Modeling ant pontoon bridging using a robophysical active matter system*

Laboratory experiments demonstrate that when groups of fire ants [S. invicta] detect food located on the surface of water a short distance from a rigid boundary, the ants self-assemble into a floating pontoon bridge, allowing the colony to reach the food. The bridge grows from the boundary via a complex interplay of individual water walking, collective clumping and edge connection. Once formed, the ants rapidly travel across the bridge to and from the food source. To begin to robophysically model the bridge formation behavior we use BOBbots [Li et al, Sci. Adv. 2021] an active matter system composed of 6 cm diameter disc-shaped robots which move via vibratory motors and connect via loose magnets. To systematically determine individual behavioral rules of the BOBbots leading to the emergent bridge-building behavior, we first test bridge-building in a Discrete Element Method simulation of the BOBbots. Simulations qualitatively replicate aspects of the bridge formation dynamics including slow growth from a boundary and stochastic clumping and aggregation. Study of the directed aggregation (bridging) effect revealed by the simulation gives insight into relevant ant behaviors which contribute to formation of these on-demand biological pontoon bridges.