Collective Motion of Vibrated Polar Disks

We experimentally study a monolayer of vibrated disks with a built-in polar asymmetry which enables them to move quasi-balistically on a large persistence length. Alignment occurs during collisions as a result of self-propulsion and hard core repulsion. Varying the amplitude of the vibration, we observe the onset of large-scale collective motion and the existence of giant number fluctuations with a scaling exponent in agreement with the predicted theoretical value.Comment: 4 pages, 4 figure

Macroscopic chaos in globally coupled maps

We study the coherent dynamics of globally coupled maps showing macroscopic chaos. With this term we indicate the hydrodynamical-like irregular behaviour of some global observables, with typical times much longer than the times related to the evolution of the single (or microscopic) elements of the system. The usual Lyapunov exponent is not able to capture the essential features of this macroscopic phenomenon. Using the recently introduced notion of finite size Lyapunov exponent, we characterize, in a consistent way, these macroscopic behaviours. Basically, at small values of the perturbation we recover the usual (microscopic) Lyapunov exponent, while at larger values a sort of macroscopic Lyapunov exponent emerges, which can be much smaller than the former. A quantitative characterization of the chaotic motion at hydrodynamical level is then possible, even in the absence of the explicit equations for the time evolution of the macroscopic observables.Comment: 24 pages revtex, 9 figures included. Improved version also with 1 figure and some references adde

Clustering data by inhomogeneous chaotic map lattices

A new approach to clustering, based on the physical properties of inhomogeneous coupled chaotic maps, is presented. A chaotic map is assigned to each data-point and short range couplings are introduced. The stationary regime of the system corresponds to a macroscopic attractor independent of the initial conditions. The mutual information between couples of maps serves to partition the data set in clusters, without prior assumptions about the structure of the underlying distribution of the data. Experiments on simulated and real data sets show the effectiveness of the proposed algorithm.Comment: 8 pages, 6 figures. Revised version accepted for publication on Physical Review Letter

Control of silver-polymer aggregation mechanism by primary particle spatial correlations in dynamic fractal-like geometry

Silver nanocrystals have been prepared by reacting silver nitrate with ascorbic acid in aqueous solution containing a low concentration of a commercial polynaphtalene sulphonate polymer (Daxad 19). Various crystalline morphologies have been obtained simply by tuning the reaction temperature. We have investigated the nanoparticle formation mechanism at three different temperatures by in situ and time resolved Small Angle X ray Scattering measurements. By modeling the scattering intensity with interacting spherical particles in a fractal-like polymer-Ag matrix, we found signatures of nucleation, growth and assembly of primary particles of about 15-20 nm. We observed how the time evolution of both spatial correlations between primary particles and the dynamic fractal geometry of the polymer-Ag matrix could influence and determine both the aggregation mechanism and the morphology of forming nanostructures in solution