Drag reduction in minichannel laminar flow past superhydrophobic surfaces

Abstract

We present friction drag reduction in minichannel laminar flow past superhydrophobic metallic surfaces. Femtosecond laser surface processing (FLSP), a one-step scalable method, was used to create permanent microscale and nanoscale structures on 316 stainless steel plates. The resulting superhydrophilic plates were covered by tall (215 μm) or short (27 μm) structures. The FLSP plates were then transitioned to superhydrophobic by evaporative deposition of a fluorinated silane, a low surface energy coating. Using purified water, the friction factor was obtained by measuring pressure drop along a minichannel with a rectangular cross section for flow rates corresponding to Reynolds numbers from 45 to 250. The superhydrophobic FLSP plates with the tall or short microstructures were used on the bottom surface of the minichannel. Pressure drop reduction was observed when compared to a smooth unprocessed stainless steel surface at the same flow rate. Compared to the drag experienced by the smooth unprocessed surface, the superhydrophobic FLSP surfaces exhibited a drag reduction over the entire range of Reynolds numbers tested. The drag reduction was attributed to the slip velocity created by the presence of an air layer (plastron) between the water and the superhydrophobic surfaces. The superhydrophobic FLSP surface with the tall microstructures exhibited drag reduction that was almost constant at approximately 15% for the entire range of Reynolds numbers tested. The drag reduction for the surface with the short microstructures decreased monotonically with Reynolds number and ranged from about 31% at Reynolds number of 45 to about 25% at Reynolds number of 250.