Biomedical subjects

Mario Nicodemi

Publications and source records attributed to Mario Nicodemi.

9 recordsLinked to original sources

PubMed2006

Thermodynamics and statistical mechanics of dense granular media.

By detailed molecular dynamics and Monte Carlo simulations of a model system we show that granular materials at rest can be described as thermodynamics systems. First, we show that granular packs can be characterized by few parameters, as much as fluids or solids. Then, in a second independent step, we demonstrate that these states can be described in terms of equilibrium distributions which coincide with the statistical mechanics of powders first proposed by Edwards. We also derive the system equation of state as a function of the "configurational temperature," its new intensive thermodynamic parameter.

Journal Article↗

PubMed2006

Dynamically induced effective interaction in periodically driven granular mixtures.

We show that a granular mixture subject to horizontal oscillations can be reduced to a monodisperse system of particles interacting via an effective interaction. This interaction is attractive at short distances and strongly anisotropic, and its features explain the system rich phenomenology, including segregation and stripe pattern formation. Finally, we show that a modified Cahn-Hilliard equation, which takes into account the characteristics of the effective interaction, is capable of describing the dynamics of the mixture.

Algorithms↗

PubMed2006

Granular species segregation under vertical tapping: effects of size, density, friction, and shaking amplitude.

We present extensive molecular dynamics simulations on species segregation in a granular mixture subject to vertical taps. We discuss how grain properties, e.g., size, density, friction, as well as shaking properties, e.g., amplitude and frequency, affect such a phenomenon. Both the Brazil nut effect (larger particles on the top, BN) and the reverse Brazil nut effect (larger particles on the bottom, RBN) are found and we derive the system comprehensive "segregation diagram" and the BN to RBN crossover line. We also discuss the role of friction and show that particles which differ only for their frictional properties segregate in states depending on the tapping acceleration and frequency.

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PubMed2005

Shear instabilities in granular mixtures.

Dynamical instabilities in fluid mechanics are responsible for a variety of important common phenomena, such as waves on the sea surface or Taylor vortices in Couette flow. In granular media dynamical instabilities have just begun to be discovered. Here we show by means of molecular dynamics simulation the existence of a new dynamical instability of a granular mixture under oscillating horizontal shear, which leads to the formation of a striped pattern where the components are segregated. We investigate the properties of such a Kelvin-Helmholtz-like instability and show how it is connected to pattern formation in granular flow and segregation.

Complex Mixtures↗

PubMed2005

Slow relaxation and compaction of granular systems.

Granular materials are of substantial importance in many industrial and natural processes, yet their complex behaviours, ranging from mechanical properties of static packing to their dynamics, rheology and instabilities, are still poorly understood. Here we focus on the dynamics of compaction and its 'jamming' phenomena, outlining recent statistical mechanics approaches to describe it and their deep correspondence with thermal systems such as glass formers. In fact, granular media are often presented as ideal systems for studying complex relaxation towards equilibrium. Granular compaction is defined as an increase of the bulk density of a granular medium submitted to mechanical perturbation. This phenomenon, relevant in many industrial processes and widely studied by the soil mechanics community, is simple enough to be fully investigated and yet reveals all the complex nature of granular dynamics, attracting considerable attention in a broad range of disciplines ranging from chemical to physical sciences.

Journal Article↗

PubMed2004

Glass-glass transition and new dynamical singularity points in an analytically solvable p-spin glasslike model.

We introduce and analytically study a generalized p-spin glasslike model that captures some of the main features of attractive glasses, recently found by mode coupling investigations, such as a glass-glass transition line and dynamical singularity points characterized by a logarithmic time dependence of the relaxation. The model also displays features not predicted by the mode coupling scenario that could further describe the attractive glasses behavior, such as aging effects with new dynamical singularity points ruled by logarithmic laws or the presence of a glass spinodal line.

Journal Article↗

PubMed2004

Segregation in fluidized versus tapped packs.

We compare the predictions of two different statistical mechanics approaches, corresponding to different physical measurements, proposed to describe binary granular mixtures subjected to some external driving (continuous shaking or tap dynamics). In particular we analytically solve at a mean field level the partition function of a simple hard sphere lattice model under gravity and focus on the phenomenon of size segregation. We find that the two approaches lead to similar results and seem to coincide in the limit of very low shaking amplitude. However, they give different predictions of the crossovers from Brazil nut effect to reverse Brazil nut effect with respect to the shaking amplitude, which could be detected experimentally.

Journal Article↗

PubMed2003

Peak effect in a driven lattice gas model.

We study the peak effect (PE), i.e., a sharp peak observed in the critical current as a function of the particle density, discovered in transport properties of a driven lattice gas model. We show that the PE corresponds to a first-order phase transition found in the undriven system at equilibrium, which in turn gives rise to an "anomalous" second peak in magnetic hysteresis loops. We also explain the "history" dependent phenomena observed in the PE region by investigating the system characteristic time scales, which diverge at low T and have a broad maximum as a function of the external field around the PE. The model we consider can be related to a coarse grained description of vortex lines in superconductors and we discuss the relations of the PE described here and the one experimentally observed in these systems.

Journal Article↗

PubMed2002

Thermodynamics and statistical mechanics of frozen systems in inherent states.

We discuss a statistical mechanics approach in the manner of Edwards to the "inherent states" (defined as the stable configurations in the potential energy landscape) of glassy systems and granular materials. We show that at stationarity the inherent states are distributed according a generalized Gibbs measure obtained assuming the validity of the principle of maximum entropy, under suitable constraints. In particular, we consider three lattice models (a diluted spin glass, a monodisperse hard-sphere system under gravity, and a hard-sphere binary mixture under gravity) undergoing a schematic "tap dynamics," showing via Monte Carlo calculations that the time averages of macroscopic quantities over the tap dynamics and over such a generalized distribution coincide. We also discuss about the general validity of this approach to nonthermal systems.

Journal Article↗