Fig. 1. Multi-level micro/nano structure formed on surface of zinc foil after nanosecond laser drilling. (a) Schematic of nanosecond laser machining system; (b) SEM image of top surface of zinc foil; (c) locally enlarged view of Fig. 1(b); (d) SEM image of bottom surface of zinc foil; (e) locally enlarged view of Fig. 1(d)

Fig. 2. Effect of pulse energy on surface morphology and wettability of zinc foil. (a) Cross-sectional view of tapered pore formed by nanosecond laser drilling; (b) diameter of micropore on sample surface versus pulse energy; (c) water contact angle versus laser energy; (d) bubble contact angle versus laser energy

Fig. 3. Effect of chemical composition on surface wettability of zinc foil. (a) Water contact angle on top surface of pristine zinc foil; (b) water contact angle on top surface of zinc foil after laser drilling; (c) water contact angle of heated zinc foil; (d) EDS result of O1s peak on top surface of pristine zinc foil; (e) EDS result of O1s peak on top surface of zinc foil after laser drilling; (f) EDS result of O1s peak on top surface of heated zinc foil

Fig. 4. Asymmetric wettability caused by morphology difference between upper and lower surfaces of zinc foil. Morphologies of microporous edge on (a) top and (b) bottom surfaces; (c) water contact angle and (d) bubble contact angle on top surface of heated zinc foil; (e) water contact angle and (f) bubble contact angle on bottom surface of heated zinc foil

Fig. 5. Analysis of dynamic behavior of bubbles on Janus zinc foil. (a) Intraoperative imaging for bubbles ejected from hydrophilic surface to superhydrophobic one; (b) intraoperative imaging for bubbles ejected from superhydrophobic surface to hydrophilic one; (c) laser energy with respect to passing time of bubbles on zinc foil; schematic of unidirectional transport mechanism of bubbles on heated zinc foil when superhydrophobic surface is placed (d) downwards and (e) upwards