  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.
