  We present a microfluidic platform for two-dimensional manipulation of water
droplets immersed in bulk oil-based ferrofluid. Although non-magnetic, the
droplets are exclusively controlled by magnetic fields, without any
pressure-driven flow. The diphasic fluid layer is trapped in a submillimeter
Hele-Shaw chamber that includes permalloy tracks on its substrate. An in-plane
rotating magnetic field magnetizes the permalloy tracks, thus producing local
magnetic gradients, while an orthogonal magnetic field magnetizes the bulk
ferrofluid. To minimize the magnetostatic energy of the system, droplets are
attracted towards the locations of the tracks where ferrofluid is repelled.
Using this technique, we demonstrate synchronous generation and propagation of
water droplets, analyze PIV data of the bulk ferrofluid flow and study the
kinematics of propagation. In addition, we show controlled break-up of droplets
and droplet-to-droplet interactions. Finally, we discuss future applications
owing to the potential biocompatibility of the droplets.
