PMMA Microparticles Show Negative Coefficient of Friction and Angle of Repose

Engineering researchers at Lehigh University, including Professor James F. Gilchrist and Dr. Samuel Wilson-Whitford, synthesized PMMA microparticles exhibiting uphill granular flow. This development was recently published in the journal Nature Communications. These microparticles are made from spheres of polymethyl methacrylate (PMMA), which are then coated on one hemisphere with a layer of iron. This layer is then oxidized to form a 100 nanometer thick layer of iron oxide on the surface of the hemisphere. This creates a magnetic dipole moment on each particle which is off-center from its center of mass and thus can interact with magnetic fields to rotate the particles. 

When not exposed to a magnetic field, these microparticles behave like ordinary granular substances. However, when they are exposed to a rotating magnetic field, the microspheres start to spin due to their heterogenous coating providing a net torque on the particles. These particles can then cohere into temporary doublets that spin in the direction of the magnetic field rotation, allowing them to effectively "climb" over each other. This causes the spheres to form a fast-moving surface layer that flows uphill on top of a slower-moving layer that is forced downhill. This behavior is very unusual for granular particles to have, as it effectively gives them a negative angle of repose and a negative coefficient of friction.