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 $\tau = 70 \pm 15$~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

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

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

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

Random Convex Hulls and Extreme Value Statistics

In this paper we study the statistical properties of convex hulls of $N$ random points in a plane chosen according to a given distribution. The points may be chosen independently or they may be correlated. After a non-exhaustive survey of the somewhat sporadic literature and diverse methods used in the random convex hull problem, we present a unifying approach, based on the notion of support function of a closed curve and the associated Cauchy's formulae, that allows us to compute exactly the mean perimeter and the mean area enclosed by the convex polygon both in case of independent as well as correlated points. Our method demonstrates a beautiful link between the random convex hull problem and the subject of extreme value statistics. As an example of correlated points, we study here in detail the case when the points represent the vertices of $n$ independent random walks. In the continuum time limit this reduces to $n$ independent planar Brownian trajectories for which we compute exactly, for all $n$, the mean perimeter and the mean area of their global convex hull. Our results have relevant applications in ecology in estimating the home range of a herd of animals. Some of these results were announced recently in a short communication [Phys. Rev. Lett. {\bf 103}, 140602 (2009)].Comment: 61 pages (pedagogical review); invited contribution to the special issue of J. Stat. Phys. celebrating the 50 years of Yeshiba/Rutgers meeting

Effects of countdown displays in public transport route choice under severe overcrowding

The paper presents a route choice model for dynamic assignment in congested, i.e. overcrowded, transit networks where it is assumed that passengers are supported with real-time information on carrier arrivals at stops. If the stop layout is such that passenger congestion results in First-In-First-Out (FIFO) queues, a new formulation is devised for calculating waiting times, total travel times and route splits. Numerical results for a simple example network show the effect of information on route choice when heavy congestion is observed. While the provision of information does not lead to a remarkable decrease in total travel time, with the exception of some particular instances, it changes the travel behaviour of passengers that seem to be more averse to queuing at later stages of their journey and, thus, prefer to interchange at less congested stations

Channel Length Scaling of MoS2 MOSFETs

In this article, we investigate electrical transport properties in ultrathin body (UTB) MoS2 two-dimensional (2D) crystals with channel lengths ranging from 2 {\mu}m down to 50 nm. We compare the short channel behavior of sets of MOSFETs with various channel thickness, and reveal the superior immunity to short channel effects of MoS2 transistors. We observe no obvious short channel effects on the device with 100 nm channel length (Lch) fabricated on a 5 nm thick MoS2 2D crystal even when using 300 nm thick SiO2 as gate dielectric, and has a current on/off ratio up to ~109. We also observe the on-current saturation at short channel devices with continuous scaling due to the carrier velocity saturation. Also, we reveal the performance limit of short channel MoS2 transistors is dominated by the large contact resistance from the Schottky barrier between Ni and MoS2 interface, where a fully transparent contact is needed to achieve a high-performance short channel device.Comment: 22 pages, 6 figures; ACS Nano, ASAP, 201