Reducing drag in fluid flow has been one of the most widely studied topics in fluid dynamics due to the significant impact on improving operational efficiencies and cutting cost in applications from the aerospace, automotive and naval industries. Over the past two decades, superhydrophobic surfaces have been in the spotlight due to their ability to reduce frictional drag on the wall surface in both laminar and turbulent flows. Despite the extensive work on superhydrophobic surfaces, there are still a number of open questions remaining. In this dissertation, we investigate how a moving contact line interacts with a superhydrophobic surface by performing the first dynamic contact angle measurements to better understand the dynamics of droplets and streams on the surfaces. Our measurements found that the dynamic advancing contact angles on a superhydrophobic surface remains constant independent on capillary number while the dynamic receding contact angles decreases with capillary number but at a rate much slower than on a smooth surface. Furthermore, we investigated the role of the air-water interface shapes on the drag reduction. A novel microfluidic device was designed to incorporate superhydrophobic pillars. The shape of the air-water interface was changed with change to the static pressure in the channel. Slip along interface trapped within the superhydrophobic surface was found to result in significant drag reduction. However, the changes in flow geometry due to changes in bubble shape dominated effects due to slip. Reducing the bubble size amplified drag reduction, while increasing bubble size reduced drag reduction and even resulted in drag enhancement. In this dissertation, we also studies liquid-infused superhydrophobic surfaces as an alternative to the air-infused superhydrophobic surfaces. In the studies presented here, various immiscible oils were infused into the structures of precisely patterned and randomly rough superhydrophobic surfaces. A series of experiments were performed to investigate how liquid-infused surface affect drag reduction and droplet impact dynamics. The pressure drop reduction and slip length on the liquid-infused surfaces in microchannels were found to increase as the ratio between viscosity of water and the infused oil was increased. The longevity of these surfaces was also studied with the most effective surface found to be randomly rough. The effect of the viscosity ratio was also investigated on the droplet impact dynamics onto liquid-infused superhydrophobic surfaces. The increase in the viscosity ratio was found to increase a maximum diameter and a spreading/retraction rates of droplets. Taken together, the experimental research presented in this dissertation have allowed us to better understand and optimize the design of air-infused and liquid-infused superhydrophobic surfaces for drag reduction, droplet spreading and liquid mobility. With this new-found knowledge, a sense of new innovative ideas and applications has been or soon will be realized
