Wetting characteristics of Colocasia esculenta (Taro) leaf and a bioinspired surface thereof.

We investigate wetting and water repellency characteristics of Colocasia esculenta (taro) leaf and an engineered surface, bioinspired by the morphology of the surface of the leaf. Scanning electron microscopic images of the leaf surface reveal a two-tier honeycomb-like microstructures, as compared to previously-reported two-tier micropillars on a Nelumbo nucifera (lotus) leaf. We measured static, advancing, and receding angle on the taro leaf and these values are around 10% lesser than those for the lotus leaf. Using standard photolithography techniques, we manufactured bioinspired surfaces with hexagonal cavities of different sizes. The ratio of inner to the outer radius of the circumscribed circle to the hexagon (b/a) was varied. We found that the measured static contact angle on the bioinspired surface varies with b/a and this variation is consistent with a free-energy based model for a droplet in Cassie-Baxter state. The static contact angle on the bioinspired surface is closer to that for the leaf for b/a ≈ 1. However, the contact angle hysteresis is much larger on these surfaces as compared to that on the leaf and the droplet sticks to the surfaces. We explain this behavior using a first-order model based on force balance on the contact line. Finally, the droplet impact dynamics was recorded on the leaf and different bioinspired surfaces. The droplets bounce on the leaf beyond a critical Weber number (We ~  1.1), exhibiting remarkable water-repellency characteristics. However, the droplet sticks to the bioinspired surfaces in all cases of We. At larger We, we recorded droplet breakup on the surface with larger b/a and droplet assumes full or partial Wenzel state. The breakup is found to be a function of We and b/a and the measured angles in full Wenzel state are closer to the predictions of the free-energy based model. The sticky bioinspired surfaces are potentially useful in applications such as water-harvesting.

Umbrella leaves-Biomechanics of transition zone from lamina to petiole of peltate leaves.

In this study we aim to show how the peltate leaves of Colocasia fallax Schott and Tropaeolum majus L., despite their compact design, achieve a rigid connection between petiole and lamina. We have combined various microscopy techniques and computed tomography (CT) scanning for the analysis of the basic structure of the plant's stabilization system. Mechanical tests yielded key mechanical parameters and allowed us to assess the mode of failure. The results of the tests were further processed in a finite element method (FEM) analysis. We were able to show that both plants are able to endure high loads irrespective of the different composition of the supporting structure. C. fallax forms many separate branched strands, whereas T. majus forms fewer strands of greater diameter interconnected in the centre of the transition area, forming a bundle of irregular orientation. This results in different ways to dissipate loads on the lamina. In C. fallax we observed the outer strands of the strengthening tissue under high stress while the inner bundle carries little load. In T. majus the load is distributed more evenly through the juncture in the middle of the transition area. Potential applications include the construction of biomimetical flying roofs.

Nanostructure on taro leaves resists fouling by colloids and bacteria under submerged conditions.

The antifouling and self-cleaning properties of plants such as Nelumbo nucifera (lotus) and Colocasia esculenta (taro) have been attributed to the superhydrophobicity resulting from the hierarchical surface structure of the leaf and the air trapped between the nanosized epicuticular wax crystals. The reported study showed that the nanostructures on the taro leaf surfaces were also highly resistant to particle and bacterial adhesion under completely wetted conditions. Adhesion force measurements using atomic force microscopy revealed that the adhesion force on top of the papilla as well as the area around it was markedly lower than that on the edge of an epidermal cell. The decreased adhesion force and the resistance to particle and bacterial adhesion were attributed to the dense nanostructures found on the epidermal papilla and the area surrounding it. These results suggest that engineered surfaces with properly designed nanoscale topographic structures could potentially reduce or prevent particle/bacterial fouling under submerged conditions.

Surface chemical modification of poly(dimethylsiloxane)-based biomimetic materials: oil-repellent surfaces.

The oil-repellent performance of a poly(dimethylsiloxane)-based biomimetic replica (PDMS-replica) was tuned by modifying its surface chemical composition. PDMS-replica possessing a complementary combination of hierarchical roughness and mixed -CF(3) and -SiCH(3) terminal functionality was prepared in the presence of a surface-modifying agent, using nanocasting based on soft lithography. PDMS-replica showed superhydrophobicity and enhanced oil repellency, theta(oil) approximately 86 degrees . PDMS-replica was further modified with silica nanoparticles followed by chemical vapor deposition of (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane. The -CF(3) terminal, silica-modified PDMS-replica (i.e., PDMS-replica(silica/CF(3))) showed both superhydrophobic and high oil-repellent properties (advancing theta(oil) approximately 120 degrees ). During the process of each chemical transformation, the surface pattern present on PDMS-replica was preserved and monitored using scanning electron microscopy. Surface chemical compositions of PDMS-replica and PDMS-replica(silica/CF(3)) were determined using X-ray photoelectron spectroscopy. Understanding the extent of adhesion on a biomimetic replica possessing different surface chemical compositions and roughness would provide fundamental information for various applications.

Surface characterization and adhesion and friction properties of hydrophobic leaf surfaces.

Super-hydrophobic surfaces as well as low adhesion and friction are desirable for various industrial applications. Certain plant leaves are known to be hydrophobic in nature. These leaves are hydrophobic due to the presence of microbumps and a thin wax film on the surface of the leaf. The purpose of this study is to fully characterize the leaf surface and to separate out the effects of the microbumps and the wax on the hydrophobicity. Furthermore, the adhesion and friction properties of the leaves, with and without wax, are studied. Using an optical profiler and an atomic/friction force microscope (AFM/FFM), measurements on the hydrophobic leaves, both with and without wax, were made to fully characterize the leaf surface. Using a model that predicts contact angle as a function of roughness, the roughness factor for the hydrophobic leaves has been calculated, which is used to calculate the contact angle for a flat leaf surface. It is shown that both the microbumps and the wax play an equally important role in the hydrophobic nature as well as adhesion and friction of the leaf. This study will be useful in developing super-hydrophobic surfaces.