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Randomness and disorder in materials can play a major role in determining their properties.

A polycrystal modelled through the Potts model; different colours correspond to the different domains.

Pure single crystals in nature are very rare (e.g. diamonds are very precious) and most of materials are polycrystals, amorphous or glasses. In addition, they can feature in the form grains instead of a bulk of matter. This often translates into the displaying of strong fluctuations of the material response to external requests.

In order to understand how disorder acts in determining materials' properties even simple experiments and models can be very fruitful, especially when complemented with concepts and tools of statistical physics.

A spring model for the fracture of a disorder solid. More stressed regions are in red, less stressed in blue.

In a homogeneous body a uniform load applied to a surface (the upper one in the figures) generates a regular and quickly decaying deformation field, like in the left picture. When inhomgeneities are present, the field becomes irregular and penetrates deeper in the medium. The right picture represents the deformation field  in a lattice where 10% of the sites, have an elastic modulus 2/3 smaller than the others. The defects are placed at random.

Laws of thermodynamics do not prevent the possibility of extracting work from systems that are isothermic and in a stationary state as long as these are not at equilibrium. These states require something similar to a spontaneous breaking of symmetry and these pertain to the realm of Brownian motors.

Macroscopic realizations have been developed in the field of granular mechanics, a field of great relevance both industrially [due to the ubiquity of granular materials (GMs)] and theoretically (for the challenging properties and behavior generally exhibited by GMs). In our laboratory, in collaboration with R. Balzan and V. Loreto from La Sapienza, we realised an experimental apparatus (Balzan et al., PRE 83,031310 (2011)) in which a ratchet probe is immersed in a vibrated dense granular, in order to study how chaotic granular motion can indeed propel an asymmetric probe in a persistent direction. More recently, in collaboration with A. Gnoli (ISC Tor Vergata) and A. Puglisi (ISC La Sapienza) a gas granular version of the experiment has been realised and is currently under investigation. See the movie.

A new realization of a granular ratchet has been completed in collaboration with A. Puglisi and A. Gnoli at the Physics Dept. of the University La Sapienza