EPFL doctorate Award 2006 - Patrick Mayor

"Pour une étude expèrimentale de trés haut niveau du comportement dynamique de la matière granulaire et de son approche vers l'état statique de verre granulaire, qui a grandement contribué à éclaircir la notion fondamentale de température dans un système hors équilibre."

Fluid and Glassy Phases of Vibrated Granular Matter Studied with a Torsion Oscillator

Understanding the behaviour of granular media, either at rest or moving under external driving, is a difficult task, although it is important and of very practical interest.

The properties of shaken granular materials are investigated with an immersed torsion oscillator sensitive to the grains' motion. The resulting angular deflections are analyzed to give information on the granular agitation. The system can also be used in forced mode, allowing to study the linear response to a perturbation.

In the case of strong vibrations, the combination of both the forced and free oscillations provides a way to test the fluctuation-dissipation theorem, which is surprisingly valid, even for this complex nonequilibrium system. Thus, the motion of the oscillator is very similar to the Brownian motion of a pollen grain due to the molecular agitation of a surrounding liquid: here, the agitation is due to the grains of the "granular bath", and it is possible to introduce global parameters such as "effective temperature" and "effective viscosity"

On the other hand, for weaker vibrations (with accelerations below that of gravity), the angular fluctuations of the oscillator become very small, and the "jamming route" to the frozen static state is reminiscent of a glass transition.