Droplet manipulation on a hydrophobic textured surface with roughened patterns

A novel concept is proposed and verified, experimentally and theoretically, to manipulate droplets without external power sources. The proposed device is a hydrophobic surface containing specific roughness gradients, which is composed of several textured regions with gradually increased structural roughness. Hydrophobic materials of four types, photoresist AZ6112, Teflon, Parylene C, and plasma polymerization fluorocarbon film (PPFC)-are adopted to fabricate the textured surfaces, and are tested. Actuating forces come from the different Laplace pressures exerted on a droplet across various hydrophobic surfaces, whereas resistance forces come from the contact-angle hysteresis. Two patterns of devices are shown in this article-chain-shaped and concentric circular. The former functions as a droplet transport route and the latter provides both transport and orientation functions. Theoretical estimation and experimental verification of the droplet motion, including actuation and resistance forces, on the device are conducted. Optimal design is achieved based on accurate estimations of the acting forces. The proposed device provides a simplified fabrication process and shows superior biocompatibility for droplet manipulation in microfluidic systems.