Selective handling of droplets in a microfluidic device using magnetic rails

Droplet microfluidics is currently undergoing an explosive development due to its ability to compartmentalize samples in picolitre to nanolitre volumes, transport them without dispersion and perform high-throughput analysis. The precise manipulation of single droplets, however, still requires complex chips, such as microelectrode arrays, or equipment, such as laser-based sorting. We report here a very simple proof of concept of an innovative and active technology which allows the individual manipulation of single droplets. This technology combines ferromagnetic rails and magnetic nanolitre droplets. Ferromagnetic rails are used to locally create magnetic potential wells. When the field is turned OFF, the hydrodynamic drag force transports the magnetic droplets according to the flow velocity profile. By switching ON the magnetic field, droplets experience a magnetic force that affects their trajectory when passing over the magnetized rail. The combination of the drag force exerted by the oil flow and the magnetic force resulting from the magnetized rail leads to a deflection force that guides the droplet along the rail, thus imposing a deterministic trajectory. The magnetic rails networks offer a spatially and temporally addressable guidance and sorting of individual magnetic droplets by synchronizing field activation and droplets positions. Numerical simulations were performed to evaluate spatial distribution of both drag and magnetic forces within the microdevice. The influence of different parameters such as magnetic flux density magnitude, flow rate and orientation of the rail has been investigated. Finally, selective droplet sorting, parking and merging were demonstrated and the monitoring of parallelized enzymatic reactions was performed.