Dynamics of colloidal rods rotating in viscoelastic media

Abstract

We experimentally investigate the in-plane rotational motion of ferromagnetic colloidal rods immersed in viscoelastic media and subjected to a rotating magnetic field. Unexpectedly, we observe significant angular velocity even at field frequencies an order of magnitude exceeding the step-out frequency, a regime where rods typically cease rotating in Newtonian fluids. This anomalous behavior arises from the interplay between the rapid rod actuation driven by the external field and the slower microstructural relaxation of the viscoelastic fluid. A minimal model incorporating memory effects quantitatively captures our experimental findings. Our study demonstrate a rather general case of microrheological probe dynamics in viscoelastic media where the behavior beyond step-out frequency depends strongly on the rheological parameters medium. Additionally, we derive an analytical expression for the rod orientation in the high-frequency limit, providing a potential method for extracting rheological parameters.