Abstract

Active particles often swim in non-Newtonian and possibly even anisotropic fluids. We show how anisotropic orientational order can not only steer the swimming direction, but also provide new propulsion modes of microswimmers that are not able to move in isotropic fluids. Specifically, we derive the mobility tensor of passive spheres in nematic fluids though a non-equilibrium multipolar analysis of the director field, effectively finding a perturbative solution to the coupled nematodynamic equations for the director and the flow field. The mobility tensor links the swimming velocity and the rotation rate with the forces and torques applied at given viscoelastic material parameters and equilibrium multipolar moments, showing for example how continuous propulsion can be achieved by applying a torque on a rotationally symmetric colloidal particle. Beyond driven particles, we can expand the approach using a squirmer model for active microswimmers, determining how swimming direction and speed can be tuned by the nematic parameters. The result is a contribution towards developing the rules for propulsion in complex fluids and tuning the mutual microswimmer interactions. [1] E. Caf, Ž. Kos, to be submitted.[2] T. Yao, Ž. Kos, Q. X. Zhang, Y. Luo, E. B. Steager, M. Ravnik, K. J. Stebe, Topological defect-propelled swimming of nematic colloids, Science Advances 8 (2022) eabn8176.[3] S.-J. Kim, Ž. Kos, E. Um, J. Jeong, Symmetrically pulsating bubbles swim in an anisotropic fluid by nematodynamics, Nature communications 15 (2024) 1220.