Shear-thinning rheology reverses wall-induced motion of low-Reynolds-number propellers

We experimentally investigate how shear-thinning rheology modifies the near-wall motion of low-Reynolds-number propellers. Using a magnetic actuation platform, we examine two representative systems: a rotating spherical roller and a helical propeller near a solid boundary. In a Newtonian fluid, the spherical roller exhibits forward wall-induced translation, while the helical propeller displays forward propulsion accompanied by lateral drift. In shear-thinning fluids, however, increasing the actuation frequency qualitatively alters these behaviors. For the spherical roller, the direction of wall-induced translation reverses beyond a critical frequency, producing backward motion. For the helical propeller, shear-thinning rheology reverses the lateral drift while simultaneously enhancing forward propulsion. These results provide direct experimental evidence that shear rate-dependent viscosity can fundamentally modify boundary-mediated propulsion at low Reynolds number, with implications for the design and control of microrobots operating in complex biological fluids.