Real microrobots are too small to carry their own motor or battery, so they are steered instead by an external rotating magnetic field generated by Helmholtz coil pairs. Each microrobot embeds a permanent magnetic dipole that tries to align with the field. When the field rotates, the dipole is dragged around with it, and each robot's slightly asymmetric (chiral, screw-like) shape converts that forced rotation into forward translation along the rotation axis — the same trick a corkscrew uses to advance through cork as it turns.
torque = m × B(t)
v_swim ≈ k · f_rot (below step-out frequency)
v_swim ↓ sharply (above step-out — sync lost)
- Rotation speed — how fast the external field spins. Swarm speed rises with it up to a step-out frequency; push past that and viscous drag can no longer keep the dipoles synchronized with the field, so average speed collapses even though the field spins faster.
- Field azimuth / elevation — the orientation of the rotation axis, i.e. the corkscrew's heading. Every robot in the field responds identically — there is no individual addressing, only whole-swarm steering.
- Click inside the fluid channel — sets a target point; the controller works out the axis orientation needed to drive the whole swarm there and dials the sliders to match, the way an operator would aim real Helmholtz coils to steer a drug-delivery swarm toward a tissue site.
Real-world relevance: at micron scale, viscous forces dominate over inertia (low Reynolds number), so ordinary flagellar swimming is inefficient — magnetically-driven corkscrew propulsion is one of the few practical ways to move untethered microrobots through blood vessels or other fluids for targeted drug delivery and microsurgery.