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

Abstract

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.