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Myrmecological Rheology: A study of the material properties of Fire Ant aggregations

Mike Tennenbaum, Zhongyang Liu, David Hu, and Alberto Fernandez-Nieves

Why Fire ants?
We are investigating the properties of Fire ant aggregations because of several survival traits that fire ants display. Fire ants will join together to form bridges and towers to find food. These can be composed of thousands of ants. Fire ants will also form rafts to survive floods. Rafts can be made from up to the entire colony.

Fig. 1:
Ant raft

Fig. 2:
Ant Bridge

Fire ants display both liquid-like and solid-like behavior. In Vid. 1 a Teflon rod fall over inside a beaker containing Fire ants. This shows liquid like behavior in two ways: the ants flow around the rod as it falls over, and the ants have taken the shape of the container they are in. Both of these are properties of liquids but Fire ants are not solely a liquid.

In Vid. 2 an ant ball is compressed with a petri dish. Not only are the ants now holding their shape but when compressed with the petri dish they return to their original shape. These are the properties of an elastic solid.

Vid. 1:
Liquid aspects of the ant aggregation

Vid. 2:
Elastic aspects of the ant aggregation.

Both the Teflon rod test and the petri dish test show different aspects of the material properties of the ant aggregations. To look more closely at these different aspects of the material properties of ant aggregations we use parallel plate rheology. This allows for a more quantitative description of the solid-like and liquid-like behavior. In parallel plate rheology, a sample is placed between two plates and a strain is applied to the sample and the resulting stress is measured. With ants however, we need a bit more, otherwise the ants will walk out of the rheometer. To forestall this, we use a containment cylinder. Another modification we have to make to the rheometer in order for it to work with ants, is to add Velcro (the loop side) to both the top and bottom plate. This is so the ants can grip the top and bottom plates, to validate the no-slip condition necessary for rheology. Fig. #3 shows the rheometer setup without the containment cylinder. With both of these modifications we are able to perform rheological experiments on ants. From these we get G' and G'', the storage and loss shear moduli of the sample.

Fig. 3: Schematic of ants in the rheometer Fig. 4: Frequency sweep of live ants (red) and dead ants (black).

Fig. 4 shows the storage modulus and loss modulus as a function of frequency for live and dead ants. The dead ants are more elastic over the entire frequency range. Exhibited by a higher G' than G''. The live ants show congruence of the storage and loss modulus ver the whole range. The dead ant behavior is independent of frequency. The live also show a dependence of frequency. As frequency is increased both the storage and loss modulus increase. At high frequency, the live ant behavior approaches that of the dead ants.

Future Work
We intend to look at the effects of density on the frequency sweeps for both live and dead ants. So far these tests have all been oscillatory and in the linear regime. We plan on investigating the ant aggregation using steady state rheology and also looking into the non-linear regime.
We also want to look at the ant motions that make up the aggregations material properties. To do so we are taking videos of ant motions.

[1] Mlot, N. J., Tovey, C. A. & Hu, D. L. Fire ants self-assemble into waterproof rafts to survive floods. Proceedings of the National Academy of Sciences 108, 7669{7673 (2011).
[2] The Ant Raft
[3] Photography by Tim Nowack Photography

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Soft Condensed Matter Laboratory, School of Physics, Georgia Institute of Technology
770 State Street NW, Atlanta, GA, 30332-0430, USA
Phone: 404-385-3667 Fax: 404-894-9958
alberto.fernandez [at] physics.gatech.edu