Fig. 1. Scanning electron microscopy image (a) and UV-Vis absorption spectrum (b) of Fe₃O₄ nanoparticle.

Fig. 2. (a) Schematic diagram of experimental system for light driven flow in droplet; (b) flow in a thin layer of droplet; (c) light intensity with Gaussian distribution in horizontal section.

Fig. 3. The spatial distribution of Fe₃O₄ particles driven by different light sources: (a) Bright field; (b) UV irradiation, 3 s; (c) UV irradiation, 6 s; (d) blue irradiation, 3 s; (e) blue irradiation, 6 s; (f) green irradiation, 3 s; (g) green irradiation, 6 s.

Fig. 4. The velocity field of Fe₃O₄ particles driven by different light sources: (a) UV irradiation, 3 s; (b) blue irradiation, 3 s; (c) green irradiation, 3 s.

Fig. 5. Maximum velocity of Fe₃O₄ particles under different light source and concentration.

Fig. 6. Temperature field (a) and the change of temperature with radius (b) on the surface of droplets under UV light.

Fig. 7. (a) A comparison between theoretical and experimental velocities of Fe₃O₄ particles; (b) velocity distribution and streamline corresponding to theoretical model.

Fig. 8. Three equivalent forces that induce vortex: (a) Surface tension; (b) concentrating physical strength; (c) surface pressure.