Self-Guided Navigation of Magnetic Microspheres on Topographic Landscapes

The directed propulsion of magnetic
microrobots through
structured
environments often requires real-time feedback between external sensors
and the applied field. This requirement, however, can be relaxed to
enable self-guided propulsion by coupling field-driven motion to gradients
in the local environment. We show that rotating fields direct the
migration of ferromagnetic spheres up local gradients in the topography
of a solid substrate. We quantify the speed and direction of particle
migration as a function of the rotation frequency and incline angle.
These observations are explained by a dynamic model that describes
particle motion through the fluid due to the magnetic torque and gravitational
force. We demonstrate how “topotaxis” can direct the
simultaneous navigation of multiple particles on patterned arrays
of concave bowls and convex domes without knowledge of the particle
locations or the surface topography. These results highlight opportunities
for designing time-varying fields to achieve other self-guided behaviors
conditioned on local environmental cues.