Biomedical subjects

G D'Anna

Publications and source records attributed to G D'Anna.

13 recordsLinked to original sources

PubMed2003

Observing brownian motion in vibration-fluidized granular matter.

Observation of the rotational brownian motion of a very fine wire immersed in a gas led to one of the most important ideas of equilibrium statistical mechanics. Namely, the many-particle problem of a large number of molecules colliding with the wire can be represented by just two macroscopic parameters: viscosity and temperature. Interest has arisen in the question of whether this idea (mathematically developed in the Langevin model and the fluctuation-dissipation theorem) can also be used to describe systems that are far from equilibrium. Here we report an experimental investigation of an archetypal non-equilibrium system, involving a sensitive torsion oscillator immersed in a granular system of millimetre-size grains that are fluidized by strong external vibrations. The vibro-fluidized granular medium is a driven environment, with continuous injection and dissipation of energy, and the immersed oscillator can be seen as analogous to an elastically bound brownian particle. By measuring the noise and the susceptibility, we show that the experiment can be treated (to a first approximation) with the equilibrium formalism. This gives experimental access to a granular viscosity and an effective temperature; however, these quantities are anisotropic and inhomogeneous. Surprisingly, the vibro-fluidized granular matter behaves as a 'thermal' bath satisfying a fluctuation-dissipation relation.

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PubMed2001

Vogel-Fulcher-Tammann-type diffusive slowdown in weakly perturbed granular media.

We study the process by which a perturbed granular medium of millimetric-size particles reaches a "frozen" static configuration by decreasing perturbation intensity. The granular system is perturbed by isolated taps or by continuous vibrations. The granular process is observed by an immersed oscillator, which is used as a low-frequency "thermometer." A diffusive noise is seen until well below the fluidization limit, and the approach to the frozen configuration arises according to a modified Vogel-Fulcher-Tammann (VFT) behavior. As a function of an empirical control parameter with unit of time one recovers a standard VFT form.

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PubMed2001

The jamming route to the glass state in weakly perturbed granular media.

It has been suggested that a common conceptual framework known as 'jamming' (refs 1 and 2) may be used to classify a wide variety of physical systems; these include granular media, colloidal suspensions and glass-forming liquids, all of which display a critical slowdown in their dynamics before a sudden transition to an amorphous rigid state. Decreasing the relevant control parameter (such as temperature, drive or inverse density) may cause geometrical constraints to build up progressively and thus restrict the accessible part of the system's phase space. In glass-forming liquids (thermal molecular systems), jamming is provided by the classical vitrification process of supercooling, characterized by a rapidly increasing and apparently diverging viscosity at sufficiently low temperatures. In driven (athermal) macroscopic systems, a similar slowdown has been predicted to occur, notably in sheared foam or vibrated granular media. Here we report experimental evidence for dynamic behaviour, qualitatively analogous to supercooling, in a driven granular system of macroscopic millimetre-size particles. The granular medium is perturbed by isolated tapping or continuous vibration, with the perturbation intensity serving as a control parameter. We observe the random deflection of an immersed torsion oscillator that moves each time the grains rearrange, like a 'thermometer' sensing the granular noise. We caution that our granular analogy to supercooling is based on similarities in the dynamical behaviour, rather than quantitative theory.

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PubMed2001

Vibration-induced jamming transition in granular media.

The quasistatic frequency response of a granular medium is measured by a forced torsion oscillator method, with forcing frequency f(p) in the range 10(-4) Hz to 5 Hz, while weak vibrations at high-frequency f(s), in the range 50 Hz to 200 Hz, are generated by an external shaker. The intensity of vibration Gamma is below the fluidization limit. A loss factor peak is observed in the oscillator response as a function of Gamma or f(p). In a plot of ln f(p) against 1/Gamma, the position of the peak follows an Arrhenius-like behavior over four orders of magnitude in f(p). The data can be described as a stochastic hopping process involving a probability factor exp(-Gamma(j)/Gamma) with Gamma(j) a f(s)-dependent characteristic vibration intensity. An f(s)-independent description is given by exp(-tau(j)/tau), with tau(j) an intrinsic characteristic time, and tau=Gamma(n)/2pif(s), n=0.5-0.6, an empirical control parameter with unit of time. tau is seen as the effective average time during which the perturbed grains can undergo structural rearrangement. The loss factor peak appears as a crossover in the dynamic behavior of the vibrated granular system, which, at the time scale 1/f(p), is solid-like at low Gamma, and the oscillator is jammed into the granular material, and is fluid-like at high Gamma, where the oscillator can slide viscously.

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PubMed2000

Critical dynamics of burst instabilities in the Portevin-Le Ch atelier effect.

We investigate the Portevin-Le Châtelier effect (PLC), by compressing Al-Mg alloys in a very large deformation range, and interpret the results from the viewpoint of phase transitions and critical phenomena. The system undergoes two dynamical phase transitions between intermittent (or "jerky") and "laminar" plastic dynamic phases. Near these two dynamic critical points, the order parameter 1/tau of the PLC effect exhibits large fluctuations, and "critical slowing down" (i.e., the number tau of bursts, or plastic instabilities, per unit time slows down considerably).

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PubMed2000

Mechanical properties of granular media, including snow, investigated by a low-frequency forced torsion pendulum

The oscillating probe of a low-frequency forced torsion pendulum is immersed into various granular media, such as natural sand, glass beads, and granular snow. A first layer of particles is in general solidly bound to the probe surface. The principle of operation and a rheological model are presented. The measured dynamic moduli systematically show a peak of the loss factor and a step in the absolute modulus. The effect of moisture-induced aging in glass beads of small size and the effect of sintering of ice grains in snow are investigated. The response of the pendulum is determined by the long-range statistical properties of force chains opposing the rotation of the pendulum, and by the tribological processes that take place at the grain contacts.

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PubMed1996