Liquid Springs for High-Speed Contamination-Free Manipulation of Droplets and Solid Particles

Accurate and flexible control of droplets is essential in many industrial applications, such as water harvesting, chemical assays, and biological detection. Magnetic force-based methods have been broadly exploited to fulfill the goals due to the advantage of non-contact, easy control, and long-range navigation. Nevertheless, it still suffers from some challenges, such as sample fouling, and the paradox between the droplet efficient motion and the large volume. Here a ferrofluid-based liquid spring to achieve contamination-free and fast droplet transportation on non-wetting solid surfaces is proposed. The liquid spring is based on the actuation of a ferrofluid droplet in an external uniform magnetic field. The actuation enables the liquid spring to propel tiny objects, including the non-magnetic miscible droplets and water-repellent solid particles, with adjustable motion velocity. Furthermore, the magnetic force, applied on the ferrofluid via an additional permanent magnet, makes it possible to navigate the liquid spring in a programmable way. With the aid of the liquid spring, the single or multiple droplets/solid particles advancing, on-demand droplets coalescence, and out-of-plane droplet motion are achievable.