  Swimming microorganisms often have to propel in complex, non-Newtonian
fluids. We carry out experiments with self-propelling helical swimmers driven
by an externally rotating magnetic field in shear-thinning, inelastic fluids.
Similarly to swimming in a Newtonian fluid, we obtain for each fluid a
locomotion speed which scales linearly with the rotation frequency of the
swimmer, but with a prefactor which depends on the power index of the fluid.
The fluid is seen to always increase the swimming speed of the helix, up to 50%
faster and thus the strongest of such type reported to date. The maximum
relative increase for a fluid power index of around 0.6. Using simple scalings,
we argue that the speed increase is not due to the local decrease of the flow
viscosity around the helical filament but hypothesise instead that it
originates from confinement-like effect due to viscosity stratification around
the swimmer.
