(443j) Uphill Granular Heaping Flow Via Magnetically-Driven Particle Rotation

Gravity-driven granular flows, commonly found in nature in the form of avalanches and landslides and industrial processing, have long piqued the interest of scientists and engineers. Their actuation simplicity and response complexity have designated them as a cornerstone system of study within the soft matter and complex systems communities. Heap formation is a typical behavior of poured granular systems. With continuous addition of a granular material, such as sand, to a flat surface, as a heap forms a continuous gravity driven boundary flow can be observed in which grains are transported from an uphill to a downhill position. The heap itself grows as grains are transferred from the boundary layer to the underlying static heap. The dissipation of potential energy through frictional particle-particle and hydrodynamic particle-fluid interactions within the boundary layer shear flow leads to a fundamental slope angle, known as the angle of repose and the kinematics of the boundary layer itself have been well-documented through granular rheology of these inertial systems.

What has not previously been explored is the behavior of granular systems where shear is introduced to the system through torque applied directly to the particles themselves. In this work, a granular system of 50 micron ferromagnetic Janus particles is prepared. These responsive Janus particles are rotated through the application of magnetic torque by an external rotating magnetic field resulting in uphill granular heaping which for all purposes demonstrates quintessential granular behavior, such as boundary layer flow, but against gravity, giving rise to a negative angle of repose (downhill to uphill). It is proposed that the behavior is governed frictional control as a direct result of the magnetic field strength. These observations elucidate a whole new way to observe granular systems and have a significant impact on the way in which granular systems could be applied.