Experiments on granular materials

Even if the initial motivation for the study of granular materials was the large number of problems encountered in industrial processes, the experimental approach to the granular state has not been reduced to a mere reproduction of applicative situations: the strong theoretical questions arisen in the last two decades have guided the experiments toward more refined and often simpler, more essential, set-ups which could shed light on new fundamental mechanisms. Numerical work has been invaluable as a source of new problems and interesting questions (for example molecular dynamics simulations of diluted granulars systems have stimulated the measurement of velocity distributions, predicting a non-Maxwellian behavior), while at the same time some phenomena observed in experiments have led to the birth of new theoretical paradigms (e.g. sand-pile avalanches have inspired the debate on self organized criticality).

For a long time the term granular materials has designed anything similar to sand, so that in all workshops and conferences this subject was treated as a coherent field of study with its coherent scientific community and its coherent set of questions and possible answers. This was not true and it is now recognized: there is nothing like a Universal Granular Behavior, there is probably nothing similar to a Granular Equation of State and, above all, the mechanisms that must be taken into account in order to explain some typical granular phenomena are often useless in the study of other typical granular phenomena. This means that the study and the modeling of internal stress propagation in grain silos is of little help for the investigation of surface waves or for the pattern formation in rotating drums, as well as the hydrodynamic description of dilute and quasi-elastic granular flows cannot work in more dense and inelastic situations (this fact can be less obvious). As a consequence of this, it has grown the necessity of more specialized workshops and of a better definition of granular sub-topics. One important example has been the large increase of interest in the physics of Granular Gases [182], i.e. assemblies of grains in dilute packing, characterized by the high ratio between the number of binary interactions (inelastic collisions) and that of non-binary interactions (frequent in more dense situations). Many new phenomena in this context (cluster and shear instabilities, non-Maxwellian velocities, and much more) have made their first appearance in computer simulations and then have been observed in real experiments. It must be finally underlined that also a too specialistic approach can be dangerous. An essential feature of granular systems is the strong enhancement of fluctuations and therefore their non-equilibrium character: this means for example that, even in a dilute experimental set-up, regions with very high density can appear, leading to a dramatic failure of the assumption of ``gas-like'' behavior.

In the first section we review a number of real and numerical experiments performed on granulars that come to rest in a metastable equilibrium, e.g. studies on the Janssen effect and on the distribution of internal stresses, measures of the slow compaction under vibration and finally observation of avalanches in sand piles. The second section focuses on experiments concerning the fundamental physics of granular flows, taking into account systems subject to different kinds of external driving: shear (by means of Couette rheometers, inclined channel and so on), vibration and air fluidizing are the most common setups used to study granular dynamics.