The Active Sands of Time: Flow of Granular Microrollers through Funnels

Granular materials play a vital role in agriculture, industry, medicine, and filtration. The behavior of granular media is very complex. Since they behave as a fluid when moving at a high enough speed, they exhibit distinct velocity profiles in certain flows, and can exhibit turbulent and laminar flows, the same as fluids. This work explores the area of “active granular media” where magnetically responsive microroller particles rotate in response to an oscillating field. Being able to magnetically activate granular particles and initiate motion opens a vast spectrum of possibilities for applications in biomedicine and industry. What is unclear is to what degree can systems of many microrollers behave as granular media. This work will observe the flow of functionalized 44 µm polymer particles in funnels and how they mix and disperse within a fluid with various magnetic fields to further explore their viability for navigating complex systems. The polymer particles have evaporated Fe₂O₃ on half of their surface, creating a north and south pole in the metal, allowing them to be moved magnetically. Submerged in ethanol, the passive granular matter without magnetic manipulation settled at an angle of repose of 47° whereas microrollers can fluidize in the ethanol and flow through the funnel in complex ways, even stagnating away from the orifice or flowing deterministically toward the orifice. In trials with applied magnetic fields that individually rotated microrollers clockwise on the left and counterclockwise on the right side of the opening, all of the microrollers moved through the orifice. These preliminary results suggested there is significant impact on the flow rate, final distribution, and fluidization of the particles in the system. Microroller movement prevailed even in a container with flat surfaces surrounding the orifice. Given the properties of these particles and their ability for collective movement, potential use in biomedical and industrial fields shows promise.