Textured surfaces for oil droplet transport

Abstract

Droplet-based microfluidic systems bring the advantage of handling samples in discrete forms and these systems can be built as a surface-based platform, eliminating the need of a carrier fluid. Droplet transport on surfaces can be achieved by creating local energy gradients by using electrowetting, thermocapillary forces, chemical gradients and surface texture. These methods are developed for aqueous droplets; while controlled transportation of oil droplets remains as a challenge due to their low surface tension. One of the major challenges in achieving controlled oil droplet manipulation is to obtain oil repellent -oleophobic- surfaces. In this work, pillar arrays with mushroom shaped profiles were designed, fabricated, and tested to study oil wetting on textured surfaces. Contact angles higher than 170° were achieved for hexadecane droplets in Fakir state. Based on these findings, surface texture ratchet tracks that are composed of an array of arc shaped pillars were built to generate local energy gradients to manipulate oil droplets in a continuous fashion. Motion of hexadecane droplets at a speed of 7 mm/s was achieved. The results of this study are crucial for applications of oil droplet transport such as in biochemistry, smart surface development, wearable device design as well as microsystem packaging.