Mechanics of fire ant aggregations: An experimental study of active matter
Tennenbaum, Michael Jacob
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Fire ant aggregations are inherently active materials. Each ant converts its own stored energy into motion, and it is these motions that contribute to the bulk material properties of the aggregation. However, the level of activity is not constant in time. This allows us measure the material properties of this active material at different activity levels. As the aggregation changes in time we monitor the changes in activity through the mechanics of the aggregation, the normal force, and by looking at a 2D system. We find that the frequency response of active aggregations at low volume fraction is similar to that of critical gels whereas inactive aggregations are frequency independent. At high volume fraction active ants lose their frequency dependence. We construct a model of the level of activity that is dependent on the number of currently active ants in the system. The activity also affects how homogeneous the system is which we see from the 2D system. Active aggregations are more homogeneous than inactive aggregations. We apply large amplitude oscillations to the aggregation and find that at large strain amplitude the effect of the activity is washed out. In that case there is no difference in the level of viscous or elastic nonlinearity with activity. At intermediate strain amplitudes we do see an effect but only in the viscous nonlinearity. We attribute this to linking/unlinking events taking place in the aggregation. The elastic nonlinearity is dependent on the rate at which linking and unlinking events occur but the level of viscous nonlinearity is related not to the rate but to the number of linking/ unlinking events. This number increases when changing from an inactive to an active aggregation and when increasing the effective volume fraction. Activity also affects how the aggregation responds to applied stresses, at low applied stresses active ants flow in the direction of the applied stress but at intermediate applied stresses, active ants resist the applied stress. At high applied stress the aggregation is not able to resist the applied stress and flows like a simple liquid. If instead of a stress, a strain rate is applied we see no difference between a live and a dead aggregation. We find that the viscosity of the aggregation shear thins massively. Overall, we learn that the level of activity affects the mechanics of the system. The activity level changes naturally in ant aggregations in time which allows us to measure the mechanics at different activity levels. We show that it is possible to overwhelm an active system through external perturbations such that the activity does not play a role in the mechanics. However, within the realm where the activity plays a role, it can be used to tune the material properties of the system without changing the structure of the system itself.