1,275 research outputs found
Remarks on Bootstrap Percolation in Metric Networks
We examine bootstrap percolation in d-dimensional, directed metric graphs in
the context of recent measurements of firing dynamics in 2D neuronal cultures.
There are two regimes, depending on the graph size N. Large metric graphs are
ignited by the occurrence of critical nuclei, which initially occupy an
infinitesimal fraction, f_* -> 0, of the graph and then explode throughout a
finite fraction. Smaller metric graphs are effectively random in the sense that
their ignition requires the initial ignition of a finite, unlocalized fraction
of the graph, f_* >0. The crossover between the two regimes is at a size N_*
which scales exponentially with the connectivity range \lambda like_* \sim
\exp\lambda^d. The neuronal cultures are finite metric graphs of size N \simeq
10^5-10^6, which, for the parameters of the experiment, is effectively random
since N<< N_*. This explains the seeming contradiction in the observed finite
f_* in these cultures. Finally, we discuss the dynamics of the firing front
Leaders of neuronal cultures in a quorum percolation model
We present a theoretical framework using quorum-percolation for describing
the initiation of activity in a neural culture. The cultures are modeled as
random graphs, whose nodes are excitatory neurons with kin inputs and kout
outputs, and whose input degrees kin = k obey given distribution functions pk.
We examine the firing activity of the population of neurons according to their
input degree (k) classes and calculate for each class its firing probability
\Phi_k(t) as a function of t. The probability of a node to fire is found to be
determined by its in-degree k, and the first-to-fire neurons are those that
have a high k. A small minority of high-k classes may be called "Leaders", as
they form an inter-connected subnetwork that consistently fires much before the
rest of the culture. Once initiated, the activity spreads from the Leaders to
the less connected majority of the culture. We then use the distribution of
in-degree of the Leaders to study the growth rate of the number of neurons
active in a burst, which was experimentally measured to be initially
exponential. We find that this kind of growth rate is best described by a
population that has an in-degree distribution that is a Gaussian centered
around k = 75 with width {\sigma} = 31 for the majority of the neurons, but
also has a power law tail with exponent -2 for ten percent of the population.
Neurons in the tail may have as many as k = 4, 700 inputs. We explore and
discuss the correspondence between the degree distribution and a dynamic
neuronal threshold, showing that from the functional point of view, structure
and elementary dynamics are interchangeable. We discuss possible geometric
origins of this distribution, and comment on the importance of size, or of
having a large number of neurons, in the culture.Comment: Keywords: Neuronal cultures, Graph theory, Activation dynamics,
Percolation, Statistical mechanics of networks, Leaders of activity, Quorum.
http://www.weizmann.ac.il/complex/tlusty/papers/FrontCompNeuro2010.pd
An experimental investigation of chatter effects on tool life
Tool wear is one of the most important considerations in machining operations as it affects surface quality and integrity, productivity and cost. The most commonly used model for tool life analysis is the one proposed by F.W. Taylor about a century ago. Although the extended form of this equation includes the effects of important cutting conditions on tool wear, tool life studies are mostly performed under stable cutting conditions where the effect of chatter vibrations are not considered. This paper presents an empirical attempt to understand tool life under vibratory cutting conditions. Tool wear data are collected in turning and milling on different work materials under stable and chatter conditions. The effects of cutting conditions as well as severity of chatter on tool life are analyzed. The results indicate significant reduction in tool life due to chatter as expected. They also show that the severity of chatter, and thus the vibration amplitude, strongly reduces the life of cutting tools. These results can be useful in evaluating the real cost of chatter by including the reduced tool life. They can also be useful in justifying the cost of chatter suppression and more rigid machining systems
Percolation in living neural networks
We study living neural networks by measuring the neurons' response to a
global electrical stimulation. Neural connectivity is lowered by reducing the
synaptic strength, chemically blocking neurotransmitter receptors. We use a
graph-theoretic approach to show that the connectivity undergoes a percolation
transition. This occurs as the giant component disintegrates, characterized by
a power law with critical exponent is independent of the
balance between excitatory and inhibitory neurons and indicates that the degree
distribution is gaussian rather than scale freeComment: PACS numbers: 87.18.Sn, 87.19.La, 64.60.Ak
http://www.weizmann.ac.il/complex/tlusty/papers/PhysRevLett2006.pd
Compliance error compensation in robotic-based milling
The paper deals with the problem of compliance errors compensation in
robotic-based milling. Contrary to previous works that assume that the
forces/torques generated by the manufacturing process are constant, the
interaction between the milling tool and the workpiece is modeled in details.
It takes into account the tool geometry, the number of teeth, the feed rate,
the spindle rotation speed and the properties of the material to be processed.
Due to high level of the disturbing forces/torques, the developed compensation
technique is based on the non-linear stiffness model that allows us to modify
the target trajectory taking into account nonlinearities and to avoid the
chattering effect. Illustrative example is presented that deals with
robotic-based milling of aluminum alloy
On Bootstrap Percolation in Living Neural Networks
Recent experimental studies of living neural networks reveal that their
global activation induced by electrical stimulation can be explained using the
concept of bootstrap percolation on a directed random network. The experiment
consists in activating externally an initial random fraction of the neurons and
observe the process of firing until its equilibrium. The final portion of
neurons that are active depends in a non linear way on the initial fraction.
The main result of this paper is a theorem which enables us to find the
asymptotic of final proportion of the fired neurons in the case of random
directed graphs with given node degrees as the model for interacting network.
This gives a rigorous mathematical proof of a phenomena observed by physicists
in neural networks
Model and parameter dependence of heavy quark energy loss in a hot and dense medium
Within the framework of the Langevin equation, we study the energy loss of
heavy quark due to quasi-elastic multiple scatterings in a quark-gluon plasma
created by relativistic heavy-ion collisions. We investigate how the initial
configuration of the quark-gluon plasma as well as its properties affect the
final state spectra and elliptic flow of D meson and non-photonic electron. We
find that both the geometric anisotropy of the initial quark-gluon plasma and
the flow profiles of the hydrodynamic medium play important roles in the heavy
quark energy loss process and the development of elliptic flow. The relative
contribution from charm and bottom quarks is found to affect the transverse
momentum dependence of the quenching and flow patterns of heavy flavor decay
electron; such influence depends on the interaction strength between heavy
quark and the medium.Comment: 16 pages, 7 figure
Nematic-Wetted Colloids in the Isotropic Phase: Pairwise Interaction, Biaxiality and Defects
We calculate the interaction between two spherical colloidal particles
embedded in the isotropic phase of a nematogenic liquid. The surface of the
particles induces wetting nematic coronas that mediate an elastic interaction.
In the weak wetting regime, we obtain exact results for the interaction energy
and the texture, showing that defects and biaxiality arise, although they are
not topologically required. We evidence rich behaviors, including the
possibility of reversible colloidal aggregation and dispersion. Complex
anisotropic self-assembled phases might be formed in dense suspensions.Comment: 4 pages, 6 figure
Electromagnetic Calorimeter for HADES
We propose to build the Electromagnetic calorimeter for the HADES di-lepton
spectrometer. It will enable to measure the data on neutral meson production
from nucleus-nucleus collisions, which are essential for interpretation of
dilepton data, but are unknown in the energy range of planned experiments (2-10
GeV per nucleon). The calorimeter will improve the electron-hadron separation,
and will be used for detection of photons from strange resonances in elementary
and HI reactions.
Detailed description of the detector layout, the support structure, the
electronic readout and its performance studied via Monte Carlo simulations and
series of dedicated test experiments is presented.
The device will cover the total area of about 8 m^2 at polar angles between
12 and 45 degrees with almost full azimuthal coverage. The photon and electron
energy resolution achieved in test experiments amounts to 5-6%/sqrt(E[GeV])
which is sufficient for the eta meson reconstruction with S/B ratio of 0.4% in
Ni+Ni collisions at 8 AGeV. A purity of the identified leptons after the hadron
rejection, resulting from simulations based on the test measurements, is better
than 80% at momenta above 500 MeV/c, where time-of-flight cannot be used.Comment: 40 pages, 38 figures version2 - the time schedule added, information
about PMTs in Sec.III update
Molecular Model of the Contractile Ring
We present a model for the actin contractile ring of adherent animal cells.
The model suggests that the actin concentration within the ring and
consequently the power that the ring exerts both increase during contraction.
We demonstrate the crucial role of actin polymerization and depolymerization
throughout cytokinesis, and the dominance of viscous dissipation in the
dynamics. The physical origin of two phases in cytokinesis dynamics ("biphasic
cytokinesis") follows from a limitation on the actin density. The model is
consistent with a wide range of measurements of the midzone of dividing animal
cells.Comment: PACS numbers: 87.16.Ka, 87.16.Ac
http://www.ncbi.nlm.nih.gov/pubmed/16197254
http://www.weizmann.ac.il/complex/tlusty/papers/PhysRevLett2005.pd
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