198 research outputs found
Linear response to leadership, effective temperature and decision making in flocks
Large collections of autonomously moving agents, such as animals or
micro-organisms, are able to 'flock' coherently in space even in the absence of
a central control mechanism. While the direction of the flock resulting from
this critical behavior is random, this can be controlled by a small subset of
informed individuals acting as leaders of the group. In this article we use the
Vicsek model to investigate how flocks respond to leadership and make
decisions. Using a combination of numerical simulations and continuous modeling
we demonstrate that flocks display a linear response to leadership that can be
cast in the framework of the fluctuation-dissipation theorem, identifying an
'effective temperature' reflecting how promptly the flock reacts to the
initiative of the leaders. The linear response to leadership also holds in the
presence of two groups of informed individuals with competing interests,
indicating that the flock's behavioral decision is determined by both the
number of leaders and their degree of influence.Comment: 8 pages (incl. supplementary information), 8 figures, Supplementary
movies can be found at
http://wwwhome.lorentz.leidenuniv.nl/~giomi/sup_mat/20151108
Excitable Patterns in Active Nematics
We analyze a model of mutually-propelled filaments suspended in a
two-dimensional solvent. The system undergoes a mean-field isotropic-nematic
transition for large enough filament concentrations and the nematic order
parameter is allowed to vary in space and time. We show that the interplay
between non-uniform nematic order, activity and flow results in spatially
modulated relaxation oscillations, similar to those seen in excitable media. In
this regime the dynamics consists of nearly stationary periods separated by
"bursts" of activity in which the system is elastically distorted and solvent
is pumped throughout. At even higher activity the dynamics becomes chaotic.Comment: 4 pages, 4 figure
Bubble Raft Model for a Paraboloidal Crystal
We investigate crystalline order on a two-dimensional paraboloid of
revolution by assembling a single layer of millimeter-sized soap bubbles on the
surface of a rotating liquid, thus extending the classic work of Bragg and Nye
on planar soap bubble rafts. Topological constraints require crystalline
configurations to contain a certain minimum number of topological defects such
as disclinations or grain boundary scars whose structure is analyzed as a
function of the aspect ratio of the paraboloid. We find the defect structure to
agree with theoretical predictions and propose a mechanism for scar nucleation
in the presence of large Gaussian curvature.Comment: 4 pages, 4 figure
Crystalline Order On Riemannian Manifolds With Variable Gaussian Curvature And Boundary
We investigate the zero temperature structure of a crystalline monolayer
constrained to lie on a two-dimensional Riemannian manifold with variable
Gaussian curvature and boundary. A full analytical treatment is presented for
the case of a paraboloid of revolution. Using the geometrical theory of
topological defects in a continuum elastic background we find that the presence
of a variable Gaussian curvature, combined with the additional constraint of a
boundary, gives rise to a rich variety of phenomena beyond that known for
spherical crystals. We also provide a numerical analysis of a system of
classical particles interacting via a Coulomb potential on the surface of a
paraboloid.Comment: 12 pages, 8 figure
The mean field infinite range p=3 spin glass: equilibrium landscape and correlation time scales
We investigate numerically the dynamical behavior of the mean field 3-spin
spin glass model: we study equilibrium dynamics, and compute equilibrium time
scales as a function of the system size V. We find that for increasing volumes
the time scales increase like . We also present an
accurate study of the equilibrium static properties of the system.Comment: 6 pages, 9 figure
Hemodynamic sensor in cardiac implantable electric devices: the endocardial accelaration technology
There have been substantial progresses in the technology of cardiac implantable electric devices (CIEDs) during the past decades. One of the progresses is represented by the development of a hemodynamic sensor embedded at the tip of a pacing lead that measures myocardial contractility by the analysis of myocardial mechanical vibrations occurring during the cardiac cycle. This sensor, providing continuous hemodynamic monitoring, could play an important role in clinical practice because of several clinical applications in CIEDs recipients. The objectives of this work are to report how this sensor operates and to review the main findings about its clinical applications
Toroidal Crystals
Crystalline assemblages of identical sub-units packed together and
elastically bent in the form of a torus have been found in the past ten years
in a variety of systems of surprisingly different nature, such as viral
capsids, self-assembled monolayers and carbon nanomaterials. In this Letter we
analyze the structural properties of toroidal crystals and we provide a unified
description based on the elastic theory of defects in curved geometries. We
find ground states characterized by the presence of 5-fold disclinations on the
exterior of the torus and 7-fold disclinations in the interior. The number of
excess disclinations is controlled primarily by the aspect ratio of the torus,
suggesting a novel mechanism for creating toroidal templates with precisely
controlled valency via functionalization of the defect sites.Comment: 4 pages, 4 figure
Orientational correlations in active and passive nematic defects
We investigate the emergence of orientational order among +1/2 disclinations
in active nematic liquid crystals. Using a combination of theoretical and
experimental methods, we show that +1/2 disclinations have short-range
antiferromagnetic alignment, as a consequence of the elastic torques
originating from their polar structure. The presence of intermediate -1/2
disclinations, however, turns this interaction from anti-aligning to aligning
at scales that are smaller than the typical distance between like-sign defects.
No long-range orientational order is observed. Strikingly, these effects are
insensitive to material properties and qualitatively similar to what is found
for defects in passive nematic liquid crystals.Comment: 6 pages, 4 figure
The importance of thermal history: costs and benefits of heat exposure in a tropical, rocky shore oyster
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