Fig. 1: Magnetic actuation of additive-free droplets on ferrofluid-infused slippery surface.

a Collective actuation of numerous micro-sized droplets. The growing black part at the center was the ferrofluid (12 v/v% Fe₃O₄ nanoparticles) raised by the electromagnet. b The actuation force is provided by the ferromagnetic wetting ridge that spontaneously forms around the deposited droplet. The dotted lines and the solid arrows illustrate the magnetic induction lines and the droplet moving direction, respectively. c Snapshots of a typical actuation process. Red dotted arrow indicates the droplet moving direction. Inset: magnified optical image of the deposited droplet (rose red dashed rectangle), showing the wetting ridge (blue dashed rectangle). d Confocal fluorescence image of the wetting ridge. e The driving velocity increases with the concentration of the magnetic particles (Fe₃O₄ nanoparticles) in the ferrofluid (drop volume: 2 µL). f The velocity also increases with the droplet radius for different liquids, presumably because the ridge volume increases with the droplet size, and g it increases with the surface tension of the droplet (orange solid squares), presumably because a liquid with higher surface tension generally raises higher ridge (blue open squares). Dashed lines are eye-guide. The error bars represent the standard deviation (n = 3).