Jeremie BecRapid growth of large aggregates by correlated coalescences in turbulent flowThe 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 ChibbaroFluctuations in Lagrangian convective turbulenceThe 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-flowsWe 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 GuardianiAn Integrated Computational Approach to characterize the unfoldingpathway 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 chainsOne-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 LiviDynamical phases in HMF dynamics of neural networksAbstract: 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 auctionsIn 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 PolettiniThe stochastic thermodynamics of efficiency fluctuations: Exact results and perspectivesEfficiency 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 PradosPulling experiments of biomolecules: what can be learnt from simplemodelsIn 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. TorciniIntermittent chaotic chimeras for coupled rotatorsTwo 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) |