Magnetic-Actuated Jumping of Droplets on Superhydrophobic Grooved Surfaces: A Versatile Strategy for Three-Dimensional Droplet Transportation.

ABSTRACT

On-demand droplet transportation is of great significance for numerous applications. Although various strategies have been developed for droplet transportation, out-of-surface three-dimensional (3D) transportation of droplets remains challenging. Here, a versatile droplet transportation strategy based on magnetic-actuated jumping (MAJ) of droplets on superhydrophobic grooved surfaces (SHGSs) is presented, which enables 3D, remote, and precise manipulation of droplets even in enclosed narrow spaces. To trigger MAJ, an electromagnetic field is utilized to deform the droplet on the SHGS with the aid of an attached magnetic particle, thereby the droplet acquires excess surface energy. When the electromagnetic field is quickly removed, the excess surface energy is partly converted into kinetic energy, allowing the droplet to jump atop the surface. Through high-speed imaging and numerical simulation, the working mechanism and size matching effect of MAJ are unveiled. It is found that the MAJ behavior can only be observed if the sizes of the droplets and the superhydrophobic grooves are matched, otherwise unwanted entrapment or pinch-off effects would lead to failure of MAJ. A regime diagram which serves as a guideline to design SHGSs for MAJ is proposed. The droplet transportation capacities of MAJ, including in-surface and out-of-surface directional transportation, climbing stairs, and crossing obstacles, are also demonstrated. With the ability to remotely manipulate droplets in enclosed narrow spaces without using any mechanical moving parts, MAJ can be used to design miniaturized fluidic platforms, which exhibit great potential for applications in bioassays, microfluidics, droplet-based switches, and microreactions.