(522d) “Moonwalking”: A New Mode of Dissipative Active Motion of Rotating Colloidal Particles in Complex Fluids

One of the general techniques of driving magnetic colloidal particles is by actuating them by an out-of-plane rotating magnetic field near a substrate or a wall. The particle experiences different hydrodynamic resistances near its bottom and top, which results in net translation. We report here a new pattern of dissipative active motion of a new class of micro-scale colloidal rotators in complex fluids where they rotate and translate in directions some of which might not be intuitively anticipated. These micro-scale rotators are synthesized by assembling iron oxide nanoparticles inside polydimethylsiloxane (PDMS) microdroplets, after which the host droplets are cured in the presence of a static magnetic field. Rotating magnetic fields can induce a strong torque on these microbeads via the embedded anisotropic chain-like structures. As a result, they exhibit a net forward translation as they rotate clockwise near a glass substrate in Newtonian fluids. Surprisingly, in shear-thinning fluids, we encountered a fascinating new backward movement pattern of these microbeads, which we refer to as 'moonwalking.' In this new dynamic pattern, the translation is opposite to the one expected by the rotational clockwise direction of the particles. Our COMSOL simulation results show that this reversive motion results from difference of the shear forces acting on the top and the bottom of the particles due to localized non-uniform shear thinning around the particle. We further take advantage of light-tunable rheological properties of a photo-rheological (PR) fluid, and show that these particles can change their translation behavior from “moonwalking” to rolling in a PR fluid upon UV irradiation. The new moonwalking phenomenon has the potential to deepen our understanding of fundamental principles of transport of autonomous particles in soft viscoelastic media and may open the door to future applications of active rollers in advanced biomedical applications.