10:00-10:40 | Umberto Marini Bettolo Marconi - Università di Camerino & INFN
Dynamics of Fluids in Nanospaces ![]()
By combining methods of kinetic and density functional theory,
we present a description of molecular fluids
which accounts for their microscopic structure and thermodynamic
properties as well as for the hydrodynamic behavior.
We focus on the evolution of the one particle phase space
distribution, $f(r,v,t)$ rather than on the evolution of the average
particle density, $\rho(r,t)$,
which features in dynamic density functional theory
often employed to describe colloidal systems.
In order to describe with sufficient accuracy the fluid structure
at length scales comparable with the size of the particles we shall resort
to methods similar to those of density functional theory (DFT)
employed in the study of equilibrium and non equilibrium properties.
In the case of hard-core
fluids, DFT and its dynamical extension give excellent results and
can be extended to more realistic fluids by using the van der Waals
picture of decomposing the total inter-particle potential into a
short-range repulsive potential and a long-range attractive potential
tail. The first is treated by means of a reference hard-sphere system
whilst the second is considered within the random phase
approximation (RPA).
A simple analysis of the equations is used to derive explicit
expressions both for equilibrium thermodynamic quantities, such as
pressure, compressibility etc., and for non equilibrium transport
coefficients.
In the second part of our presentation we shall introduce a
a multicomponent extension of our theory and
describe miscible and immiscible liquid mixtures
under inhomogeneous, non steady conditions typical of confined
fluid flows.
We first derive from a microscopic level
the evolution equations of the phase space distribution function of
each component in terms of a set of self consistent fields,
representing both body forces and viscous forces. Secondly, we solve
numerically
the resulting governing equations by means of the
Lattice Boltzmann method whose implementation contains novel
features with respect to existing approaches. Our model incorporates
hydrodynamic flow, diffusion, surface tension,and the possibility for
global and local viscosity variations. We validate our model by
studying the bulk viscosity dependence of the mixture on concentration,
packing fraction and size ratio. Finally we consider inhomogeneous systems
and study the
dynamics of mixtures in slits of molecular thickness and relate structural
and flow properties.
The resulting equation for $f(r,v,t)$ is studied
in two different physical limits:
diffusive dynamics, typical of
colloidal fluids without hydrodynamic interaction, where particles
are subject to overdamped motion resulting from the coupling with a
solvent at rest, and inertial dynamics, typical of
molecular fluids .
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10:40-11:00 | Cesare Nardini - Università di Firenze e ENS Lyon
STOCHASTICALLY PERTURBED LONG-RANGE INTERACTING SYSTEMS AND 2D FLUID
MODELS ![]()
Long-range interacting systems include neutral plasma, gravitational
systems, two-dimensional and geophysical fluid models. We consider
long-range interacting systems perturbed by external stochastic forces.
Unlike the case of short-range systems, where stochastic forces usually
act locally on each particle, here we consider perturbations by external
stochastic fields. The system reaches stationary states where the external
forces balance the dissipation on average. These states do not respect
detailed balance and support non-vanishing fluxes of conserved quantities.
I will discuss how the classical tools of kinetic theory can be extended
to describe these systems when the large structures evolve slowly.
Comparisons with numerical simulations in a case of a particularly simple
system, show an excellent agreement between the theory and simulations. I
will also discuss some results on non-equilibrium phase transitions that
we numerically observed in these systems.
|
11:00-11:20 | Aurelio Patelli, Università di Firenze
Linear response theory for long range interacting systems ![]()
Long-range interacting systems, while relaxing to equilibrium, often get
trapped in long-
lived quasistationary states which have lifetimes that diverge with the
system size. In this
work, we address the question of how a long-range system in a
quasistationary state (QSS)
responds to an external perturbation. We consider a long-range system that
evolves under
deterministic Hamilton dynamics. The perturbation is taken to couple to
the canonical
coordinates of the individual constituents. Our study is based on
analyzing the Vlasov
equation for the single-particle phase space distribution. The QSS
represents stable sta-
tionary solution of the Vlasov equation in the absence of the external
perturbation. In
the presence of small perturbation, we linearize the perturbed Vlasov
equation about the
QSS to obtain a formal expression for the response observed in a
single-particle dynami-
cal quantity. We apply our analysis to a paradigmatic model, the
Hamiltonian mean-field
model, that involves particles moving on a circle under Hamilton dynamics.
Our prediction
for the response of some representative QSSs in this model (the water-bag
QSS and the
Gaussian QSS) is found to be in good agreement with N-particle simulations
for large N.
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11:20-11:50 | coffee break |
11:50-12:10 | Samir Suweis - Università di Padova
Emerging patterns in Ecology:
the species persistence-time distributions ![]()
Natural ecosystems are characterized by striking diversity of form
and functions and yet exhibit deep symmetries emerging across
scales of space, time and organizational complexity. Species-area
relationships and species-abundance distributions are examples
of emerging patterns irrespective of the details of the underlying
ecosystem functions. We present empirical and theoretical evidence
for a new macro-ecological pattern related to the distributions of
local species persistence times, defined as the timespans between
local colonization and extinctions in a given geographic region, and
empirically estimated from local observations of species' presence-
absence time series. Empirical distributions exhibit power-law scaling
limited by a cut-off determined by the rate of emergence of new
species. The scaling exponents depend solely on the structure of the
spatial interaction network, regardless of the details of the ecological
interactions, suggesting similarities between ecosystem dynamics
and critical systems in physics. We also present generalize and solve
analytically a related sampling problem. The framework developed
here also allows to link the cut-off timescale with the spatial scale
of analysis, and the persistence-time distribution to the species-area
relationship. We conclude that the inherent coherence obtained
between spatial and temporal macro-ecological patterns points at a
seemingly general feature of the dynamical evolution of ecosystems.
|
12:10-12:30 | Piero Olla - ISAC CNR Cagliari
Activation of phytoplankton blooms by seasonal forcing
and demographic noise. ![]()
Population models, such as those for plankton dynamics,
are often based on a mean-field approximation of individual
behaviors. A weakly stable mean-field configuration, however,
can be destabilized by demographic noise. In certain cases,
such destabilization persists even in the thermodynamic limit.
It is shown how this effect can be exploited, in a simple
predator-prey model, to produce behaviors similar to algal
blooms.
|
12:30-12:50 | Lucia Pettinato, Università di Firenze
Spectral methods for DNA promoter analysis ![]()
Joining two different spectral methods, we are allowed to arrange the
promoters of a given species in equivalence classes, each of which is
found to be characterized by particular regular subsequences.
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12:50-14:40 | lunch break |
14:40-15:20 | Bernardo Spagnolo, Università di Palermo
Fluctuations and nonlinearity in classical and quantum systems ![]()
The role of interplay between noise sources and nonlinearity is
investigated in three different
classical and quantum systems.
(i) The role of a non-Gaussian Lévy noise on the nonlinear
transient
dynamics of a short
overdamped Josephson junction is analyzed. The mean escape time of the
junction is investigated
considering Gaussian, Cauchy-Lorentz and
Lévy-Smirnov probability
distributions of the noise
signals. In these conditions we find resonant activation and the first
evidence of noise enhanced
stability in a metastable system in the presence of
Lévy
noise. For
Cauchy-Lorentz noise source,
trapping phenomena and power law dependence on the noise intensity are
observed.
(ii) The phenomena of dissonance and consonance in a simple auditory
sensory model composed
of three neurons are considered. Two of them, here so-called sensory
neurons, are driven by noise
and subthreshold periodic signals with different ratio of frequencies, and
its outputs plus noise are
applied synaptically to a third neuron, so-called interneuron. We propose
a theoretical analysis
with a probabilistic approach to investigate the interspike intervals
(ISI) statistics of the spike train
generated by the interneuron. We find that at the output of the
interneuron, inharmonious signals
give rise to blurry spike trains, while the harmonious signals produce
more regular, less noisy, spike
trains. Theoretical results are compared with numerical simulations.
(iii) Finally the dynamics of a quantum particle subject to an asymmetric
bistable potential and
interacting with a thermal reservoir is investigated. We obtain the time
evolution of the population
distributions in the position eigenstates of the particle, for different
values of the coupling strength
with the thermal bath. The calculation is carried out by using the
Feynman-Vernon functional under
the discrete variable representation.
|
15:20-15:40 | Giuseppe Luca Celardo, Università Cattolica di Brescia
Superradiance Transition in Photosynthetic Light-Harvesting Complexes ![]()
We investigate the role of long-lasting quantum coherence in the
efficiency of energy transport at room temperature in
Fenna-Matthews-Olson photosynthetic complexes. The dissipation due to
the coupling of the complex to a reaction center is analyzed using an
effective non-Hermitian Hamiltonian. We show that, as the coupling to
the reaction center is varied, the maximum efficiency in energy
transport is achieved at the superradiance transition, characterized
by a segregation of the imaginary parts of the eigenvalues of the
effective non-Hermitian Hamiltonian. This approach allows one to study
various couplings to the reaction center. We show that the maximal
efficiency at room temperature is sensitive to the coupling of the
system to the reaction center.
|
15:40-16:00 | Luca Guido Molinari - Università di Milano
Identities and inequalities for Transfer Matrices ![]()
General properties are presented for transfer matrices
originating from the eigenvalue equation of Hamiltonian matrices
with band structure (for example: tight binding model for crystals,
Anderson model for localization). Their eigenvalues are linked by
a spectral identity. A general statement by Demko, Moss and Smith
on the exponential decay of matrix elements of the inverse of a
band matrix, translates into statements on the matrix elements
and singular values of transfer matrices.
|
16:00-16:20 | Stefano Mostarda - Freiburg Institute of Advanced Studies
Complex network analysis of quantum transport ![]()
Quantum transport is deeply influenced by interference. When considering
excitation propagation through a quantum multi-body system, interference
is determined by the spatial
disposition of the components.
Notwithstanding, a clear link between structure and fast, efficient transport is still missing. Here, we present a complex network analysis of quantum transport to elucidate the relationship between efficiency and structural organisation. Our analysis reveals a well defined classification related to the dynamical behaviour. |