Event

Experiments with fire-ant columns reveal similarities and differences with granular columns. For granular columns, we will show that narrowing the column diameter can result in novel behavior relative to how we think of wider columns, which were studied more than a century ago by German engineer Janssen; the key is the force-chain structure. We will then show that fire-ant columns also exhibit force chains; however, as an active granular system, there are apparent fluctuations and activity within the system. In 2D, these effects become most noticeable and manifest in the form of activity waves that propagate along the vertical direction. Ultimately, these waves reflect the existence of activity cycles, whereby the fire-ant system changes from “active”, where all the ants move, to “inactive”, where a large fraction of the ants cluster and remain stationary. Our findings indicate that the “inactive” states can be thought of resulting from a motility-induced attraction reflecting the social interactions between the ants.