Giant magnetoresistance monitoring of magnetic picodroplets in an integrated microfluidic system

This letter describes the integration of giant magnetoresistance (GMR) sensors with a microfluidic system for the velocity and size monitoring, and enumeration of flowing magnetic entities. We have fabricated a microdevice that enables: (1) controlled formation of picoliter-sized droplets of a ferrofluid separated by a nonmagnetic oil; and (2) continuous-flow sensing of these ferrofluid droplets. It is shown that the flow velocity, droplet size, and droplet-formation frequency can readily be determined from the GMR response. These results are validated by comparisons to fluorescence microscopy data.