34 research outputs found
Nonequilibrium polarity-induced mechanism for chemotaxis: emergent Galilean symmetry and exact scaling exponents
A generically observed mechanism that drives the self-organization of living
systems is interaction via chemical signals among the individual elements --
which may represent cells, bacteria, or even enzymes. Here we propose a novel
mechanism for such interactions, in the context of chemotaxis, which originates
from the polarity of the particles and which generalizes the well-known
Keller--Segel interaction term. We study the resulting large-scale dynamical
properties of a system of such chemotactic particles using the exact stochastic
formulation of Dean and Kawasaki along with dynamical renormalization group
analysis of the critical state of the system. At this critical point, an
emergent "Galilean" symmetry is identified, which allows us to obtain the
dynamical scaling exponents exactly; these exponents reveal superdiffusive
density fluctuations and non-Poissonian number fluctuations. We expect our
results to shed light on how molecular regulation of chemotactic circuits can
determine large-scale behavior of cell colonies and tissues.Comment: The first three authors contributed equall
Contributions of plasma physics to chaos and nonlinear dynamics
This topical review focusses on the contributions of plasma physics to chaos
and nonlinear dynamics bringing new methods which are or can be used in other
scientific domains. It starts with the development of the theory of Hamiltonian
chaos, and then deals with order or quasi order, for instance adiabatic and
soliton theories. It ends with a shorter account of dissipative and high
dimensional Hamiltonian dynamics, and of quantum chaos. Most of these
contributions are a spin-off of the research on thermonuclear fusion by
magnetic confinement, which started in the fifties. Their presentation is both
exhaustive and compact. [15 April 2016
Heat Transfer and Reconnection Diffusion in Turbulent Magnetized Plasmas
It is well known that magnetic fields constrain motions of charged particles,
impeding the diffusion of charged particles perpendicular to magnetic field
direction. This modification of transport processes is of vital importance for
a wide variety of astrophysical processes including cosmic ray transport,
transfer of heavy elements in the interstellar medium, star formation etc.
Dealing with these processes one should keep in mind that in realistic
astrophysical conditions magnetized fluids are turbulent. In this review we
single out a single transport process, namely, heat transfer and consider how
it occurs in the presence of the magnetized turbulence. We show that the
ability of magnetic field lines to constantly change topology and connectivity
is at the heart of the correct description of the 3D magnetic field
stochasticity in turbulent fluids. This ability is ensured by fast magnetic
reconnection in turbulent fluids and puts forward the concept of reconnection
diffusion at the core of the physical picture of heat transfer in astrophysical
plasmas. Appealing to reconnection diffusion we describe the ability of plasma
to diffuse between different magnetized eddies explaining the advection of the
heat by turbulence. Adopting the structure of magnetic field that follows from
the modern understanding of MHD turbulence, we also discuss thermal
conductivity that arises as electrons stream along stochastic magnetic field
lines. We compare the effective heat transport that arise from the two
processes and conclude that in many astrophysically-motivated cased eddy
advection of heat dominates. Finally, we discuss the concepts of sub and
superdiffusion and show that the subdiffusion requires rather restrictive
settings. At the same time, accelerated diffusion or superdiffusion of heat is
possible on the scales less than the injection scale of the turbulence.Comment: 25 pages, 8 figures, chapter 9 in "Heat Conduction", ed. V.S.
Vikhrenko, ISBN 978-952-307-404-
Interstellar Turbulence II: Implications and Effects
Interstellar turbulence has implications for the dispersal and mixing of the
elements, cloud chemistry, cosmic ray scattering, and radio wave propagation
through the ionized medium. This review discusses the observations and theory
of these effects. Metallicity fluctuations are summarized, and the theory of
turbulent transport of passive tracers is reviewed. Modeling methods, turbulent
concentration of dust grains, and the turbulent washout of radial abundance
gradients are discussed. Interstellar chemistry is affected by turbulent
transport of various species between environments with different physical
properties and by turbulent heating in shocks, vortical dissipation regions,
and local regions of enhanced ambipolar diffusion. Cosmic rays are scattered
and accelerated in turbulent magnetic waves and shocks, and they generate
turbulence on the scale of their gyroradii. Radio wave scintillation is an
important diagnostic for small scale turbulence in the ionized medium, giving
information about the power spectrum and amplitude of fluctuations. The theory
of diffraction and refraction is reviewed, as are the main observations and
scintillation regions.Comment: 46 pages, 2 figures, submitted to Annual Reviews of Astronomy and
Astrophysic
Large Scale Stochastic Dynamics
In focus are interacting stochastic systems with many components, ranging from stochastic partial differential equations to discrete systems as interacting particles on a lattice moving through random jumps.
More specifically one wants to understand the large scale behavior, large in spatial extent but also over long time spans, as entailed by the characterization of stationary measures, effective macroscopic evolution laws, transport of conserved fields, homogenization, self-similar structure and scaling, critical dynamics, aging, dynamical phase transitions, large deviations, to mention only a few key items
Anomalous statistics of laser-cooled atoms in dissipative optical lattices
Diffusion occurs in numerous physical systems throughout nature, drawing its
generality from the universality of the central limit theorem. Around a century
ago it was realized that an extension to this type of dynamics can be obtained
in the form of "anomalous" diffusion where distributions are allowed to have
heavy, power-law tails. Due to a unique feature of its momentum-dependant
dissipative friction force, the physical system of laser-cooled atomic
ensembles can be used as a test bed for such dynamics. The interplay between
laser cooling and anomalous dynamics bears deep, predictive implications on
fundamental concepts in both equilibrium and non-equilibrium statistical
physics. The high degree of experimental control available in cold-atom
experiments allows for tuning of the parameters of the friction force,
revealing transitions in the dynamical properties of the system. Rare events,
in both the momentum and spatial distributions, are described by non-normalized
states using tools adapted from infinite ergodic theory. This leads to new
results, both experimental and theoretical, illuminating the various features
of the system
Kinetics of phase separation and thin film formation of multiresponsive polymers
In this thesis, we were interested in studying the deposition of layers formed by stimuli-responsive colloidal soft particles using a surface acoustic wave method: “Quartz Crystal Microbalance with Dissipation Monitoring, (QCM-D)”. This thesis is divided into four chapters: chapter 1 gives an introduction to the structural properties of polymers and glasses in solution and at interfaces, chapter 2 presents the principles of the QCM-D monitoring technique in terms of the physical meaning of its main output (the normalized resonance frequency shift Δfn/n and dissipation factor shift ΔDn) and important parameters to consider while carrying out our study (temperature and light control). Chapter 3 is dedicated to deposition control of soft layers formed out of dual stimuli-responsive colloidal particles of Poly (triethylene glycol acrylate-co-Spiropyran acrylate) (P(TEGA-co-SPA)). In this chapter, we examine the concomitant effect of light and temperature in order to manipulate the formation process of soft glassy films and their subsequent structural response. Chapter 4 presents the kinetics of the deposition of Poly(N-isopropylacrylamide) P(NIPAAm) soft glassy layers. In this chapter, we focus on the effect of temperature and concentration on the isothermal glass formation below and above the phase separation temperature using a twostep kinetic model. We also provide a subsequent analysis of the viscoelastic properties of the deposited layers in non-isothermal mode and compare it with their properties in isothermal mode. The present study extends the state of the art in two main disciplines. The first discipline is fundamental research in glass sciences and the second discipline is the research of multi stimuli-responsive polymers, where our findings would be of great interest for applications that need remote-controlled switching, especially in microfluidic chips and biomedical applications
Recent Advances in Single-Particle Tracking: Experiment and Analysis
This Special Issue of Entropy, titled “Recent Advances in Single-Particle Tracking: Experiment and Analysis”, contains a collection of 13 papers concerning different aspects of single-particle tracking, a popular experimental technique that has deeply penetrated molecular biology and statistical and chemical physics. Presenting original research, yet written in an accessible style, this collection will be useful for both newcomers to the field and more experienced researchers looking for some reference. Several papers are written by authorities in the field, and the topics cover aspects of experimental setups, analytical methods of tracking data analysis, a machine learning approach to data and, finally, some more general issues related to diffusion
Magnetic Field Line Random Walk and Solar Energetic Particle Path Lengths: Stochastic Theory and PSP/ISoIS Observation
Context:In 2020 May-June, six solar energetic ion events were observed by the
Parker Solar Probe/ISoIS instrument suite at 0.35 AU from the Sun. From
standard velocity-dispersion analysis, the apparent ion path length is 0.625 AU
at the onset of each event. Aims:We develop a formalism for estimating the path
length of random-walking magnetic field lines, to explain why the apparent ion
pathlength at event onset greatly exceeds the radial distance from the Sun for
these events. Methods:We developed analytical estimates of the average increase
in pathlength of random-walking magnetic field lines, relative to the
unperturbed mean field. Monte Carlo simulations of fieldline and particle
trajectories in a model of solar wind turbulence are used to validate the
formalism and study the path lengths of particle guiding-center and
full-orbital trajectories. The formalism is implemented in a global solar wind
model, and results are compared with ion pathlengths inferred from ISoIS
observations. Results:Both a simple estimate and a rigorous theoretical
formulation are obtained for fieldlines' pathlength increase as a function of
pathlength along the large-scale field. From simulated fieldline and particle
trajectories, we find that particle guiding centers can have pathlengths
somewhat shorter than the average fieldline pathlength, while particle orbits
can have substantially larger pathlengths due to their gyromotion with a
nonzero effective pitch angle. Conclusions:The long apparent path length during
these solar energetic ion events can be explained by 1) a magnetic field line
path length increase due to the field line random walk, and 2) particle
transport about the guiding center with a nonzero effective pitch angle. Our
formalism for computing the magnetic field line path length, accounting for
turbulent fluctuations, may be useful for application to solar particle
transport in general