Synergistic binding ability of electrostatic tweezers and femtosecond laser-structured slippery surfaces enabling unusual droplet manipulation applications†

We propose a novel contactless droplet manipulation strategy that combines electrostatic tweezers (ESTs) with lubricated slippery surfaces. Electrostatic induction causes the droplet to experience an electrostatic force, allowing it to move with the horizontal shift of the EST. Because both the EST and the slippery operating platform prepared by a femtosecond laser exhibit a strong binding effect on droplets, the EST droplet manipulation features significant flexibility, high precision, and can work under various operating conditions. The EST can manipulate droplets with a wide volume range (500 nL–1 mL), droplets hanging on tilted or even inverted surfaces, multiple droplets in parallel, corrosive droplets, low-surface-tension organic droplets (e.g., ethanol), and even droplets in a sealed space from the outside. The EST operation method is suitable for various slippery substrates prepared by femtosecond laser processing and can also be used to manipulate small solid spheres other than liquids. Additionally, a self-powered EST system is also designed without the need for high-voltage static electricity, allowing even fingers to serve as EST sources for droplet manipulation. The flexible and precise manipulation performance allows this technology to be applied in a variety of applications. For example, a new digital microfluidic (DMF) technology based on an EST array has been successfully validated and is expected to replace traditional electrowetting-on-dielectric technology in the future.