Spatial, dynamical and spatiotemporal heterogeneities in granular media

We provide an overview of some recent results on heterogeneities in granular media; an attempt is made, in keeping with the review nature of this article, to include both experimental and theoretical results on this emergent topic. In the context of spatial heterogeneity, we first discuss bridges, which are one of its most ubiquitous and widely studied instances; we next mention work on force chains, which have also attracted a lot of recent interest, and discuss possible connections between force chains and bridges. Next, we remark on the widespread prevalence of dynamical heterogeneities in a wide range of soft matter systems, ranging from granular media through pastes to glasses. We review in detail two experiments of recent interest, both of which suggest that analogies among such systems can be usefully exploited to learn more about each one. Finally, we review a model of spatiotemporal inhomogeneities, where temporal heterogeneities occur at distinct spatial points in a granular system, and in particular examine its relevance to recent experiments.