Fig. 2: Versatility of the actuation.

a–c Actuation of biological droplets of Dulbecco’s modified Eagle medium (DMEM, a), fetal bovine serum (FBS, b), and buffy coat (c). d, e Schematic illustration and fluorescence image showing the actuation of solid samples (dried E.coli cells). E.coli was dyed with propidium iodide (a dye that visualizes dead cells). f, g Actuation of a droplet inside a straight (f) or curved (g) tube with the inner surface infused with ferrofluid, which is useful in closed microfluidics. h Actuation of a droplet hanging upside down on the rod with the outer surface infused with ferrofluid. i, j On an open surface, the droplet could be transported against gravity, either when the surface was tilted by 45° (i) or 90° (j). The black arrows and the symbol g indicate the direction of the gravitational force. k By programmed sequential actuation of neibhourghing electromagnets, the droplet was guided to walk along a M-shape path. Electromagnets were beneath the droplets’ snapshots. The droplet was dyed with fluorescent molecules. l The fluorescence image of the final state in k did not show any residue fluorescence on the walked path, suggesting the anti-fouling property. Dotted arrows indicate the moving direction of the actuated droplet.