222 research outputs found
Fast determination of coarse grained cell anisotropy and size in epithelial tissue images using Fourier transform
Mechanical strain and stress play a major role in biological processes such
as wound healing or morphogenesis. To assess this role quantitatively, fixed or
live images of tissues are acquired at a cellular precision in large fields of
views. To exploit these data, large numbers of cells have to be analyzed to
extract cell shape anisotropy and cell size. Most frequently, this is performed
through detailed individual cell contour determination, using so-called
segmentation computer programs, complemented if necessary by manual detection
and error corrections. However, a coarse grained and faster technique can be
recommended in at least three situations. First, when detailed information on
individual cell contours is not required, for instance in studies which require
only coarse-grained average information on cell anisotropy. Second, as an
exploratory step to determine whether full segmentation can be potentially
useful. Third, when segmentation is too difficult, for instance due to poor
image quality or too large a cell number. We developed a user-friendly, Fourier
transform-based image analysis pipeline. It is fast (typically cells per
minute with a current laptop computer) and suitable for time, space or ensemble
averages. We validate it on one set of artificial images and on two sets of
fully segmented images, one from a Drosophila pupa and the other from a chicken
embryo; the pipeline results are robust. Perspectives include \textit{in vitro}
tissues, non-biological cellular patterns such as foams, and stacks.Comment: 13 pages; 9 figure
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
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
Random Convex Hulls and Extreme Value Statistics
In this paper we study the statistical properties of convex hulls of
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 independent random walks. In the continuum time limit this reduces to
independent planar Brownian trajectories for which we compute exactly, for
all , 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
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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
Integrated Circuits Based on Bilayer MoS
Two-dimensional (2D) materials, such as molybdenum disulfide (MoS2), have been shown to exhibit excellent electrical and optical properties. The semiconducting nature of MoS2 allows it to overcome the shortcomings of zero-bandgap graphene, while still sharing many of graphene’s advantages for electronic and optoelectronic applications. Discrete electronic and optoelectronic components, such as field-effect transistors, sensors, and photodetectors made from few-layer MoS2 show promising performance as potential substitute of Si in conventional electronics and of organic and amorphous Si semiconductors in ubiquitous systems and display applications. An important next step is the fabrication of fully integrated multistage circuits and logic building blocks on MoS2 to demonstrate its capability for complex digital logic and high-frequency ac applications. This paper demonstrates an inverter, a NAND gate, a static random access memory, and a five-stage ring oscillator based on a direct-coupled transistor logic technology. The circuits comprise between 2 to 12 transistors seamlessly integrated side-by-side on a single sheet of bilayer MoS2. Both enhancement-mode and depletion-mode transistors were fabricated thanks to the use of gate metals with different work functions.United States. Office of Naval Research (Young Investigator Program)Microelectronics Advanced Research Corporation (MARCO) (Focus Center for Materials, Structure and Device (MARCO MSD))National Science Foundation (U.S.) (NSF DMR 0845358)United States. Army Research Offic
A Mixture of “Cheats” and “Co-Operators” Can Enable Maximal Group Benefit
It is commonly assumed that the world would be best off if everyone co-operates. Experimental and mathematical analysis of “co-operation” in yeast show why this isn't always the case
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