A microfluidic device for generating monodisperse superparamagnetic droplets and rapidly manipulating desired droplets into designated sub-microchannels by an external magnetic force is described. Superparamagnetic magnetite (Fe₃O₄) nanoparticles are synthesized by a chemical co-precipitation method. They are well dispersed in the water-phase to form a superparamagnetic fluid that is sheared into picoliter-volume monodisperse superparamagnetic droplets by the oil-phase in a T-junction PDMS microchannel. Superparamagnetic droplets always flow into sub-microchannel 1 due only to laminar flow without a magnetic field. But they are deflected from the direction of laminar flow by a perpendicular magnetic field. The results show that the deflection is proportional to the magnetic field gradient and magnetic nanoparticle concentration, and it is closely related to the magnet position. The observed experimental results make a good match with theoretical analysis. Single or bulk superparamagnetic droplets are successfully manipulated into the designated sub-microchannels 2 and 3 respectively, only by changing the positions of the magnet. Relatively high efficiency is obtained with more than 10 superparamagnetic droplets precisely manipulated per second. This simple and robust apparatus has wide applications in high throughput drug delivery/screening, immunoassay, cell research and synthesis of magnetic microparticles due to good biological compatibility and monodispersity of superparamagnetic droplets.

Multiple essential microdroplet operation units, including splitting, dispensing, oil-phase exchange, trapping, release and demulsification, were successfully implemented by combining hydrophobic ferrofluid with microfluidic chips.