Superhydrophobic Friction Reduction Microtextured Surfaces

Superhydrophobic surfaces are surfaces with fluid contact angles larger than 150°. Superhydrophobicity can be achieved by chemically modifying the surface or introducing texturing which increases the real or effective area of the surface. In this work we focus on the latter approach. If the texturing leads to a Cassie non-wetting state, the surface can also exhibit drag reduction characteristics. Thus, for the same energy input drag reducing surfaces lead to higher flow velocities or, conversely, in order to achieve the same flow rates and velocities, drag reducing surfaces require less energy input. In order to optimize the surface topography of the superhydrophobic surface, a stratified two-phase model of flow between flat plates was developed to simulate the friction reduction characteristics of the surface as a function of varying ‘fluid to gas’ ratios. The Stokes flow equation was used to derive velocity profiles with appropriate slip/no-slip conditions within the flow. Non-dimensional formulations were used to optimize the liquid flow rate as a function of the gas layer thickness. Based on these formulations, a pressure drop reduction of 72% is achieved when the air layer height to the total channel height is 7%. The results of the theoretical model were also compared against experimental measurements of microfluidic channels with different substrate surface topographies. Two different types of silicon substrates were used: one with flat plane topography and one with a micropillar array. The substrates were irreversibly bonded to PDMS (poly-dimethylsiloxane; Dow Corning) microfluidic channel replicas and were silanized to further enhance hydrophobicity. Flow was induced using a constant pressure source and the flow rate was measured using a high-precision scale. As expected, the experimental results deviated somewhat from the expected theoretical model due to the presence of the micropillar obstruction in the air layer. It was also observed that there was a certain ‘pillar-to-channel height’ ratio that minimized the pressure drop for a given flow rate.