Scanning electron microscope (SEM) images of the surface morphology of the two different types of superhydrophobic coatings of SH-1 [(a), (c), (e), and (g)] and SH-2 [(b), (d), (f), and (h)]. For the SEM images, the coatings were prepared on aluminum substrates (1 in. × 1 in.). The same coatings were used for the large-area flat plate flow models.
Measurement of contact angles on superhydrophobic coating samples. Receding and advancing contact angles are measured by reduction and expansion of the droplet volume, respectively, by using a dispenser needle.
Schematics of wetting states on a planar (a) and structured (b)-(c) hydrophobic surfaces. (a) Conventional non-wetting state on a planar surface with a contact angle θ ref. (b) Superhydrophobic state in a Wenzel mode on a structured hydrophobic surface with an apparent contact angle θ w. (c) Superhydrophobic state in a Cassie mode on a structured hydrophobic surface with an apparent contact angle θ c.
Flat plate model. (a) Schematic of the dimensions of a flat plate and its edges. (b) Bare aluminum plate. (c) Superhydrophobic-coated aluminum plate (SH-2).
(a) Schematics (side and front views) of towing tank experimental apparatus. (b) A flat plate during a towing test for drag measurement (c) A force sensor mounted in a flexure unit which is located above the pivot axis and under the vertical pivot arm. (d) An inclinometer clipped at the plate pivoting frame is to measure a roll angle (enhanced online);. [URL: http://dx.doi.org/10.1063/1.4791602.1] [URL: http://dx.doi.org/10.1063/1.4791602.2]doi: 10.1063/1.4791602.1.
Results of hydrodynamic drag test at full speed range (1-30 ft/s). Symbols represent the experimental data (●: control plate, □: SH-1 coating, and ○: SH-2 coating). The error bar indicates ±5% for experimental uncertainty. Dotted lines represent theoretical drag coefficient lines [Eq. (4) ] fitted to the experimental data. The δC D-rough values were estimated for the data indicating fully turbulent flow regime (Re > 3 × 106).
Results of hydrodynamic drag test at lower speed range (1-5 ft/s). Symbols represent the experimental data (●: control plate, □: SH-1 coating, and ○: SH-2 coating). The error bar indicates ±5% for experimental uncertainty. Dotted lines represent theoretical drag coefficient lines [Eq. (4) ] fitted to the experimental data measured at the full speed range.
Micrograph images of a flat plate surface (SH-2) after flow tests. (a)-(b) After towing experiment at 2 ft/s. Many streak lines (dark color) shown in the images are those of air bubbles depleted from the surface by the wall shear flow. The upward-inclined trajectories of the streak lines are due to the combined effect of a buoyant force. The other area of the surface (light color) still retains air on the surface. (c)-(d) After towing experiment at 30 ft/s. At the higher shear flow, the amount of retained air on the surface (e.g., indicated by an arrow in (c)) is significantly reduced. The other area is wet since the air is swept off from the surfaces due to high wall shear rate.
Water droplets placed on the dried superhydrophobic plates after the flow tests. (a) SH-1 coating. (b) SH-2 coating.
Recent experimental studies of hydrodynamic drag reduction on superhydrophobic surfaces in high Reynolds number turbulent flows.
Summary of contact angle measurement data and estimated air fraction of superhydrophobic coatings.
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