Applied Math/ Mathematical Biology Joint Seminar Henry Fu, Department of Mechanical Engineering, University of Utah Monday, November 1, 2021 3:05pm on Zoom "Physical constraints on the propulsion of microrobots Abstract: Currently, artificially propelled magnetic micro- and nanoparticles are of interest for potential applications such as hyperthermic therapy, drug delivery, and microsurgery. Here, we focus on propulsion via rotation of rigid magnetic particles by an external magnetic field, which is attractive since it can be scaled down to micron sizes, and since magnetic fields permeate through many media including biological tissue. I will discuss our work exploring the fundamental physical constraints on this type of propulsion. By analogy with biological swimming, which is constrained by the kinematic reversibility of Stokes flow, it was expected that magnetically rotated micropropulsion required chiral geometries, like that of a helical bacterial flagellum. In contrast, we have shown that propulsion by magnetic rotation is possible for achiral geometries. Still, the propulsion of the simplest geometries, such as spheres, would seem to be prohibited by fore-aft symmetry along the rotation axis. We have found that in nonlinearly viscoelastic fluids, a symmetry breaking propulsion is possible for rotating microspheres. Finally, most models of magnetically rotating microrobots assume a permanent magnetic moment, but actual magnetic materials have moments which depend on the externally applied field. We show that such magnetic response leads to limits on the propulsion velocity achievable by rotating propulsion.