The Impact of Polymer Fluid Microstructure on Achiral Microswimmer Propulsion*

Rigid microscale swimmers consisting of geometries with two or fewer symmetrical axes offer a simple design approach for producing effective microswimmers. However, for efficient propulsion in complex media, swimmer geometry must also account for interactions within fluidic environments which often contain colloids and polymers that can alter swimming kinematics. Here we explore the effects of fluid microstructure on two geometrically distinct achiral microswimmers made of aggregated magnetic microbeads that have arc structures with a single axis of symmetry. Polymer solutions of varying number average molecular weight (M_n) are used to create Newtonian solutions with differing local microstructures. When actuated using a uniform rotational magnetic field, swimmer propulsion efficiency varies significantly with M_n. Local viscoelastic effects are proposed to contribute to the modulation of achiral swimmers’ gait and efficiency in high M_n polymer solutions. To investigate this, dilute polymer solutions that exhibit either predominantly shear thinning behavior or have significant elasticity are explored. Both types of viscoelastic fluid are observed to affect the achiral kinematics; however, these effects vary with swimmer geometry. This work provides insight into designing achiral microswimmers with enhanced propulsion efficiency given a priori knowledge of the fluidic environment.