Fig. 1. Preparation and characterization of dual-circular track. (a) Preparation process of dual-circular track; (b) SEM image of dual-circular track boundary; (c) EDS image of the aluminum sheet surface; (d) schematic diagram of structure parameters for dual-circular track; (e) CA of liquid droplet on the surface after hydrophobic treatment; (f) CA of underwater bubble on the surface after hydrophobic treatment

Fig. 2. Mechanism of bubble transportation. (a) Schematic diagram of bubbles with different volumes at circular dot positions; (b) schematic diagram of bubble volume when the circular dot diameter is 3 mm; (c) schematic diagram of underwater bubble transportation driven by internal Laplace pressure gradient; (d) schematic diagram of underwater bubble transportation on dual-circular track

Fig. 3. Influence of circles with different diameters on bubble transportation time

Fig. 4. Influence of device parameters on microfluidic transportation rate. (a) Track length; (b) bubble volume; (c) track width

Fig. 5. Bubble transportation on different circular tracks designed based on the dual-circular track structure. (a) Three-circular-linear microchannel structure; (b) four-circular-linear microchannel structure; (c) two-circular-ring microchannel structure; (d) two-circular-arched microchannel structure