Tunable pheromone interactions among microswimmers

Indirect interactions via shared memory deposited on the field (field memory) play an essential role in collective motions. Some motile species, such as ants and bacteria, use attractive pheromones to complete many tasks. Mimicking these kinds of collective behavior at the laboratory scale, we present a pheromone-based autonomous agent system with tunable interactions. In this system, colloidal particles leave phase-change trails reminiscent of the process of pheromone deposition by individual ants, and the trails attract other particles and themselves. To implement this, we combine two phys-ical phenomena: the phase change of a Ge2Sb2Te5 (GST) substrate by self-propelled Janus particles (pheromone deposition) and the AC (alternating current) electroosmotic (ACEO) flow generated by this phase change (pheromone attraction). Laser irradiation causes the GST layer to crystalize locally beneath the Janus particles, owing to the lens heating effect. Under AC field application, the high conductivity of the crystalline trail causes a field concentration and generates ACEO flow, and we introduce this flow as an attractive interaction between the Janus particles and the crystalline trail. By changing the AC frequency and voltage, we can tune the attractive flow, i.e., the sensitivity of the Janus particles to the trail, and the isolated particles undergo diverse states of motion, from self-caging to directional motion. A swarm of Janus particles also shows differ-ent states of collective motion, including colony formation and line formation. This tunability enables a reconfigurable system driven by a pheromone-like memory field.SignificanceBesides direct interactions, many creatures utilize indirect communication via pheromone -like shared memory fields, and exhibit a rich array of behavior. Field memory usually remains for a long time, affecting many individuals, or evaporates quickly, and thus updates itself over time. Therefore, controlling and quantifying momentary contributions to global behavior in the complex natural environment is challenging. Our pheromone-based autonomous agent system contains self-propelled microswimmers and a phase-change material that physically mimics the elemental process of memory-induced collective behavior. The system allows us to study not only the dynamics under continuous tuning of pheromone interaction but also the different states of collective motion. This can provide insights into how a spatiotemporally varying field affects the activity of living things.

Automated summary

Indirect interactions via shared memory deposited on the field play a crucial role in collective motions. We propose a pheromone-based autonomous agent system that mimics the collective behavior of ants and bacteria. This system combines the phase-change behavior of self-propelled Janus particles and AC electroosmotic flow to create pheromone-like trails that attract other particles.