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n this talk I will describe our work dealing with living and artificial systems which achieve propulsion at the low-Reynolds-number microscale environment.

Part one deals with the many swimming microorganisms that naturally encounter non-Newtonian, viscoelastic fluids, including mucus in airways, the stomach, and the reproductive tract. Biological environments such as mucus contain crosslinked networks of elastic fibers, with structural features at the same lengthscales as the microorganisms which swim through them. Thus one must go beyond homogeneous continuum descriptions of the medium to understand physiologically relevant situations. Using the simplest representations of microscopic swimmers in heterogeneous environments, I will explain the physical principles linking the forces on heterogeneous structures and changes in swimming behavior. I will discuss how these apply to more realistic biological scenarios and experiments underway to visualize these forces and test the usefulness of this linkage.

Part two deals with swimming microrobots which are being developed for applications in drug delivery, therapeutics, micromanufacturing, and sensing. So far, many magnetically actuated artificial microswimmers have relied on either swimmer flexibility or chiral geometry to overcome constraints on swimming strategies at low Reynolds numbers and achieve propulsion. However, being either flexible or chiral is not a necessary condition for propulsion of microswimmers rotated by external fields. We analyze achiral, rigid swimming using experiment, numerical simulation, and symmetry analysis. Achiral rigid swimming is demonstrated with planar colloidal structures constructed of magnetic beads and rotated by a spatially uniform magnetic field. This swimming is numerically modeled using a boundary element method. Finally, symmetry analysis is used to generically determine which combinations of achiral rigid geometry and magnetic moment can achieve propulsion.