Schematic of air trapped between hydrophobic microfeatures of a superhydrophobic surface. The air-water interface produces shear-free regions resulting in a reduction in wetted area and regions that can experience significant slip in flows. (b) Micrograph of a superhydrophobic microridge geometry containing wide ridges spaced apart. Features are approximately deep.
Water and ethanol droplets resting on a superhydrophobic surface. The water drops stand off the surface in the Cassie state while ethanol fully wets the surface in the Wenzel state. Microridges run front to back and the air-water interfaces they support are visible under the water drops.
Cross section of flow cell used for PIV with a PDMS superhydrophobic surface on the bottom and a smooth acrylic surface on top. The bottom surface was interchangeable and was replaced with a number of different superhydrophobic PDMS surfaces.
Cross section of flow cell used for pressure drop measurements. Superhydrophobic surfaces were fitted to both the top and the bottom surfaces of the channel.
(a) Velocity profiles over a microridge surface showing the development of significant slip velocities with increasing Reynolds number from 2700 (△) to 8200 (◼). (Inset) Velocity profiles near the wall demonstrating prominent slip velocities. Reynolds numbers are 2700 (△), 3900 (▲), 4840 (◇), 5150 (◆), 6960 (◻), and 8200 (◼). For clarity, the modified Spalding fits (◻) from Eq. (3) are only overlaid on the profiles corresponding to and . (b) Velocity profiles over the microridge surface demonstrate slip velocity behavior consistent with that observed on the 60-60 surface, but reduced in magnitude. Reynolds numbers range from 4970 (○) to 7930 (▽). Larger feature spacing performs better for a given Reynolds number. Reynolds numbers are 4970 (○), 5400 (◆), 6800 (△), 7160 (◼), and 7930 (▽) The modified Spalding fits (◼) are overlaid on the profile corresponding to .
Pressure drop measurements for flow through a rectangular channel with a smooth walls (△) and with two walls containing superhydrophobic microridges with and (◼). The Colebrook line (–—) is shown for a smooth channel.
Wall shear stress measured from PIV as a function of Reynolds number for a channel with a single superhydrophobic surface. Results are presented for both the smooth top wall (△) and the superhydrophobic bottom wall containing wide ridges spaced apart (●). Drag reduction is seen only on the superhydrophobic wall, the smooth wall being in good agreement with the Colebrook prediction for a smooth channel (—).
Coefficient of friction for various surfaces calculated from both PIV and pressure measurements. Smooth surfaces (△) and superhydrophobic surfaces containing wide microridge spaced apart (●) are shown for PIV measurements of a channel with a single superhydrophobic wall. Pressure drop measurements from channels with two smooth walls and two superhydrophobic walls containing and microridges (○) and microridges (◼) are also shown. The predictions of the friction coefficient for a smooth channel are also shown (—) in both the laminar and turbulent regimes. Transition occurs around .
Drag reduction as a function of Reynolds number for a channel with (a) a single superhydrophobic wall (●) and (b) two superhydrophobic walls containing and microridges (○) and and microridges (◼).
The microridge spacing in wall units, , as a function of Reynolds number. The data are taken from PIV measurements from a channel with a single superhydrophobic surface of and microridges (●) and from pressure measurements for flow through a channel with two superhydrophobic walls containing and microridges (○) and and microridges (◼). A spacing of corresponds to the thickness of the viscous sublayer. Only points in the turbulent regime are shown.
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