DRAG REDUCTION IN A SUPERHYDROPHOBIC MICROCHANNEL: EFFECTS OF INTERFACIAL POSITION

It has long been known that hydrophobic surfaces with roughness features of an appropriately chosen length scale interact with water in a highly non-wetting manner. This so-called superhydrophobic behavior manifests itself in a number of ways, including the formation of nearly spherical drops of water that are able to move on these surfaces with negligible friction and very small contact-angle hysteresis. One also finds these surfaces in nature, for example on the leaves of plants such as the lotus. The behavior of water flowing in a confined microchannel with superhydrophobic walls is equally interesting due to the potential for drag reduction. This paper describes the testing of microchannels constructed using a variety of post-structured superhydrophobic surfaces fabricated using deep reactive ion etching of silicon combined with a hydrophobic fluoropolymer coating. Pressure-flow characteristics were measured for samples of various roughness dimensions. A major challenge in correlating theory with experiment, however, is uncertainty in the location of the gas-liquid interface and the associated gas-liquid-solid contact line within the roughness features comprising the superhydrophobic surface. We present experimental results based on laser-scanning confocal microscopy that measure of the location of the gas-liquid interface and associated contact line for a fluid flowing in a parallel-plate microchannel test rig with a superhydrophobic wall. The gas-liquid interface and contact line location data are then used to correlate experimental flow and pressure drop data with a theoretical model based on porous flow theory.