Oil droplet manipulation on superomniphobic textured surfaces

Abstract

Microfluidic systems are mostly composed of closed microchannels in which flow is generated by syringe or pressure pumps. The flow in these channels can be droplet-based however access to each droplet individually in these systems is not possible. As an alternative approach to these channel-based devices, droplets can also be manipulated on surfaces by generating surface energy gradients. Since in these systems droplets can be handled individually and samples can be carried in small packages, these systems can perform more controlled operations. For instance, the concentration and volume of the samples can be adjusted more precisely. These systems can be very useful for biological analysis as well as chemical synthesis. Until now, transport of water droplets by using surface energy gradients has been demonstrated in literature. On the other hand, controlled transport of oil droplets on surfaces remained as a challenging task because of their low surface tension. In addition, in the literature, most of the work about oil droplet transportation was carried out in an aqueous environment, and therefore it restricts its potential for applications. This work demonstrates the transportation of microliter sized oil droplets by utilizing textured superomniphobic surfaces in a controlled way for the first time. By applying vertical vibration to the surface, oil droplets overcome hysteresis and move by following the textured tracks. Superoleophobicity is required to decrease the affinity of oil on the surface so that the motion of droplets can be achieved. This system has advantages such as the ability to control droplet motion individually by using a single input (vertical vibration) as well as mixing droplets in precise ratios, preventing clogging in channels and cross contamination as well as eliminating the usage of syringe pumps. In this project, initial focus was on examining the topography effect on superoleophobicity and fabricating superomniphobic surfaces. Surfaces were fabricated on silicon wafers by using conventional lithography technique. In this stage, two different microstructure profile was used on the surfaces: mushroom microstructure and straight sided microstructure. It was observed that mushroom microstructures were required to maintain superoleophobicity. Also, the effect of side length of microstructures, the distance between the microstructures and TiO2 coating on wettability were investigated. In order to achieve oil droplet transportation, superomniphobic textured surfaces were developed and these surfaces were tested by applying vertical vibration. As a final aim of this project, these surfaces were used for the nanoparticle synthesis.