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

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