Title: 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. Symmetries have long been used to understand when propulsion is possible in microscale systems, and I will discuss our work to find the simplest possible rotated microrobots in light of physical constraints. First, 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.  By contrast, we have shown that propulsion by magnetic rotation is possible for achiral geometries. Second, propulsion would seem to be prohibited by geometries with fore-aft symmetry along their rotation axis, such as a rotating sphere.  We have shown that in nonlinearly viscoelastic fluids, a symmetry breaking propulsion is possible for rotating microspheres.  Finally, time permitting, 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.