299 research outputs found
A migrating epithelial monolayer flows like a Maxwell viscoelastic liquid
We perform a bidimensional Stokes experiment in an active cellular material:
an autonomously migrating monolayer of Madin-Darby Canine Kidney (MDCK)
epithelial cells flows around a circular obstacle within a long and narrow
channel, involving an interplay between cell shape changes and neighbour
rearrangements. Based on image analysis of tissue flow and coarse-grained cell
anisotropy, we determine the tissue strain rate, cell deformation and
rearrangement rate fields, which are spatially heterogeneous. We find that the
cell deformation and rearrangement rate fields correlate strongly, which is
compatible with a Maxwell viscoelastic liquid behaviour (and not with a
Kelvin-Voigt viscoelastic solid behaviour). The value of the associated
relaxation time is measured as ~min, is observed to be
independent of obstacle size and division rate, and is increased by inhibiting
myosin activity. In this experiment, the monolayer behaves as a flowing
material with a Weissenberg number close to one which shows that both elastic
and viscous effects can have comparable contributions in the process of
collective cell migration.Comment: 17 pages, 15 figure
Invariant vector fields and the prolongation method for supersymmetric quantum systems
The kinematical and dynamical symmetries of equations describing the time
evolution of quantum systems like the supersymmetric harmonic oscillator in one
space dimension and the interaction of a non-relativistic spin one-half
particle in a constant magnetic field are reviewed from the point of view of
the vector field prolongation method. Generators of supersymmetries are then
introduced so that we get Lie superalgebras of symmetries and supersymmetries.
This approach does not require the introduction of Grassmann valued
differential equations but a specific matrix realization and the concept of
dynamical symmetry. The Jaynes-Cummings model and supersymmetric
generalizations are then studied. We show how it is closely related to the
preceding models. Lie algebras of symmetries and supersymmetries are also
obtained.Comment: 37 pages, 7 table
Measuring the Boltzmann constant by mid-infrared laser spectroscopy of ammonia
We report on our ongoing effort to measure the Boltzmann constant,
using the Doppler broadening technique on ammonia. This paper presents some of
the improvements made to the mid-infrared spectrometer including the use of a
phase-stabilized quantum cascade laser, a lineshape analysis based on a refined
physical model and an improved fitting program 2 increasing the confidence in
our estimates of the relevant molecular parameters, and a first evaluation of
the saturation parameter and its impact on the measurement of k B. A summary of
the systematic effects contributing to the measurement is given and the optimal
experimental conditions for mitigating those effects in order to reach a
competitive measurement of at a part per million accuracy level are
outlined
Barreras de acceso y disponibilidad: servicio de laboratorios clínicos de emergencia, en hospitales públicos
Objective. To describe the barriers to access and availability in emergency clinical laboratories in two type IV hospitals with different geographical locations in the Zuliana Region. Method. Descriptive and comparative study, with a non-experimental cross-sectional design. Non-probabilistic sampling with a sample size of 290 service users, 80 from Hospital 1 (H1) and 210 from Hospital 2 (H2). The data were obtained through a mixed survey of own elaboration, validated by agreement between experts. Descriptive statistics and comparison of means were used, using the T Student technique at 95% confidence. Results. The main access barrier found was geographical, in H1 (97.5%) and H2 (71.9%). Differences were found in the form of transportation and time required to reach the health center; in H1 the patients used their vehicles (48.8%) with a transfer time of 13.25±4.71 minutes; and in H2 public transport is mainly used (74.3%) with longer transfer time (50.09±34.4 min). The main administrative barrier was the lack of basic supplies (97.5% H1 vs 98.1% H2). There are economic barriers in both health centers: 53.8% in H1 and 56.7% in H2. There was greater availability of tests in the emergency laboratory of H1 (85.6%) than in H2 (31.72%). Conclusion. There are geographical, administrative and economic barriers to access and differences in the availability of supplies between the laboratories of the two hospitals.Objetivo. Describir las barreras de acceso y disponibilidad en laboratorios clínicos de emergencia en dos hospitales tipo IV con diferente localización geográfica en la Región Zuliana. Método. Estudio descriptivo y comparativo, con diseño no experimental transversal. Muestreo no probabilístico con tamaño muestral de 290 usuarios del servicio, 80 del Hospital 1 (H1) y 210 del Hospital 2 (H2). Los datos fueron obtenidos a través de encuesta mixta de elaboración propia, validada mediante concordancia entre expertos. Se utilizaron estadísticos descriptivos y comparación de medias, mediante la técnica T Student al 95% de confianza. Resultados. La principal barrera de acceso encontrada fue la geográfica, en el H1 (97,5 %) y el H2 (71,9 %). Se hallaron diferencias en la forma de traslado y tiempo requerido para llegar al centro de salud; en el H1 los pacientes utilizan sus vehículos (48,8 %) con tiempo de traslado 13,25±4,71 minutos; y en el H2 se utiliza principalmente transporte público (74,3 %) con tiempo de traslado mayor (50,09±34,4 min). La principal barrera administrativa fue la falta de insumos básicos (97,5% H1 vs 98,1% H2). Existen barreras económicas en ambos centros de salud: 53,8% en el H1 y 56,7% para el H2. Se evidenció mayor disponibilidad de pruebas en el laboratorio de emergencias del H1 (85,6 %) que en el H2 (31,72%). Conclusión. Existen barreras de acceso de tipo geográficas, administrativas, económicas y diferencias en la disponibilidad de insumos entre los laboratorios de los dos hospitales
Selfoscillations of Suspended Carbon Nanotubes with a Deflection Sensitive Resistance under Voltage Bias
We theoretically investigate the electro-mechanics of a Suspended Carbon
Nanotube with a Deflection Sensitive Resistance subjected to a homogeneous
Magnetic Field and a constant Voltage Bias. We show that, (with the exception
of a singular case), for a sufficiently high magnetic field the
time-independent state of charge transport through the nanotube becomes
unstable to selfexcitations of the mechanical vibration accompanied by
oscialltions in the voltage drop and current across the nanotube.Comment: 4 pages, 1 figur
Insulating behavior in ultra-thin bismuth selenide field effect transistors
Ultrathin (~3 quintuple layer) field-effect transistors (FETs) of topological
insulator Bi2Se3 are prepared by mechanical exfoliation on 300nm SiO2/Si
susbtrates. Temperature- and gate-voltage dependent conductance measurements
show that ultrathin Bi2Se3 FETs are n-type, and have a clear OFF state at
negative gate voltage, with activated temperature-dependent conductance and
energy barriers up to 250 meV
The Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors
We present low temperature electrical transport experiments in five field
effect transistor devices consisting of monolayer, bilayer and trilayer MoS2
films, mechanically exfoliated onto Si/SiO2 substrate. Our experiments reveal
that the electronic states in all films are localized well up to the room
temperature over the experimentally accessible range of gate voltage. This
manifests in two dimensional (2D) variable range hopping (VRH) at high
temperatures, while below \sim 30 K the conductivity displays oscillatory
structures in gate voltage arising from resonant tunneling at the localized
sites. From the correlation energy (T0) of VRH and gate voltage dependence of
conductivity, we suggest that Coulomb potential from trapped charges in the
substrate are the dominant source of disorder in MoS2 field effect devices,
which leads to carrier localization as well.Comment: 10 pages, 5 figures; ACS Nano (2011
Evaluating QBF Solvers: Quantifier Alternations Matter
We present an experimental study of the effects of quantifier alternations on
the evaluation of quantified Boolean formula (QBF) solvers. The number of
quantifier alternations in a QBF in prenex conjunctive normal form (PCNF) is
directly related to the theoretical hardness of the respective QBF
satisfiability problem in the polynomial hierarchy. We show empirically that
the performance of solvers based on different solving paradigms substantially
varies depending on the numbers of alternations in PCNFs. In related
theoretical work, quantifier alternations have become the focus of
understanding the strengths and weaknesses of various QBF proof systems
implemented in solvers. Our results motivate the development of methods to
evaluate orthogonal solving paradigms by taking quantifier alternations into
account. This is necessary to showcase the broad range of existing QBF solving
paradigms for practical QBF applications. Moreover, we highlight the potential
of combining different approaches and QBF proof systems in solvers.Comment: preprint of a paper to be published at CP 2018, LNCS, Springer,
including appendi
Incorporating risk in field services operational planning process
© Springer Nature Switzerland AG 2018. This paper presents a model for the risk minimisation objective in the Stochastic Vehicle Routing Problem (SVRP). In the studied variant of SVRP, service times and travel times are subject to stochastic events, and a time window is constraining the start time for service task. Required skill levels and task priorities increase the complexity of this problem. Most previous research uses a chance-constrained approach to the problem and their objectives are related to traditional routing costs whilst a different approach was taken in this paper. The risk of missing a task is defined as the probability that the technician assigned to the task arrives at the customer site later than the time window. The problem studied in this paper is to generate a schedule that minimises the maximum of risks and sum of risks over all the tasks considering the effect of skill levels and task priorities. The stochastic duration of each task is supposed to follow a known normal distribution. However, the distribution of the start time of the service at a customer site will not be normally distributed due to time window constraints. A method is proposed and tested to approximate the start time distribution as normal. Moreover, a linear model can be obtained assuming identical variance of task durations. Additionally Simulated Annealing method was applied to solve the problem. Results of this work have been applied to an industrial case of SVRP where field engineering individuals drive to customer sites to provide time-constrained services. This original approach gives a robust schedule and allows organisations to pay more attention to increasing customer satisfaction and become more competitive in the market
Hirota's virtual multi-soliton solutions of N=2 supersymmetric Korteweg-de Vries equations
We prove that Mathieu's N=2 supersymmetric Korteweg-de Vries equations with
a=1 or a=4 admit Hirota's n-supersoliton solutions, whose nonlinear interaction
does not produce any phase shifts. For initial profiles that can not be
distinguished from a one-soliton solution at times t<<0, we reveal the
possibility of a spontaneous decay and, within a finite time, transformation
into a solitonic solution with a different wave number. This paradoxal effect
is realized by the completely integrable N=2 super-KdV systems, whenever the
initial soliton is loaded with other solitons that are virtual and become
manifest through the tau-function as the time grows.
Key words and phrases: Hirota's solitons, N=2 supersymmetric KdV,
Krasil'shchik-Kersten system, phase shift, spontaneous decay.Comment: Proc. 5th International Workshop `Nonlinear Physics: Theory and
Experiment' (June 12-21, 2008; Gallipoli, Italy), 11 page
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