Abstract

Experiments were performed to characterize hydraulic jumps that form due to liquid jet impingement on superhydrophobic surfaces with alternating micro-ribs and cavities. If the surface is unimmersed, a surface tension based transition into droplets occurs, so a known depth of water was imposed downstream from the hydraulic jump to ensure the existence of a hydraulic jump. The surfaces are characterized by the cavity fraction, which is defined as the width of a cavity divided by the combined width of a cavity and an adjoining rib. Four different surface designs were studied, with respective cavity fractions of 0 (smooth surface), 0.5, 0.8, and 0.93. Each surface was tested in its naturally hydrophilic state where water was allowed to flood the cavities, as well as with a hydrophobic coating which prevented water from entering the cavities and created a liquid-gas interface over much of the surface. The experimental data spans a Weber number range (based on the jet velocity and radius) of 3x102 to 1.05x103 and a corresponding Reynolds number range of 1.15x104 to 2.14x104. While smooth surfaces always result in circular transitions, for any rib and cavity patterned surface the flow exhibits a nearly elliptical transition from the thin film, where the major axis of the ellipse is parallel to the ribs, concomitant with greater slip in that direction. When the downstream depth is small and a superhydrophobic surface is used, the water is completely expelled from the surface, and the thin film breaks up into droplets due to surface tension interactions. When the downstream depth is large or the surface is hydrophilic a hydraulic jump exists. When the water depth downstream of the jump increases, the major and minor axis of the jump decreases due to an increase in hydrostatic force, following classical hydraulic jump behavior. The experimental results indicate that for a given cavity fraction and downstream depth, the radius of the jump increases with increasing Reynolds number. The jump radius perpendicular to the ribs is notably less than that for a smooth surface, and this radius decreases with increasing cavity fraction. When comparing flow over superhydrophobic (coated) surfaces to patterned, hydrophilic (uncoated) surfaces, a general increase is seen in the radial location of the hydraulic jump in the direction of the ribs, while no statistically significant change is seen in the direction perpendicular to the ribs.

Degree

MS

College and Department

Ira A. Fulton College of Engineering and Technology; Mechanical Engineering

Rights

http://lib.byu.edu/about/copyright/

Date Submitted

2012-09-20

Document Type

Thesis

Handle

http://hdl.lib.byu.edu/1877/etd5646

Keywords

Michael Johnson, fluids, liquid jet impingment, superhydrophobic, Cassie, Wenzel

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