Fabrication of Salvinia-inspired surfaces for hydrodynamic drag reduction by capillary-force-induced clustering

Abstract

For decades, bioinspired functional materials have been attracting the interest of many researchers for their remarkable characteristics. In particular, some plant leaves are well known for their inherent superhydrophobic nature. Salvinia molesta, a free-floating aquatic fern, has egg-beater-shaped hierarchical trichomes on its surface of leaves. Due to the unique structure and complex wettability of the hairs, this plant has the ability to maintain a stable thick air layer upon the structure when it is submerged underwater. Often referred to as the "Salvinia Effect," this property is expected to be suitable for use in hydrodynamic drag reduction. However, due to the complex shape of the trichome, currently applied fabrication methods are using a three-dimensional printing system, which is not applicable to mass production because of its severely limited productivity. In this work, artificial Salvinia leaf inspired by S. molesta was fabricated using a conventional soft lithography method assisted with capillary-force-induced clustering of micropillar array. The fabrication method suggested in this work proposes a promising strategy for the manufacturing of Salvinia-inspired hydrodynamic drag reduction surfaces.

Fig. 1. Overall concept of the research.

a Hairs on the Salvinia leaves (left) and Salvinia-inspired hierarchical microstructures (right). Reproduced with permission. B. Rice, sarracenia.com, Bugwood.org. Copyright 2010, Wiley. b Schematics showing the concept of the fabrication of Salvinia-inspired HDR surfaces.

Fig. 2. Design of the master mold.

a, b Schematics of major design variables for CFIC, where ${\mathit{d}}_{\mathit{t}}$ is the micropillar tip diameter, $\mathit{h}$ is the height of the subpillar, ${\mathit{r}}_{\mathit{c}}$ is the center-to-center radius, and $\mathit{\delta }$ is the deflection of a pillar. c, d Schematics of a hierarchical pillar being bent to capillary interaction force when partially immersed in a liquid. e Three-dimensional plot of the critical conditions for each collapse type. f Plot depicting critical conditions for CFIC and clustering angle of the pillar tips. The design point should lie in the desirable design region to induce the clustering of upper pillars while avoiding lateral collapse. g Top wireframe view of the designed hierarchical pillar array template. h Isotropic view of the designed pillar array template.

Fig. 3. Microscopic images of micropillars clustering as PVA solution dry out.

a Micropillars at their initial position with pillar tips partially immersed by PVA solution. b Micropillars being bent due to the capillary force of PVA solution. c Micropillar tips making contact with one another. d Clustered micropillars right after the PVA solution dried off the surface.

Fig. 4. Schematics of the hierarchical pillar structure at each fabrication step.

Each schematic shows the hierarchical pillar structure at each fabrication step, from the original master mold fabricated by 2PP 3D printing system (leftmost) to the final step (rightmost). Insets show schematics of the tip part of the clustered micropillars.

Fig. 5. SEM images of the fabricated sample surface.

a Clustered hierarchical pillar arrays. b Hydrophobic wax and hydrophilic tips on a single clustered structure.

Fig. 6. Microscopic images of a single clustered structure pushing and pulling the water meniscus.

Salvinia-like behavior with the water-air interface is shown when a single clustered structure of the fabricated Salvinia-inspired surfaces comes into contact with water. Scale bars represent 300 μm.

Fig. 7. Apparent viscosity of 40 wt% glycerol on the reference plate and the Salvinia-inspired surfaces.

Apparent viscosity of 40 wt% glycerol measured by the rotational rheometer on the reference plate (black squares), Salvinia-inspired surfaces (blue circles). Mean values for each case are indicated by solid lines, and the known value of the liquid is indicated by the dashed line.