Fig. 3.

Droplet movement regulation and solid actuation. a Dependence of the instantaneous rotational speed of the droplet (ω) on the radius of the spiral (R). In these tests, the Weber number of the droplet is 93. b Various morphologies of the gyrating droplets by impacting on high-adhesive patterns with different spiral numbers N of 2, 3, 5, and 6, respectively. c Scheme of the droplet actuator where a chemically heterogeneous thin solid sitting on a magnetically-levitated pyrolytic graphite (PG) flake. Water droplet is released and impacts on the center of the solid. d Sequenced images to show a water droplet (4.8 mg in mass) impacting on the PG-supported heterogeneous solid (94.3 mg in total mass). The substrate is driven to rotate in a desirable direction for more than 8000 ms. The Weber number of the droplet is 93. The thicknesses of the PG flake and the patterned solid are 0.5 mm and 50 μm, respectively. In b and d, the blue dashed arcs represent the high-adhesive pattern on the low-adhesive surface