Flexible conformable hydrophobized surfaces for turbulent flow drag reduction

Abstract

In recent years extensive work has been focused onto using superhydrophobic surfaces for drag reduction applications. Superhydrophobic surfaces retain a gas layer, called a plastron, when submerged underwater in the Cassie-Baxter state with water in contact with the tops of surface roughness features. In this state the plastron allows slip to occur across the surface which results in a drag reduction. In this work we report flexible and relatively large area superhydrophobic surfaces produced using two different methods: Large roughness features were created by electrodeposition on copper meshes; Small roughness features were created by embedding carbon nanoparticles (soot) into Polydimethylsiloxane (PDMS). Both samples were made into cylinders with a diameter under 12 mm. To characterize the samples, scanning electron microscope (SEM) images and confocal microscope images were taken. The confocal microscope images were taken with each sample submerged in water to show the extent of the plastron. The hydrophobized electrodeposited copper mesh cylinders showed drag reductions of up to 32% when comparing the superhydrophobic state with a wetted out state. The soot covered cylinders achieved a 30% drag reduction when comparing the superhydrophobic state to a plain cylinder. These results were obtained for turbulent flows with Reynolds numbers 10,000 to 32,500.

Leidenfrost Vapor Layers Reduce Drag without the Crisis in High Viscosity Liquids

“…Such vapor layers on free-falling spheres can efficiently reduce the drag and thus shift the drag crisis to a lower Re [22,23]. Such results with Leidenfrost vapor layers give an upper bound for the drag reduction possible by gas layers or plastrons, either sustained naturally on superhydrophobic surfaces or induced by microbubble injection [24][25][26][27][28][29][30][31][32][33]. Early experiments conducted using heated spheres in the perfluorocarbon liquid FC-72 comprising mainly perfuorohexane C 6 F 14 [22], as well as in water heated to 95°C [23], appeared to suggest that the drag reduction effect of the vapor layer follows a universal dependence on Re, with deviations from the no-vapor-layer case beginning from Re > 2 × 10 4 to a fully developed effect at Re ≃ 10 5 .…”

Failure pressures and drag reduction benefits of superhydrophobic wire screens

“…Woven screens enhanced with functional surface treatments/coatings have recently be considered as a costeffective alternative for producing a SHP porous surface in many applications including but not limited to drag reduction [8][9][10][11], oil-water separation [12][13][14][15][16], self-cleaning and anti-icing [17][18][19], and device manufacturing [24][25][26][27][28]…”

“…Given the prohibitive cost associated with large-scale production of microfabricated SHP surfaces, woven screens enhanced with functional surface treatments/coatings have recently been considered as a cost-effective alternative in many applications requiring a SHP surface. SHP woven screens can be used for drag reduction [8][9][10][11], oil-water separation [12][13][14][15][16], self-cleaning and anti-icing [17][18][19], underwater protection of electronic devices [20][21], water harvesting [22], and heat transfer [23] among many other applications. There have also been interesting studies on the load-carrying properties of SHP wire screens for device manufacturing [24][25][26][27][28].…”

Superhydrophobic Civil Engineering Materials: A Review from Recent Developments

“…As can be seen in Figure 8a,b, they developed a superhydrophobic surface on paraffin ax-coated glass; it can be seen that the candle soot-coated slide demonstrated a large WCA of 162 • . Based on the development of the flame synthesis method, large numbers of researchers have tried to employ different approaches for stabilizing the flame-generated coatings, as at that time they possessed low mechanical durability [58][59][60][61][62][63][64][65][66]. Qahtan et al fabricated water jet-resistant and thermally stable superhydrophobic surfaces by a spray coating of candle soot dispersion [67].…”