Graduate Studies

 

First Advisor

George Gogos

Department

Mechanical Engineering

Date of this Version

Spring 2024

Document Type

Dissertation

Comments

Copyright 2024, Henry Ems. Used by permission

Abstract

In this work, 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.

Drag reduction/enhancement in channel turbulent flow past superhydrophobic metallic surfaces was also studied. FLSP is used on 304 stainless steel plates to create microscale and nanoscale structures. To begin, angled microstructures were created, which mimic those of shark skin. The 304 stainless steel plates for the top, bottom, and sides of the channel were functionalized. Data were collected at different Reynolds numbers by varying the mass flow rate. Data were recorded after steady state was reached. The processed plates were superhydrophilic and were used to obtain the friction factor in a rectangular channel test section over Reynolds numbers ranging from 8,000 to 13,000. For a superhydrophilic rectangular channel with angled structures, drag enhancement was measured with respect to smooth (unprocessed) surfaces over the total range of Reynolds numbers tested. After superhydrophilic testing was completed, the surfaces were coated with fluorinated silane using evaporative deposition that made the plates hydrophobic. The hydrophobic plates were then tested in the rectangular channel setup to obtain the friction factor. With the addition of an acrylic viewport, the presence of an air layer (plastron) was observed that sheds light to the friction factor data obtained for hydrophobic plates. Drag reduction was shown for Reynolds numbers that were accompanied with a thin plastron. When the plastron fully degraded, the surface was fully wetted, and the friction factor value shifted towards just below the superhydrophilic value.

In the final experiments, 304 stainless steel plates were functionalized with FLSP using the same laser parameters as the short microstructures for laminar flow. The peaks of the microstructures were nearly flush to the original surface. The processed area covered the top and bottom of the channel. For the superhydrophilic rectangular channel with short microstructures, drag enhancement was measured with respect to smooth surfaces over the total range of Reynolds numbers tested. There was an increase in drag of around 54% with respect to the smooth surface at the highest Reynolds number tested. For a hydrophobic rectangular channel with short microstructures, drag reduction was shown for the lifetime of the plastron for Reynolds numbers of 9,200, 15,000, and 19,300. The lifetime of the plastron was defined as the amount of time drag reduction was measured in the channel. The lifetime of the plastron decreased from over 6 hours for the lowest Reynolds number tested of 9,200 to 10 minutes at a Reynolds number of 19,300.

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