Abstracts


Jeremie Bec
Rapid growth of large aggregates by correlated coalescences in turbulent flow

The time evolution of a population undergoing coalescence or aggregation is classically adressed in terms of the Smoluchowski coagulation equation. This mean-field model is used to describe a broad range of
processes ranging from polymerization, emulsification and flocculation to cloud droplet growth and planet formation. It relies on the assumption that successive mergers are uncorrelated from each other. We
report evidences that such an approach fails when the coagulating species are dilute and transported by a turbulent flow. The Lagrangian motion involves correlated violent events that lead to an unexpected
fast growth of the largest particles.



Sergio Chibbaro
Fluctuations in Lagrangian convective turbulence

We investigate the fluctuation behaviour of a macroscopic system far from equilibrum: Rayleigh-Bénard Turbulence.
The fluid flow is simulated via the numerical solution of Navier-Stokes equations coupled with the temperature. The statistical observables are calculated tracking lagrangian tracers.
First, results about velocity and temperature are compared with similar experiments. Then, lagrangian structures functions are computed, showing large deviations from gaussianity,
which can be described within the multifractal formalism. Finally, fluctuation relations are considered, studying the behaviour of the local heat current, which hugely fluctuates, allowing for positive and negative events.




Ralf Eichhorn 
Separation of chiral colloidal particles in micro-flows

We study the motion of chiral colloidal particles in microfluidic devices. Using computer simulations and analytical
arguments, we reveal that the particles migrate with chirality-specific average velocities through a microfluidic
channel, provided the driving flow breaks the mirror symmetry. Experimentally, we exploit this effect to separate
micron-sized chiral particles in a helical fluid flow which is created inside a microfluidic device patterned with
slanted grooves. The experimental results are confirmed by numerical calculations, which demonstrate how the coupling of
rotational and translational degrees of freedom leads to differences in the trajectories of particles with opposite
chirality. Such an effect may be of practical relevance for the separation of chiral molecules.



Carlo Guardiani
An Integrated Computational Approach to characterize the unfolding
pathway of Maltose Binding Proteins.

Recent single-molecule force spectroscopy experiments on the Maltose
Binding Proteins (MBPs) identified four stable structural units, termed
unfoldons, that resist mechanical stress and determine the intermediates
of the unfolding pathway. The topological origin and the dynamical role
of the unfoldons were analyzed using an integrated approach which combines
a graph-theoretical analysis of the interaction network of the MBP
native-state with steered molecular dynamics simulations. The topological
analysis of the native state, while revealing the structural nature of the
unfoldons, provides a framework to interpret the MBP mechanical unfolding
pathway. Indeed, the experimental pathway can be effectively predicted by
means of molecular dynamics simulations with a simple topology-based
and low-resolution model of the MBP. The results obtained from the
coarse-grained approach are confirmed and further refined by all-atom
molecular dynamics.




Stefano Lepri
Anomalous fluctuations in nonlinear oscillator chains

One-dimensional lattices of nonliner oscillators often display anomalous energy diffusion and transport. This leads to a breakdown of macroscopic transport relations like e.g. the Fourier law.
Recently, it has been argued that such anomalous fluctuations generically belongs to the universality class of the (seemingly unrelated) Kardar-Parisi-Zhang equation. We will present some
simulation data that support this predictions.



Roberto Livi
Dynamical phases in HMF dynamics of neural networks
Abstract: The computational advantage of using the
Heterogeneos Mean Field (HMF) approximation for simulating
the dynamics of large dense random networks of inhibitory and
excitatory neurons is discussed. We also show that the HMF
method allows to solve the global inverse problem of recovering
the basic information about the network topology from the
average synaptic activity field.



Simone Pigolotti
Statistical mechanics of lowest unique bid auctions

In lowest unique bid auctions, a population of players has
to choose a natural number. The player picking the lowest unique
number wins the auction. I will discuss how the Nash equilibrium for
this game can be computed by an analogy with a physical system in the
macro-canonical ensemble. By comparing the analytical solution with
real data, I will show how population of players can reproduce the
Nash equilibrium surprisingly well, provided that the number of
players is not too large. To explain the lack of adaptation of large
population, I will present a dynamical model in which the population
is kept outside of the Nash equilibrium by a turnover between
experienced and naive players. 

S. Pigolotti, S. Bernhardsson, J. Juul, G. Galster, and P. Vivo,
"Equilibrium strategy and population-size effects in lowest unique bid
auctions", Phys. Rev. Lett. 108, 088701, 2012

J. Juul, A. Kianercy, S. Bernhardsson, and S. Pigolotti, "Replicator
dynamics with turnover of players", Phys. Rev. E, 88, 022806, 2013.




Matteo Polettini
The stochastic thermodynamics of efficiency fluctuations: Exact results and perspectives

Efficiency quantifies how worth a local gain at the expense of a global loss is. In the early times of the industrial revolution, the question of the efficiency of steam machines gave birth to thermodynamics
as a science. While large thermodynamic machines work in a typical manner, the behavior of small machines, e.g. at the molecular scale, can vary dramatically from realization to realization, and thus their
efficiencies might fluctuate. I will introduce the basic ideas of a theoretical framework, called Stochastic Thermodynamics, that describes the thermodynamics of small systems subject to fluctuations, with
special emphasis on recent developments in the study of efficiency statistics. The theory of efficiency fluctuations is enormously richer than the macroscopic thermodynamic one. In particular, I will argue
that fluctuations might enhance efficiency towards its Carnot limit at nonequilibrim phase transition.



Antonio Prados
Pulling experiments of biomolecules: what can be learnt from simple
models

In recent years, experimental techniques that make it possible to
manipulate biomolecules one by one have been devised. For example,
atomic force microscopy has been employed to investigate the
elasticity of modular proteins, which are artificially constructed
proteins comprising a certain number (8-12) of identical protein
domains. In these elasticity experiments, one of the ends of the
molecule is usually kept fixed whereas the other end is pulled.  There
are two main experimental situations: (a) length-controlled
experiments, in which the end-to-end distance of the biomolecule is
the externally controlled parameter and (b) force-controlled
experiments, when it is the applied force that is monitored.

A typical outcome of these experiments is the force-extension curve
(FEC): the force needed to stretch the biomolecule is recorded as a
function of its length. The FEC of modular proteins typically displays
a sawtooth pattern in the length-controlled situation: the unfolding
of the different units that constitute the polyprotein is accompanied
by a drop of the force. Moreover, the force at which the unfolding
takes place, which is a measure of the mechanical stability of the
polyprotein, increases with the stretching rate. 

There are also protein domains that are composed of several stable
structural units. The unfolding pathway is, basically, the order and
the way in which these "unfoldons" of the domain unravel, and
it depends on the pulling speed.  Consistently with the physical
intuition, the weakest unfoldon opens first at low pulling
rates. Nevertheless, at higher rates, the first to unfold is not the
weakest but the pulled one.

In this talk, we will discuss how some key aspects of these pulling
experiments can be understood by using very simple models. Basically,
we model each unit of the biomolecule as a "particle" in a bistable
potential, which follows an overdamped Langevin equation. Firstly, for
the analysis of the force-extension curve, the units are independent
except for the global constraint given by the length-control
condition. Secondly, we study the dependence of the unfolding pathway
on the pulling speed. Therein, it is essential that we should take
into account the spatial structure of the molecule, which introduces
additional couplings among the units.



S. Olmi, E.A. Martens, S. Thutupalli, A. Torcini
Intermittent chaotic chimeras for coupled rotators

Two symmetrically coupled populations of N rotators, oscillators with inertia m, display chaotic solutions with broken symmetry similar to the ones recently observed in experiments with mechanical pendula
[1].  We focus on chaotic chimeras, where one population is chaotic and the other fully synchronized. These states have finite life-times diverging as a power-law with N and m and an intermittent dynamics between
laminar and turbulent phases. Turbulence prevails in the thermodynamic limit. Finite size Lyapunov analyses reveal spectral properties characteristic of globally coupled chaotic systems [2].
 [1] Martens, E. A., Thutupalli, S., Fourrière, A., & Hallatschek, O.,
     "Chimera states in mechanical oscillator networks", Proceedings of the    National Academy of Sciences, 110, 10563-10567. (2013)
 [2] K. A. Takeuchi, H. Chatè, F. Ginelli, A. Politi, and A. Torcini,
     "Extensive and Sub-Extensive Chaos in Globally-Coupled Dynamical Systems", Phys. Rev. Lett. 107, 124101 (2011)

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