Fig. 1. Femtosecond laser preparation of superhydrophobic surface. (a) Femtosecond laser processing system; (b)‒(d) micro-nano structures prepared by femtosecond laser on PDMS surface; (e) contact angle of droplet on structured surface; (f) rolling angle of droplet on structured surface

Fig. 2. Effects of femtosecond laser processing parameters on wettability of PDMS surface. (a) Effect of laser power (scanning speed is 15 mm/s and scanning spacing is 20 μm); (b) effect of scanning speed (laser power is 500 mW and scanning spacing is 20 μm); (c) effect of scanning spacing (laser power is 500 mW and scanning speed is 15 mm/s)

Fig. 3. Droplet motion on superhydrophobic surface prepared by femtosecond laser driven by electrostatic tweezers. (a) Schematic of droplet controlled by electrostatic tweezers; (b)(c) electrostatic action brings droplet back to position directly below electrostatic tweezers; (d) three motion scenarios of droplets under action of electrostatic tweezers [(I) droplets cannot be moved by electrostatic tweezers, (II) droplets can be normally moved laterally by electrostatic tweezers, and (III) droplets are sucked up by electrostatic tweezers]; (e) influence of potential and height of electrostatic tweezers on droplet manipulation process; (f) electrostatic tweezers control movement of droplets on superhydrophobic plane

Fig. 4. Position changes of electrostatic tweezers and droplet during movement of droplet controlled by electrostatic tweezers (displacement of origin point is denoted as 0). (a) Single reciprocating movement of droplets controlled by electrostatic tweezers; (b) multiple reciprocating movements of droplets controlled by electrostatic tweezers

Fig. 5. Schematics of mechanism of electrostatic tweezers driving droplets on insulating superhydrophobic surface. (a)‒(d) Droplet is directly below electrostatic tweezers; (e)‒(h) droplet deviating from electrostatic tweezers

Fig. 6. Multifunctional droplet operation realized by electrostatic tweezers on superhydrophobic surface fabricated by femtosecond laser. (a)(b) Droplets with different sizes (1 μL and 1 mL) are controlled by electrostatic tweezers; (c) electrostatic tweezers manipulate droplet upward movement along inclined 8° superhydrophobic surface; (d) electrostatic tweezers manipulate movement of droplets of acid, alkali, and salt solutions; (e) electrostatic tweezers guide movement of plastic, glass and stainless steel balls with diameter of 3 mm

Fig. 7. Application of electrostatic tweezers to manipulate droplets. (a) Electrostatic tweezers are used to guide droplet to move along trajectory following letters “USTC” on superhydrophobic surface fabricated by femtosecond laser; (b) chemical microreaction based on droplet; (c) guided droplets precisely clean contaminants from superhydrophobic surface