A Wettability Gradient Synergistic Bionic Wedge-Shaped Track for Ultrafast and Long-Distance Spontaneous Transport of Droplets.

The passive surface is widely considered to be an ideal platform for self-transport of liquid due to the advantage of no external energy input. However, the short-distance and low-speed transportation of liquid caused by an insufficient driving force limit its practical application. To address the aforementioned challenges, we developed a bionic wedge-shaped track on the copper (Cu) substrate inspired by the cone-shaped thorn of cactus for liquid spontaneous transport. Furthermore, a wettability gradient (water contact angle from 154.6° to 84.9°) was prepared on the wedge-shaped track to enhance the droplet motion. Under the synergistic effect of the Laplace force and wettability gradient force, the average speed of the whole movement process of the water droplet can reach 225.2 mm/s, which was much higher than those reported in previous studies, the transportation distance exceeded 25 mm, and the track need not be wetted in advance. In addition, the effect of the apex angle of the wettability gradient wedge-shaped track (WGWT) on the droplet self-transport performance was systematically investigated; the results indicated that the transport distance and speed of the water droplet decreased with the increase of the apex angle of the WGWT. The unit length of each unit was reasonably designed according to the spreading length of 10 μL water droplets in different wettability units. In addition, the WGWT exhibited excellent fog collection performance with a water collection efficiency up to 2507.10 mg/(cm·h). Additionally, it showed effective self-transport capabilities in the transport experiment with a blood droplet, achieving a transport distance of 25 mm and an average speed of 14.2 mm/s. These results highlighted the significant potential of WGWT for applications in microfluidics, water collection, and biological detection.