On slip flow and interfacial phenomena within superhydrophobic microchannels

The unique wetting properties of superhydrophobic (SH) surfaces have attracted a considerable interest in recent years. Their ability to repel liquids produces some remarkable characteristics such as spherical droplet formations with enhanced mobility, self cleaning properties, anti-sticking abilities and reduced drag on liquid flows. These wetting phenomena are directly attributed to the Cassie-Baxter wetting condition, where a SH surface supports a fluid on top of its microstructured features and traps a gas layer within the vacant regions. While the interactions of liquid droplets on SH surfaces have been well explored, in contrast, little attention has been given to the characteristics of SH surfaces when completely immersed in a fluid. In micro-scaled environments, SH surfaces can have a considerable influence on the fluidic interactions. This thesis aims to explore some of the flow and interfacial phenomena which are created by SH surfaces located in fluid filled microchannels. The drag reducing properties of SH surfaces with arrays of circular micropillar structures were investigated by measuring the pressure drop reduction across a parallel plate channel with one SH wall surface. The results from six individual SH surfaces with di erent solid surface fractions showed very little drag reduction for laminar flows (Re<70). It was proposed that both the curvature of the liquid-gas interfaces and the development of intermediate wetting states were responsible for the reduced slip lengths. A further study confirmed that curved liquid-gas interfaces could even increase the surface friction by protruding into the channel flow area under vacuum conditions. The experimental slip length results from this study were overestimated by the slip scaling theory but compared well with results reported from similar studies in literature. Therefore, it was concluded the theoretical predictions may only be suitable for low pressure flow conditions. In order to gain a better understanding of the interfacial characteristics of SH surfaces in fluid filled microchannels, the surfaces were analysed using confocal microscopy. Laser scanning confocal microscopy (LSCM) provided a means of visualising the solid and fluid interactions in 3D. The experimental results contradicted the surface stability theory and observed the presence of a partial wetting state on each surface, where the threephase contact line was situated below the micropillar tips. For the first time, experiments performed on various SH surfaces revealed the magnitude of the partial wetting may be related to the initial channel filling flow rate. From these observations it was hypothesised the partial wetting states were caused by contact line pinning along the hierarchical roughness on the pillar edges. The final section of this study focused on the reversible wetting and dewetting of SH surfaces. Controlled transitions between Wenzel (wetted) and Cassie-Baxter (dewetted) conditions would o er a means of dynamically tuning the influence of a SH surface and broaden their applicability in pressurised microfluidic systems. The ability to reversibly wet and dewet circular micropillar structures on a SH surface was demonstrated using an innovative application of dissolved gas control and surface heating. Lateral dewetting of the SH structures was achieved through the dissolution of dissolved gases from the working fluid. The process was limited by solid-liquid interface separation (bubble formations) on surface defects and contaminants. A novel method for increasing the liquid adhesion at the pillar tips was also implemented.