Remote Manipulation of Droplets on a Self-Assembled Magnetic Responsive Film

Abstract

Manipulation of droplet is of significant interest for a broad range of applications from lab on a chip to bioinspired functional surfaces. To control and actuate droplet, micro- or nanopatterned surfaces have been suggested as a passive approach in which unique structural features or chemical gradients enable liquid wetting and spreading into specific directions. While these approaches allow for a control over liquid wetting and spreading without external power sources, they are typically slow and not reversible. A set of active droplet manipulation techniques have also been proposed, which include electrowetting, dielectrophoresis, surface acoustic waves and thermocapillary force. As compared with the passive techniques, the active ones provide an enhanced controllability over a droplet position and motion in a fast and reliable manner. Especially, the electrowetting method has demonstrates its versatility in microfluidic systems as it enables precise manipulation of discrete droplets through programmed path. Nonetheless, the electrowetting platform requires formation of electrodes and external power source for the manipulation of liquid droplet, which limits the scalability and applicability of the technique. To this end, magnetically actuating surfaces with micro- or nanoscale structures have a great potential to be used for the active manipulation of droplet because of their reversible and instantaneous structural tunability in response to remote and non-intrusive magnetic field. However, previous droplet manipulation approaches based on the magnetic force are mostly limited to the transition of wetting state without the ability to control the position and motion of discrete droplet. Although studies of the positional control of droplet has been reported, they added magnetic nanoparticles into the droplet for the magnetic control, which significantly limits the broad applications of the techniques. Here, we present a simple yet novel approach to control the motion of discrete droplet on a magnetic responsive flexbile film that has reversibly actuating hierarchical structures on the surface. In this approach, a discrete droplet of pure water can be fastly manipulated into arbitrary target locations on the flexible film only with a permanat magnet, eliminating the needs for predefined electrodes or magnetic particle mixing with a droplet. The dynamically actuated hierarchical pillar arrays on the film are formed by moldess self-assembly of mixture solution of uncurd PDMS and magnetic particles under a magnetic field. The resulting magnetic responsvie film shows reliable actuating capabilities with immediate field-responses and maximum tilting angles of ~90°. Furthermore, the magnetic responsive surfaces exhibits superhydrophobic properties regardless of tilting angles of the structures.