Large Amplitude Oscillatory Shear measurements of fire ant aggregations

Michael Tennenbaum and Alberto Fernandez-Nieves

We have previosly investigated the mechanics of fire ant aggregations using oscillatiory rheology in the linear regime. We are currently working on extending our understanding into the non-linearities of the mechanics. To do this we employ Large Amplitude Oscillatory Shear (LAOS) experiments.
In oscillatory rheology a sinusoidal strain $$\gamma(t) = \gamma_0 sin(\omega t)$$ is applied and the resulatnt stress, $$\sigma(t)$$ is measured. In the linear regime the stress is also sinusoidal, however in the on-linear regime this is not the case, figure 1.

Fig. 1: a) Stress and strain curves in the linear regime. b) stress and strain curves in the non-linear regime.

To see how the waveforms change with strain and frequency we can plot the stres curve against the strain curve to get Lissajous curves, figure 2.

Fig. 2: a) Stress plotted against strain to obtain the Lissajous curve.

We can do this test for a range of strains and frequencies and volume fractions. One volume fraction is shown in figure 3.

Fig. 3: Lots of Lissajous curves for a range of strains and frequencies.

So far we can see strain stiffening behavior at high strains and frequencies. We are working on caracterizing this behavior furthur and elucidating how this behavior arrises from the individual ants behavior.

References:
[1] M. Tennenbaum, Z. Liu, D. Hu, and A. Fernandez-Nieves, Nature materials 15, 54-59 (2016)
[2] R.H. Ewoldt, A. Hosoi, and G.H. McKinley, Journal of Rheology 52, 1427-1458 (2008)

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