Video Microscopy of Dynamically Aggregated Paramagnetic Particle Chains in an Applied Rotating Magnetic Field

A rotating unidirectional magnetic field generated by a motor-mounted permanent magnet was used to dynamically aggregate paramagnetic particles from suspension to form chains. Video microscopy and image processing were used to analyze the data. Good synchronicity was observed between the chains and the field .up to 8 Hz frequency. Chain growth is facilitated at very low rotational frequencies initially after starting from static conditions, after which the mean chain length decreases in response to increasing viscous drag forces. As the frequency is increased from low values, the mean chain length decreases initially rapidly and later more slowly at higher frequencies, thus resulting in two regimes: the fast and the slow decrease regimes. Typical structures such as linear chains and S-shaped chains are observed as well as unusual U-shaped structures. The formation of dynamic S-shaped structures can be predicted based on the flexibility of the chains and the principle of conservation of angular momentum. Formation of single-chain aggregates is observed in microwells that are up to 50 μm wide, whereas in larger microwells multichain formation was noticed. The motor-mounted permanent magnet arrangement has applications in the development of portable microchip biosensors. The arrangement proved sufficient and reliable in forming chains, which display behavior similar to that observed under more elaborate and controlled conditions.