A novel magnet-actuated droplet manipulation platform using a floating ferrofluid film.

In this paper, we propose a novel but versatile magnet-actuated platform for droplet manipulation, which uses a ferrofluid film floating on a liquid surface as magnetic actuator. In contrast to the traditional magnetic droplet manipulation, this platform can handle droplets without magnetically functionalizing them. Due to the immiscibility of the oil-based ferrofluid and water, the droplets desired to be manipulated can stably rest on the surface of the floating ferrofluid film (FFF) under the action of surface tension, thereby offering possibilities for magnetically-driven droplet manipulations. Such a floating, magnetically responsive liquid film not only offers an open surface for active 2D droplet manipulation, but also enables complex droplet manipulations in 3D space. Using FFF, we demonstrate a "full-space" droplet manipulation, including droplet transport/coalescence above FFF (i.e. in air), droplet transport/coalescence on FFF and droplet encapsulation/release under FFF (i.e. in liquid). Furthermore, we investigated the effects of the magnetic field intensity, the ferrofluid concentration, the droplet volume, and the FFF thickness on droplet kinematics. By finely tuning these operating conditions, the FFF strategy can enjoy more operational latitude than traditional droplet systems, thus allowing more versatile liquid handling.