  Mobile microrobots are envisioned to be useful in a wide range of high-impact
applications, many of which requiring cohesive group formation to maintain
self-bounded swarms in the absence of confining boundaries. Cohesive group
formation relies on a balance between attractive and repulsive interactions
between agents. We found that a balance of magnetic dipolar attraction and
multipolar repulsion between self-assembled particle chain microrobots enable
their self-organization into cohesive clusters. Self-organized microrobotic
clusters translate above a solid substrate via a hydrodynamic self-propulsion
mechanism. Cluster velocity increases with cluster size, resulting from
collective hydrodynamic effects. Clustering is promoted by the strength of
cohesive interactions and hindered by heterogeneities of individual
microrobots. Scalability of cohesive interactions allows formation of larger
groups, whose internal spatiotemporal organization undergoes a transition from
solid-like ordering to liquid-like behavior with increasing cluster size. Our
work elucidates the dynamics of clustering under cohesive interactions, and
presents an approach for addressing operation of microrobots as localized
teams.
