Jannsen’s effect in 3D ant columns

When a liquid fills a cylindrical container, the pressure at the bottom increases linearly with height; we all experience this when we dive in the sea. In contrast, when a granular material fills a cylindrical container, this linearity is lost at a certain height. In this case, the pressure eventually saturates to a constant value due to the presence of frictional forces and the formation of force chains. Interestingly, for sufficiently narrow containers, these forces can be compressive rather than supportive for a range of heights, resulting in a pressure at the bottom that is larger than the fluid pressure .. We find that if fire ants, Solenopsis Invictae, are used instead of grains, this overshoot in pressure is washed away. Remarkably, we still observe the saturation typical of granular matter, indicating that ant collectives still form force chains to support their weight for sufficiently tall columns. These chains, however, form and break, reflecting the active character of the system.