Formation and mechanics of fire ant rafts as an active self-healing membrane
The unique ability of fire ants to form a raft to survive flooding rain has enchanted biologists as well as researchers in other disciplines. It has been established during the last decade that an aggregation of fire ants exhibits viscoelasticity with respect to external compression and shearing among numerous unusual mechanical properties. In addition to clarifying that the Cheerios effect is neither sufficient nor essential for the ant raft, we perform the force-displacement and creep experiments on the ant raft and concentrate on unearthing properties that derive from the unique combination of self-healing and activeness of its constituent. Varying pull speed results in distinct mechanical responses and fracture patterns, characteristic of ductile and brittle material. By image processing, we count the number of ants that actively participate in the stress-strain relation and determine their orientation to map out the force chain. The latter information reveals that the pull force expedites the alignment of fire ants, in analogy to the effect of an electric field on liquid crystal polymers. In addition, the raft can be tailored not to transversely deform in response to the axial strain. Without resorting to specific geometry structures, this property of zero Poisson's ratio is enabled by the active recruitment of ants from the top to bottom layer to keep the raft from disintegrating. Furthermore, effective Young's modulus can also be customized and is proportion to either the raft length or its inverse, depending on whether the raft is in the elastic or plastic region.