Fabrication of superhydrophobic surfaces with high and low adhesion inspired from rose petal.

Certain rose petals are known to be superhydrophobic with high adhesion. There also exist rose petals which are superhydrophobic with low adhesion similar to lotus leaf. The purpose of this study is to characterize systematically the superhydrophobic rose petal with high and low adhesion surfaces and understand the mechanism for adhesion characteristics. Based on these, artificial superhydrophobic surfaces with high and low adhesion are fabricated using a two-step molding process and wax evaporation method. It is shown that the pitch values of microstructures and density of nanostructures play an important role in real rose petals and artificial surfaces to control their adhesion properties.

An angled nano-tunnel fabricated on poly(methyl methacrylate) by a focused ion beam.

Angled nano-scale tunnels with high aspect ratio were fabricated on poly(methyl methacrylate) (PMMA) using a focused ion beam (FIB). The fabrication parameters such as ion fluence, incidence angle, and acceleration voltage of the Ga(+) ion beam were first studied on the PMMA surface to explore the formation of the nano-scale configurations such as nano-holes and cones with diameter in the range of 50-150 nm at an ion beam acceleration voltage of 5-20 kV. It was also found that the PMMA surface exposed to FIB was changed into an amorphous graphitic structure. Angled nano-scale tunnels were fabricated with high aspect ratio of 700-1500 nm in depth and 60 nm in mean diameter at an ion beam acceleration voltage of 5 kV and under a specific ion beam current. The angle of the nano-tunnels was found to follow the incident angle of the ion beam tilted from 0 degrees to 85 degrees , which has the potential for creating a mold for anisotropic adhesives by mimicking the hairs on a gecko's feet.

Nanoscale ripples on polymers created by a focused ion beam.

We show that focused ion beam irradiation results in the creation of peculiar one- and two-dimensional nanoscale features on the surface of polyimide-a common polymer in electronics, large scale structures, and the automobile industry, as well as in biomedical applications. The role of ion beam incident angle, acceleration voltage, and fluence on the morphology of the structural features is systematically investigated, and insights into the mechanisms of formation of these nanoscale features are provided. Moreover, by using the maskless patterning method of the focused ion beam system, we have developed a robust technique for controlled modification of the polymeric surface. The technique, which is analogous to using a gray glass with varying darkness to control the radiation from the sun, but at a much smaller scale, enables the ion intensity and angle to be controlled at each surface point of the polymer, giving rise to structural surface features with desired shape and morphology.

Thermal stability of superhydrophobic, nanostructured surfaces.

The thermal stability of superhydrophobic, nanostructured surfaces after thermal annealing was explored. Flat surfaces coated with hydrophobic diamond-like carbon (DLC) via plasma polymerization of hexamethyldisiloxane (HMDSO) showed a gradual decrease in the water contact angle from 90(o) to 60(o) while nanostructured surfaces maintained superhydrophobicity with more than 150° for annealing temperatures between 25 and 300°C. It was also found that surfaces with nanostructures having an aspect ratio of more than 5.2 may maintain superhydrophobicity for annealing temperatures as high as 350°C; above this temperature, however, the hydrophobicity on surfaces with lower aspect ratio nanostructures gradually degraded. It was observed that regardless of the aspect ratios of the nanostructure, all superhydrophobic surfaces became superhydrophilic after annealing at temperatures higher than 500°C.

Bioinspired steel surfaces with extreme wettability contrast.

The exterior structures of natural organisms have continuously evolved by controlling wettability, such as the Namib Desert beetle, whose back has hydrophilic/hydrophobic contrast for water harvesting by mist condensation in dry desert environments, and some plant leaves that have hierarchical micro/nanostructures to collect or repel liquid water. In this work, we have provided a method for wettability contrast on alloy steels by both nano-flake or needle patterns and tuning of the surface energy. Steels were provided with hierarchical micro/nanostructures of Fe oxides by fluorination and by a subsequent catalytic reaction of fluorine ions on the steel surfaces in water. A hydrophobic material was deposited on the structured surfaces, rendering superhydrophobicity. Plasma oxidization induces the formation of superhydrophilic surfaces on selective regions surrounded by superhydrophobic surfaces. We show that wettability contrast surfaces align liquid water within patterned hydrophilic regions during the condensation process. Furthermore, this method could have a greater potential to align other liquids or living cells.