Flow-mediated interactions between objects are important in sedimentation and collective locomotion. New phenomena arise in systems that are dominantly coupled by rotation, as relevant to active matter physics and biological and engineering contexts. Here we establish an experimental platform for studying fluid mechanical spin-coupling effects and map out how interactions depend on proximity, confinement, and flow state. The two-body problem of a driven rotor inducing spin of a passive rotor reveals a gearlike counterrotation mode but also surprising corotation modes that occur broadly across the parameter space. Streamline analysis and a mechanical model indicate distinct transitions between modes that arise geometrically at close range, topologically due to flow reconfiguration, and inertially with increasing Reynolds number. These findings provide ways to understand, control, and exploit hydrodynamic spin-spin interactions in applications.

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• Collective behavior
• Flow-structure interactions
• Fluid-particle interactions
• Interparticle interactions
• Vortex interactions