Title: Granular dynamics of magnetically-responsive microrollers

Author (Talk): James Gilchrist, Lehigh University

Abstract:

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. In this work, a granular system of responsive 50 μm ferromagnetic Janus particles is prepared. The motion of these responsive Janus microrollers is activated through the modulation of a magnetic field that applies both torque and creates weakly attractive interparticle interactions. The torque results in particle rotation and the interparticle attraction is proportional to the friction necessary for particle mobility. The net result is flow indistinguishable from that of flowing granular media with the ability to flow uphill and through and around obstacles. Our recent paper (Wilson-Whitford, et al., Nature Communications, 2023) shows the uphill negative angle of repose can be described by μ(I) rheology where the cohesion term is a negative coefficient of friction. These microrollers demonstrate both expected flow kinematics and surprising fluid-like instabilities. The primary difference in this system as compared to typical passive granular systems is that unlike pouring or agitating granular media, the stress is both imparted and dissipated at the particle scale. Using both simple granular principles and other emergent properties of the collective dynamics of these microrollers allows design of various systems where particle transport is activated and enhanced in both classical granular systems such as drums and funnels and in novel applications navigating tortuous paths.