Activity effects on the non-linear mechanical properties of fire ant aggregations

Individual fire ants are inherently active as they are a living organism that convert stored chemical energy into motion. However, each individual ant is not equally disposed to motion at any given time. In an active aggregation, most of the constituent ants are active, and vice versa for an inactive aggregation. Here we look at the role activity plays on the non-linear mechanical behavior of the aggregation through large amplitude oscillatory shear measurements. We find that the level of viscous non-linearity can be decreased by increasing the activity or by increasing the volume fraction. The level of elastic nonlinearity is not affected by either activity or volume fraction. Applying too large of an applied strain amplitude removes the effect of activity but preserves the effect of the volume fraction. We also compare this to viscosity measurements where we see that the level of nonlinearity decreases with activity as well.