Exploring the limits of magnetic levitation: submicron particle separation and density profiling.

The size of particles plays a critical role in the time required to achieve stable levitation in magnetic levitation (MagLev) systems and significantly affects its practical applications, particularly in biological systems. There is a debate on the minimum size of the objects that can be levitated in a reasonable time period (e.g., <24 h). This challenge is particularly relevant in biological applications, where the small size of most biomolecules and the effects of Brownian motion pose significant concerns for biocompatibility. To address this issue, we have studied the lower limit of particle size possible for accurate density calculations using a ring MagLev system. Specifically, we examined commercially available polystyrene particles with known densities and identical physical properties, varying only in size from the microscale to the nanoscale. Our results demonstrate that although smaller particles (e.g., ∼200 nm) take considerably longer to achieve stable levitation compared to larger particles (e.g., 10 μm), they reach similar levitation heights, confirming the reliability of MagLev-based density measurements for submicron particles. Understanding the correlation of size and levitation time enables us to design/modify the MagLev experiments to minimize the exposure time of objects/particles to paramagnetic solutions, which is of key importance in the biomedical applications of MagLev systems.