Abstract
This work reports on the modeling of, and experiments on, wedge-shape gradients to facilitate spontaneous and fast motions for a wide range of water droplet volume on low hysteresis nanotextured surfaces. The nanotextured surfaces were prepared by phase separation of methyltrichlorosilane in anhydrous toluene at 0.014M and reacted with the glass substrates for 2 hr to produce the quasi-network nanofibers that provide the superhydrophobicity with a static CA and hysteresis of about 168o and 6o, respectively. The typical diameter and thickness of these nanofibers were 34.5±9.6 nm and 320.65 nm, respectively. Water droplets underwent spontaneous self-directed motion upon contact with a chemically patterned nanotextured surface with wedge-shape gradient. The surface exhibited two distinct wetting properties and low hysteresis. The droplet profile and velocity were related to the droplet position and the gradient angle psi. A wide range of droplet volume could be transported and a droplet velocity as high as 0.5 m/s was achieved herein. Ascension of water droplets with all-round acclivity when□psi = 8 and a subnanoliter droplet movement were also demonstrated. Based on the principle of conservation of momentum, the predicted velocity evolutions are fairly consistent with the experimental data numerically. We conclude that it is the combination of surface tension gradient and nano-wetting actuation that governs the droplet motion. The finding could provide a valuable microfluidics tool in drug discovery, DNA and protein microarray, and single cell study. For demonstration, the gradient patterns were rearranged into an array of circulating wedge-shape gradients to act as a self-directed microdroplet platform for colorimetric study. Finally, a complete self-directed microdroplet manipulation platform with acoustic micromixing capability was also proposed and successfully demonstrated.