a grafted semiflexible polymer to a uniform force field
We use the worm-like chain model to analytically calculate the linear
response of a grafted semiflexible polymer to a uniform force field. The result
is a function of the bending stiffness, the temperature, the total contour
length, and the orientation of the field with respect to that of the grafted
end. We also study the linear response of a worm-like chain with a periodic
alternating sequence of positive and negative charges. This can be considered
as a model for a polyampholyte with intrinsic bending siffness and negligible
intramolecular interactions. We show how the finite intrinsic persistence
length affects the linear response to the external field.Comment: 6 pages, 3 figure
A constrained random-force model for weakly bending semiflexible polymers
The random-force (Larkin) model of a directed elastic string subject to
quenched random forces in the transverse directions has been a paradigm in the
statistical physics of disordered systems. In this brief note, we investigate a
modified version of the above model where the total transverse force along the
polymer contour and the related total torque, in each realization of disorder,
vanish. We discuss the merits of adding these constraints and show that they
leave the qualitative behavior in the strong stretching regime unchanged, but
they reduce the effects of the random force by significant numerical
prefactors. We also show that a transverse random force effectively makes the
filament softer to compression by inducing undulations. We calculate the
related linear compression coefficient in both the usual and the constrained
random force model.Comment: 4 pages, 1 figure, accepted for publication in PR
L-MYC expression maintains self-renewal and prolongs multipotency of primary human neural stem cells
Weak point disorder in strongly fluctuating flux-line liquids
We consider the effect of weak uncorrelated quenched disorder (point defects)
on a strongly fluctuating flux-line liquid. We use a hydrodynamic model which
is based on mapping the flux-line system onto a quantum liquid of relativistic
charged bosons in 2+1 dimensions [P. Benetatos and M. C. Marchetti, Phys. Rev.
B 64, 054518, (2001)]. In this model, flux lines are allowed to be arbitrarily
curved and can even form closed loops. Point defects can be scalar or polar. In
the latter case, the direction of their dipole moments can be random or
correlated. Within the Gaussian approximation of our hydrodynamic model, we
calculate disorder-induced corrections to the correlation functions of the
flux-line fields and the elastic moduli of the flux-line liquid. We find that
scalar disorder enhances loop nucleation, and polar (magnetic) defects decrease
the tilt modulus.Comment: 15 pages, submitted to Pramana-Journal of Physics for the special
volume on Vortex State Studie
Repetitive concussive and subconcussive injury in a human tau mouse model results in chronic cognitive dysfunction and disruption of white matter tracts, but not tau pathology
Due to the unmet need for a means to study chronic traumatic encephalopathy (CTE) in vivo, there have been numerous efforts to develop an animal model of this progressive tauopathy. However, there is currently no consensus in the field on an injury model that consistently reproduces the neuropathological and behavioral features of CTE. We have implemented a repetitive Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA) injury paradigm in human transgenic (hTau) mice. Animals were subjected to daily subconcussive or concussive injuries for 20 days and tested acutely, 3 months, and 12 months post-injury for deficits in social behavior, anxiety, spatial learning and memory, and depressive behavior. Animals also were assessed for chronic tau pathology, astrogliosis, and white matter degeneration. Repetitive concussive injury caused acute deficits in Morris water maze performance, including reduced swimming speed and increased distance to the platform during visible and hidden platform phases that persisted during the subacute and chronic time-points following injury. We found evidence of white matter disruption in animals injured with subconcussive and concussive injuries, with the most severe disruption occurring in the repetitive concussive injury group. Severity of white matter disruption in the corpus callosum was moderately correlated with swimming speed, while white matter disruption in the fimbria showed weak but significant correlation with worse performance during probe trial. There was no evidence of tau pathology or astrogliosis in sham or injured animals. In summary, we show that repetitive brain injury produces persistent behavioral abnormalities as late as 1 year post-injury that may be related to chronic white matter disruption, although the relationship with CTE remains to be determined
Workflow for the Validation of Geomechanical Simulations through Seabed Monitoring for Offshore Underground Activities
Underground fluid storage is gaining increasing attention as a means to balance energy
production and consumption, ensure energy supply security, and contribute to greenhouse gas
reduction in the atmosphere by CO2 geological sequestration. However, underground fluid storage
generates pressure changes, which in turn induce stress variations and rock deformations. Numerical
geomechanical models are typically used to predict the response of a given storage to fluid injection
and withdrawal, but validation is required for such a model to be considered reliable. This paper
focuses on the technology and methodology that we developed to monitor seabed movements and
verify the predictions of the impact caused by offshore underground fluid storage. To this end, we put
together a measurement system, integrated into an Autonomous Underwater Vehicle, to periodically
monitor the seabed bathymetry. Measurements repeated during and after storage activities can be
compared with the outcome of numerical simulations and indirectly confirm the existence of safety
conditions. To simulate the storage system response to fluid storage, we applied the Virtual Element
Method. To illustrate and discuss our methodology, we present a possible application to a depleted
gas reservoir in the Adriatic Sea, Italy, where several underground geological formations could be
potentially converted into storage in the futur
Transverse fluctuations of grafted polymers
We study the statistical mechanics of grafted polymers of arbitrary stiffness
in a two-dimensional embedding space with Monte Carlo simulations. The
probability distribution function of the free end is found to be highly
anisotropic and non-Gaussian for typical semiflexible polymers. The reduced
distribution in the transverse direction, a Gaussian in the stiff and flexible
limits, shows a double peak structure at intermediate stiffnesses. We also
explore the response to a transverse force applied at the polymer free end. We
identify F-Actin as an ideal benchmark for the effects discussed.Comment: 10 pages, 4 figures, submitted to Physical Review
Variational theory of flux-line liquids
We formulate a variational (Hartree like) description of flux line liquids
which improves on the theory we developed in an earlier paper [A.M. Ettouhami,
Phys. Rev. B 65, 134504 (2002)]. We derive, in particular, how the massive term
confining the fluctuations of flux lines varies with temperature and show that
this term vanishes at high enough temperatures where the vortices behave as
freely fluctuating elastic lines.Comment: 10 pages, 1 postscript figur
Evidence for a Two-stage Melting Transition of the Vortex Matter in Bi2Sr2Ca1Cu2O8+d Single Crystals obtained by Muon Spin Rotation
From muon spin rotation measurements on under- to overdoped Bi-2212 crystals
we obtain evidence for a two-stage transition of the vortex matter as a
function of temperature. The first transition is well known and related to the
irreversibility line (IL). The second one is located below the IL and has not
been previously observed. It occurs for all three sets of crystals and is
unrelated to the vortex mobility. Our data are consistent with a two-stage
melting scenario where the intra-planar melting of the vortex lattice and the
inter-planar decoupling of the vortex lines occur independently.Comment: 9 pages and 3 figure
Soil deformation analysis through fluid-dynamic modelling and DInSAR measurements: a focus on groundwater withdrawal in the Ravenna area (Italy)
This study aims at assessing the deformation processes affecting an area NW of the
city of Ravenna (northern Italy), caused by groundwater withdrawal activities. In situ
data, geologic and structural maps, piezometric measurements, underground water
withdrawal volumes, and satellite C-band SAR data were used to jointly exploit two
different techniques: 1) fluid-dynamic and geomechanical modelling (by RSE S.p.A),
and 2) Differential Synthetic Aperture Radar Interferometry (DInSAR) analysis (by
CNR - IREA). The results of the comparative analysis presented in this work brought
new evidence about the contribution of groundwater withdrawal to the total subsidence
affecting the area during the 2000-2017 time interval. In particular, they show an
increase of the subsidence from year 2000 to 2010 and a decrease from year 2010 to
2017. These results are generally in line with groundwater withdrawal data that report
a reduction of the extracted water volumes during the considered temporal interval.
Meantime, they show a delay effect in the subsidence process, partially recovered
during the 2010-2017 thanks to a stabilisation of the extracted groundwater volumes.
The presented results shade new light on the groundwater withdrawal contribution to
the subsidence of the analysed zone, although further investigations are foreseen to
better clarify the ongoing scenario
- …