Motion analysis of magnetic microdroplets driven by magnetic microdisks in a magnetic field and a microfluidic setting

Microdroplets play an important role in lab-on-a-chip systems for biological investigations, particularly in single-cell analysis. In this study, we propose an array-based magnetophoretic platform for precisely manipulating water microdroplets encapsulating magnetic particles. The dynamical behaviors of magnetic droplets moving along the periphery of single magnetic disks in a surrounding oil phase while exposed to an external rotating magnetic field are investigated experimentally and numerically. Based on the driving frequencies of the magnetic field, three motion regimes of phase-locked, phase-slipping, and phase-insulated are identified, with two critical frequency thresholds distinguishing them. The increased magnetic field strength and volume of the encapsulated magnetic particles enhance the magnetic force on the droplet, resulting in a critical frequency rise. However, adding to the quantity of particles simultaneously raises the inertia of the droplet, causing it to slow down and effectively change the trajectory patterns of the droplet. Employing larger droplets increases the inertia, and also the drag force due to greater contact surface with the surrounding oil, resulting in a reduction in critical frequency. The findings provide essential knowledge for using droplets in magnetophoretic circuits to enable precise transport of bioparticles, which have significant applications in modern biology.