Cheerios effect drives the formation of fire ant rafts

Fire ants survive waterproof floods by linking their bodies together to form waterproof rafts. How do the ants find each other on the water surface? We performed experiments with varying numbers of ants on the water surface and found that in fact, ants in numbers less than ten cannot form stable rafts. We observe ants swim in random directions even when around neighbors and kick each other away when coming into contact. These repulsive effects result in the breakup of small rafts. Surface tension, on the other hand, attracts ants together through the Cheerio effect, which draws small floating objects together. Combining the two mechanisms, a stability criterion emerges where the rafts need to be above a critical size to generate sufficient capillary forces to remain stable. We observed this stability transition in both experiments and numerical simulations. Our work shows that ants rely on capillarity to reconcile their needs to swarm and explore their surroundings.