691 research outputs found
Differential growth of wrinkled biofilms
Biofilms are antibiotic-resistant bacterial aggregates that grow on moist
surfaces and can trigger hospital-acquired infections. They provide a classical
example in biology where the dynamics of cellular communities may be observed
and studied. Gene expression regulates cell division and differentiation, which
affect the biofilm architecture. Mechanical and chemical processes shape the
resulting structure. We gain insight into the interplay between cellular and
mechanical processes during biofilm development on air-agar interfaces by means
of a hybrid model. Cellular behavior is governed by stochastic rules informed
by a cascade of concentration fields for nutrients, waste and autoinducers.
Cellular differentiation and death alter the structure and the mechanical
properties of the biofilm, which is deformed according to Foppl-Von Karman
equations informed by cellular processes and the interaction with the
substratum. Stiffness gradients due to growth and swelling produce wrinkle
branching. We are able to reproduce wrinkled structures often formed by
biofilms on air-agar interfaces, as well as spatial distributions of
differentiated cells commonly observed with B. subtilis.Comment: 19 pages, 13 figure
Dynamics of Coherent States in Regular and Chaotic Regimes of the Non-integrable Dicke Model
The quantum dynamics of initial coherent states is studied in the Dicke model
and correlated with the dynamics, regular or chaotic, of their classical limit.
Analytical expressions for the survival probability, i.e. the probability of
finding the system in its initial state at time , are provided in the
regular regions of the model. The results for regular regimes are compared with
those of the chaotic ones. It is found that initial coherent states in regular
regions have a much longer equilibration time than those located in chaotic
regions. The properties of the distributions for the initial coherent states in
the Hamiltonian eigenbasis are also studied. It is found that for regular
states the components with no negligible contribution are organized in
sequences of energy levels distributed according to Gaussian functions. In the
case of chaotic coherent states, the energy components do not have a simple
structure and the number of participating energy levels is larger than in the
regular cases.Comment: Contribution to the proceedings of the Escuela Latinoamericana de
F\'isica (ELAF) Marcos Moshinsky 2017. (9 pages, 4 figures
Axisymmetric pulse recycling and motion in bulk semiconductors
The Kroemer model for the Gunn effect in a circular geometry (Corbino disks)
has been numerically solved. The results have been interpreted by means of
asymptotic calculations. Above a certain onset dc voltage bias, axisymmetric
pulses of the electric field are periodically shed by an inner circular
cathode. These pulses decay as they move towards the outer anode, which they
may not reach. As a pulse advances, the external current increases continuously
until a new pulse is generated. Then the current abruptly decreases, in
agreement with existing experimental results. Depending on the bias, more
complex patterns with multiple pulse shedding are possible.Comment: 8 pages, 15 figure
Igniting homogeneous nucleation
Transient homogeneous nucleation is studied in the limit of large critical
sizes. Starting from pure monomers, three eras of transient nucleation are
characterized in the classic Becker-D\"oring kinetic equations with two
different models of discrete diffusivity: the classic Turnbull-Fisher formula
and an expression describing thermally driven growth of the nucleus. The latter
diffusivity yields time lags for nucleation which are much closer to values
measured in experiments with disilicate glasses. After an initial stage in
which the number of monomers decreases, many clusters of small size are
produced and a continuous size distribution is created. During the second era,
nucleii are increasing steadily in size in such a way that their distribution
appears as a wave front advancing towards the critical size for steady
nucleation. The nucleation rate at critical size is negligible during this era.
After the wave front reaches critical size, it ignites the creation of
supercritical clusters at a rate that increases monotonically until its steady
value is reached. Analytical formulas for the transient nucleation rate and the
time lag are obtained that improve classical ones and compare very well with
direct numerical solutions.Comment: 32 pages, 6 figures, to appear in Phys. Rev.
Edge dislocations in crystal structures considered as traveling waves of discrete models
The static stress needed to depin a 2D edge dislocation, the lower dynamic
stress needed to keep it moving, its velocity and displacement vector profile
are calculated from first principles. We use a simplified discrete model whose
far field distortion tensor decays algebraically with distance as in the usual
elasticity. An analytical description of dislocation depinning in the strongly
overdamped case (including the effect of fluctuations) is also given. A set of
parallel edge dislocations whose centers are far from each other can depin
a given one provided , where is the average inter-dislocation
distance divided by the Burgers vector of a single dislocation. Then a limiting
dislocation density can be defined and calculated in simple cases.Comment: 10 pages, 3 eps figures, Revtex 4. Final version, corrected minor
error
Instrument validity: perception of virtual learning during CoVId-19
En el marco de la pandemia originada por la CoVId-19 la educación a nivel mundial, como medida de prevención, pasó de la forma presencial a la virtual. La manera intempestiva de
este cambio y sin preparación previa trajo dificultades y también algunas ventajas a los estudiantes
de doctorado. Esto obligó a algunos investigadores a crear un instrumento de medición documental
denominado “Percepción del aprendizaje virtual de doctorandos en el marco de la CoVId-19” de
cuatro dimensiones y 30 ítems en escala del tipo Likert. El objetivo de este trabajo es presentar desde
el enfoque cuantitativo la validez interna y externa del instrumento mencionado. Los resultados
muestran un coeficiente Alfa de Cronbach, inicial y final, mayor a 0,8 y un coeficiente de correlación
de Pearson diferente de cero. Se concluye que el instrumento analizado, no solo es suficiente y
pertinente, sino que también, presenta validez porque es confiable y estableIn the framework of the pandemic caused by CoVId-19, education worldwide, as a
preventive measure, went from face-to-face to virtual. The untimely manner of this change, and
without prior preparation, brought difficulties and also some advantages to doctoral students. This
forced some authors to create a documentary measurement instrument called "doctoral students’ virtual learning perception in the CoVId-19 framework" with four dimensions and 30 items on a
Likert-type scale. The objective of this work is to present from the quantitative approach the internal
and external validity of the mentioned instrument. The results show a Cronbach's alpha coefficient, initial and final, greater than 0.8 and a Pearson correlation coefficient different from zero. It is
concluded that the analyzed instrument is not only sufficient and relevant, but also has validity
because it is reliable and stabl
ISM conditions in z~0.2 Lyman-Break Analogs
We present an analysis of far--infrared (FIR) [CII] and [OI] fine structure
line and continuum observations obtained with /PACS, and CO(1-0)
observations obtained with the IRAM Plateau de Bure Interferometer, of Lyman
Break Analogs (LBAs) at . The principal aim of this work is to
determine the typical ISM properties of Main Sequence (MS)
galaxies, with stellar masses between and ,
which are currently not easily detectable in all these lines even with ALMA and
NOEMA. We perform PDR modeling and apply different IR diagnostics to derive the
main physical parameters of the FIR emitting gas and dust and we compare the
derived ISM properties to those of galaxies on and above the MS at different
redshifts. We find that the ISM properties of LBAs are quite extreme (low gas
temperature, high density and thermal pressure) with respect to those found in
local normal spirals and more active local galaxies. LBAs have no [CII] deficit
despite having the high specific star formation rates (sSFRs) typical of
starbursts. Although LBAs lie above the local MS, we show that their ISM
properties are more similar to those of high-redshift MS galaxies than of local
galaxies above the main sequence. This data set represents an important
reference for planning future ALMA [CII] observations of relatively low-mass MS
galaxies at the epoch of the peak of the cosmic star formation.Comment: 19 pages, 12 Figures,8 Tables, Accepted for publication in A&
Discrete models of dislocations and their motion in cubic crystals
A discrete model describing defects in crystal lattices and having the
standard linear anisotropic elasticity as its continuum limit is proposed. The
main ingredients entering the model are the elastic stiffness constants of the
material and a dimensionless periodic function that restores the translation
invariance of the crystal and influences the Peierls stress. Explicit
expressions are given for crystals with cubic symmetry: sc, fcc and bcc.
Numerical simulations of this model with conservative or damped dynamics
illustrate static and moving edge and screw dislocations and describe their
cores and profiles. Dislocation loops and dipoles are also numerically
observed. Cracks can be created and propagated by applying a sufficient load to
a dipole formed by two edge dislocations.Comment: 23 pages, 15 figures, to appear in Phys. Rev.
Towards a resolved Kennicutt-Schmidt law at high redshift
Massive galaxies in the distant Universe form stars at much higher rates than
today. Although direct resolution of the star forming regions of these galaxies
is still a challenge, recent molecular gas observations at the IRAM Plateau de
Bure interferometer enable us to study the star formation efficiency on
subgalactic scales around redshift z = 1.2. We present a method for obtaining
the gas and star formation rate (SFR) surface densities of ensembles of clumps
composing galaxies at this redshift, even though the corresponding scales are
not resolved. This method is based on identifying these structures in
position-velocity diagrams corresponding to slices within the galaxies. We use
unique IRAM observations of the CO(3-2) rotational line and DEEP2 spectra of
four massive star forming distant galaxies - EGS13003805, EGS13004291,
EGS12007881, and EGS13019128 in the AEGIS terminology - to determine the gas
and SFR surface densities of the identifiable ensembles of clumps that
constitute them. The integrated CO line luminosity is assumed to be directly
proportional to the total gas mass, and the SFR is deduced from the [OII] line.
We identify the ensembles of clumps with the angular resolution available in
both CO and [OII] spectroscopy; i.e., 1-1.5". SFR and gas surface densities are
averaged in areas of this size, which is also the thickness of the DEEP2 slits
and of the extracted IRAM slices, and we derive a spatially resolved
Kennicutt-Schmidt (KS) relation on a scale of ~8 kpc. The data generally
indicates an average depletion time of 1.9 Gyr, but with significant variations
from point to point within the galaxies.Comment: 6 pages, 4 figures, 2 tables, accepted by Astronomy and Astrophysic
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