Experimental Realization of a Colloidal Ratchet Effect in a non-Newtonian Fluid

Shear thinning fluids represent a class of non-Newtonian media characterized by a decrease of the apparent viscosity when increasing the shear rate. Here we experimentally demonstrate a deterministic ratchet effect in such media that enables directed transport of microscopic particles under a square-wave magnetic force. The applied modulation is designed in such a way that it does not produce any average speed when the particles are dispersed in a Newtonian fluid (e.g., water). However, in a dilute biopoly-mer solution, we observe the emergence of a net colloidal current when the forcing wave is composed of different amplitudes and time durations within a single period. The shear thinning nature of the dis-persing medium nonlinearly raises the mean speed for strong forces, breaking the spatial symmetry of the particle displacement and generating a net colloidal transport. We complement our findings with numer-ical simulations that capture well the underlying physical mechanism, showing good agreement with the experimental results. Our technique to ratchet magnetic particles could be potentially extended in active microrheology to probe other non-Newtonian, complex fluids and to infer the nonlinear properties of viscoelastic materials.