3,199 research outputs found
Xylem plasticity in Pinus pinaster and Quercus ilex growing at sites with different water availability in the Mediterranean region: relations between Intra-Annual Density Fluctuations and environmental conditions.
Fluctuations in climatic conditions during the growing season are recorded in Mediterranean tree-rings and often result in intra-annual density fluctuations (IADFs). Dendroecology and quantitative wood anatomy analyses were used to characterize the relations between the variability of IADF traits and climatic drivers in Pinus pinaster Aiton and Quercus ilex L. growing at sites with different water availability on the Elba island in Central Italy. Our results showed that both species present high xylem plasticity resulting in the formation of L-type IADFs (L-IADFs), consisting of earlywood-like cells in latewood. The occurrence of such IADFs was linked to rain events following periods of summer drought. The formation of L-IADFs in both species increased the hydraulic conductivity late in the growing season, due to their larger lumen area in comparison to "true latewood". The two species expressed greater similarity under arid conditions, as unfavorable climates constrained trait variation. Wood density, measured as the percentage of cell walls over total xylem area, IADF frequency, as well as conduit lumen area and vessel frequency, specifically in the hardwood species, proved to be efficient proxies to encode climate signals recorded in the xylem. The response of these anatomical traits to climatic variations was found to be species- and site-specific
Competing mechanisms of stress-assisted diffusivity and stretch-activated currents in cardiac electromechanics
We numerically investigate the role of mechanical stress in modifying the
conductivity properties of the cardiac tissue and its impact in computational
models for cardiac electromechanics. We follow a theoretical framework recently
proposed in [Cherubini, Filippi, Gizzi, Ruiz-Baier, JTB 2017], in the context
of general reaction-diffusion-mechanics systems using multiphysics continuum
mechanics and finite elasticity. In the present study, the adapted models are
compared against preliminary experimental data of pig right ventricle
fluorescence optical mapping. These data contribute to the characterization of
the observed inhomogeneity and anisotropy properties that result from
mechanical deformation. Our novel approach simultaneously incorporates two
mechanisms for mechano-electric feedback (MEF): stretch-activated currents
(SAC) and stress-assisted diffusion (SAD); and we also identify their influence
into the nonlinear spatiotemporal dynamics. It is found that i) only specific
combinations of the two MEF effects allow proper conduction velocity
measurement; ii) expected heterogeneities and anisotropies are obtained via the
novel stress-assisted diffusion mechanisms; iii) spiral wave meandering and
drifting is highly mediated by the applied mechanical loading. We provide an
analysis of the intrinsic structure of the nonlinear coupling using
computational tests, conducted using a finite element method. In particular, we
compare static and dynamic deformation regimes in the onset of cardiac
arrhythmias and address other potential biomedical applications
Electrocardiogram of the Mixmaster Universe
The Mixmaster dynamics is revisited in a new light as revealing a series of
transitions in the complex scale invariant scalar invariant of the Weyl
curvature tensor best represented by the speciality index , which
gives a 4-dimensional measure of the evolution of the spacetime independent of
all the 3-dimensional gauge-dependent variables except for the time used to
parametrize it. Its graph versus time characterized by correlated isolated
pulses in its real and imaginary parts corresponding to curvature wall
collisions serves as a sort of electrocardiogram of the Mixmaster universe,
with each such pulse pair arising from a single circuit or ``complex pulse''
around the origin in the complex plane. These pulses in the speciality index
and their limiting points on the real axis seem to invariantly characterize
some of the so called spike solutions in inhomogeneous cosmology and should
play an important role as a gauge invariant lens through which to view current
investigations of inhomogeneous Mixmaster dynamics.Comment: version 3: 20 pages iopart style, 19 eps figure files for 8 latex
figures; added example of a transient true spike to contrast with the
permanent true spike example from the Lim family of true spike solutions;
remarks in introduction and conclusion adjusted and toned down; minor
adjustments to the remaining tex
Neutrons from Piezonuclear Reactions
We report the results obtained by cavitating water solutions of iron salts
(iron chloride and iron nitrate) with different concentrations at different
ultrasound powers. In all cases we detected a neutron radiation well higher
than the background level. The neutron production is perfectly reproducible and
can at some extent be controlled. These evidences for neutron emission
generated by cavitation support some preliminary clues for the possibility of
piezonuclear reactions (namely nuclear reactions induced by pressure and shock
waves) obtained in the last ten years. We have been able for the first time to
state some basic features of such a neutron emission induced by cavitation,
namely: 1) a marked threshold behavior in power, energy and time; 2) its
occurring without a concomitant production of gamma radiation.Comment: 8 figures; we added some more important references; we replaced some
figures with more detailed ones; we added more comprehensive details which
could not be desclosed before as part of private patents which have been
published no
Amalgams of finite inverse semigroups
reserved3We show that the word problem is decidable for an amalgamated free product of finite inverse
semigroups (in the category of inverse semigroups). This is in contrast to a recent result of M. Sapir
that shows that the word problem for amalgamated free products of finite semigroups (in the category
of semigroups) is in general undecidable.A. Cherubini; J. Meakin; B. PiochiCherubini, Alessandra; J., Meakin; B., Pioch
Nonlinear diffusion & thermo-electric coupling in a two-variable model of cardiac action potential
This work reports the results of the theoretical investigation of nonlinear
dynamics and spiral wave breakup in a generalized two-variable model of cardiac
action potential accounting for thermo-electric coupling and diffusion
nonlinearities. As customary in excitable media, the common Q10 and Moore
factors are used to describe thermo-electric feedback in a 10-degrees range.
Motivated by the porous nature of the cardiac tissue, in this study we also
propose a nonlinear Fickian flux formulated by Taylor expanding the voltage
dependent diffusion coefficient up to quadratic terms. A fine tuning of the
diffusive parameters is performed a priori to match the conduction velocity of
the equivalent cable model. The resulting combined effects are then studied by
numerically simulating different stimulation protocols on a one-dimensional
cable. Model features are compared in terms of action potential morphology,
restitution curves, frequency spectra and spatio-temporal phase differences.
Two-dimensional long-run simulations are finally performed to characterize
spiral breakup during sustained fibrillation at different thermal states.
Temperature and nonlinear diffusion effects are found to impact the
repolarization phase of the action potential wave with non-monotone patterns
and to increase the propensity of arrhythmogenesis
pair creation by vacuum polarization around electromagnetic black holes
The concept of "dyadotorus" was recently introduced to identify in the
Kerr-Newman geometry the region where vacuum polarization processes may occur,
leading to the creation of pairs. This concept generalizes the
original concept of "dyadosphere" initially introduced for
Reissner-Nordstr\"{o}m geometries. The topology of the axially symmetric
dyadotorus is studied for selected values of the electric field and its
electromagnetic energy is estimated by using three different methods all giving
the same result. It is shown by a specific example the difference between a
dyadotorus and a dyadosphere. The comparison is made for a Kerr-Newman black
hole with the same total mass energy and the same charge to mass ratio of a
Reissner-Nordstr\"{o}m black hole. It turns out that the Kerr-Newman black hole
leads to larger values of the electromagnetic field and energy when compared to
the electric field and energy of the Reissner-Nordstr\"{o}m one. The
significance of these theoretical results for the realistic description of the
process of gravitational collapse leading to black hole formation as well as
the energy source of gamma ray bursts are also discussed.Comment: Revised version with minor corrections; to appear on Phys. Rev.
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