Material Disturbance during Collective Construction with Soft Matter

In collective construction, agents like ants and termites transport matter in their environment to create structures far larger than an individual. Instead of the deterministic approach that humans and other robot swarms employ, construction by invertebrate collectives is subject to uncertainty due to challenges in transport and cohesion of soft materials, agent interactions during collisions and crowding, and material disturbances from external and internal perturbations. Using a robophysical swarm, we seek to discover principles by which collectives can mitigate such challenges and form reliable, repeatable structures composed of soft materials. We use a minimalist two-armed, two-wheeled locomotor (24 cm long) equipped with force sensitive "jaws" to detect and transport cohesive materials (staples) from one side of a dark arena (1.8 x 1.2 m) where colored LEDs. In a cycle, a robot excavates then attempts to find the largest existing pile and place its excavated material. In early experiments, a single agent identified the primary deposition pile in 21 / 28 cycles, depositing 252 g of material per hour on average, leading to an emergent mound. In contrast, two agents using this algorithm deposited at an existing pile in only 8 / 24 cycles, depositing 142 g per hour per robot on average; importantly, this deposition was scattered in the arena. The decrease in excavation and deposition efficacy was a result of robot interference with each other and with deposited material (e.g., inadvertent scattering).