Linear Motility Maps in Nonlinear Viscous Fluids
This work extends geometric mechanics to a broader class of biological fluids and reveals a mechanism for violating the scallop theorem, which is significant for understanding and designing microswimmers in complex fluids.
The authors show that linear motility maps, which govern low-Reynolds-number locomotion, extend to power-law viscosity fluids, and that nonlinear drag in Carreau-Yasuda fluids can enable net motion from reciprocal shape changes, violating the scallop theorem. An inchworm model demonstrates direction reversal by changing speeds.
Systems moving in low Reynolds number fluid regimes are known to be governed by a ``motility map'' which linearly relates their shape change rates to they body frame velocity moving through the fluid. A consequence of this is ``Purcell's Scallop Theorem'' -- a locomotion system that undergoes shape changes that follow the same path forward and backward in time (reciprocal body deformations) cannot achieve net displacement, regardless of pacing of those changes.We show that linear-in-velocity motility maps extend to any power law viscosity (a.k.a. Ostwald--de Waele fluid), and therefore to many biological fluids in intermediate shear ranges. We also show that the linear-in-velocity property can be violated in Carreau-Yasuda fluids to produce net motion using an ``inchworm'' model consisting of two unequal masses with unequal drag coefficients performing reciprocal motions. Interestingly, the direction of motion can be switched by changing speeds. Our results show that the linear motility map of geometric mechaincs can be used to analyze and design locomotion in power-law fluids, and that some nonlinear drag relationships such as Carreau-Yasuda can be exploited to generate net locomotion in seeming violation of the ``scallop theorem''.