Abstract

You are standing in the Sahara , and far away an avalanche occurs -- would you slowly sink into the sand? The classical answer would be "no": solid and flowing granular media are easily distinguished. This may not always be the case, however. In recent experiment we have locally sheared and stirred a layer of sand and probed its mechanical response in far-away regions where the material remains stationary. In contrast to its solid-like appearance, an intruder placed into these far-away zones sinks into the sand -- it is thus entirely possible that one would sink into the Sahara under the action of far away flows, albeit very slowly.

I'll discuss how to characterize this state, what causes this behavior, and what the consequences of our observations are for our under-standing of granular flows.