Study on effects of magnetic fields of different inhomogeneous on the formation mechanism of microdroplets

Microfluidic technology is widely applied in biological detection, primarily utilizing microvalves to control and regulate fluid flow. Increasing attention and research have recently been directed toward magnetic droplet valves, which use magnetic fields to control magnetic droplets in microchannels for sealing purposes. A novel droplet formation technique has been proposed, employing a permanent magnet to attract magnetic fluid through a step emulsification process, thus controllably forming the magnetic droplets required for microvalves. However, the current understanding of the generation mechanism of magnetic fluid step emulsification remains insufficiently deep, with inadequate force analysis during the expansion stage of the magnetic fluid. This shortcoming results in an unclear comprehension of the relationship between the magnetic field and step emulsification formation, impeding the accurate prediction and control of droplet size and formation rate, thereby compromising the performance and reliability of magnetic droplet valves. Therefore, the study initially analyzes the forces acting on the magnetic fluid in a non-uniform magnetic field theoretically and systematically explores the step emulsification mechanism of magnetic fluids through a combination of numerical simulations and experimental validations. The magnetic field inhomogeneity degree directly affects the microdroplet formation process. As the lateral distance between the permanent magnet and the channel outlet increases, the magnetic field inhomogeneity degree decreases, resulting in larger droplet volumes and lower formation rates. Through theoretical analysis and experimental validation, this study provides a significant theoretical foundation and practical guidance for forming magnetic fluid in microfluidic systems.