The jamming route to the glasss tate 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(3), colloidal suspensions(4) and glass-formin g liquids(5), all of which display a critical slowdown in their dynamics be fore a sudden transition to an amorphous rigid state. Decreasing the releva nt control parameter (such as temperature, drive or inverse density) may ca use geometrical constraints to build up progressively and thus restrict the accessible part of the system's phase space. In glass-forming liquids (the rmal molecular systems), jamming is provided by the classical vitrification process of supercooling, characterized by a rapidly increasing and apparen tly diverging viscosity at sufficiently low temperatures(6,7). In driven (a thermal) macroscopic systems, a similar slowdown has been predicted to occu r, notably in sheared foam or vibrated granular media(8,9). Here we report experimental evidence for dynamic behaviour, qualitatively analogous to sup ercooling, in a driven granular system of macroscopic millimetre-size parti cles. The granular medium is perturbed by isolated tapping or continuous vi bration, 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(10,11). We caution that our granular analogy to supercooling is based on similarities in the dynamical behaviour, rather than quantitative theor y.