Various surface modification technology can form micro and nanoscale structures. The structures can change and control various properties of surface, i.e. wetting, heat flux, liquid flow, optic reflection. Wide range of industrial and everyday products can benefit from surface modifications.

Principle and theory

Surfaces with less than 90° contact angle are called hydrophilic, while the ones with more than 90° are called hydrophobic. Generally the tendency of wettability is decided by chemical composition of the surface, and micro- and nanoscale structures on the surface boosts the tendency. For example, superhydrophobic lotus leaves which slips off dirt are consist of hydrophobic wax and surface micro/nanostructures.

Unique characteristics of micro- and nanoscale structured surfaces could enhance heat transfer, reduce flow resistance, and make self-cleaning surface to improve a wide range of industrial productions.

Fabrication

1. ZnO Nanorod on Bulk-etched silicon

• Silicon etching + ZnO nanorod→Superhydrophilic surface
• Silicon etching + ZnO nanorod + SAM coating → Superhydrophobic surface

ZnO nanorod fabrication by catalytic growth method provides a generally usable surface modification technique, which can be applicable on various materials.

2. Laser Interference Lithography

The interference lithography is an effective fabrication method to form periodic structures on a large area. Coherent lights make interference pattern and selectively activates photoresist. Nanoscale patterns are developed after chemical treatment. Main advantage of this fabrication is, nanoscale lithography can be realized without photomask. Shape and period of patterns can be easily controlled by adjusting assembly of the interferometer.

3. Metal Surface Modification

- Aluminum

Anodic aluminum oxide (AAO) is a well-known method for nanostructure fabrication on aluminum surface. With appropriate parameters (type of solution, concentration, applied voltage, etc.), a well-ordered, high-aspect-ratio porous structures can be achieved on large area. Moreover, combination with microscale structure fabrication results superhydrophilic, high-evaporation surfaces.

- Zircaloy

Zircaloy is an alloy based on zirconium, usually consumed to make fuel claddings for nuclear reactor. Surfaces with superhydrophilicity and high spreading could enhance safety of the nuclear reactors. Anodic oxidation provides the Superhydrophilic, high-spreading surface to the zircaloy claddings. The nuclear fuel (e.g. uranium) dissipates tremendous amount of heat inside the claddings to boil the water. The modified surface can endure about two times higher heat flux at maximum.

4. Aerogel coating

Aerogels are a solid material, which is gas replaced liquid in gels. Extremely low density (3-150 mg/cm3) with very high porosity (90~99.8%) makes aerogel a good insulator. A thin (1 μm~10 μm) layer of aerogels can be achieved by modifying conventional fabrication method. Our aerogels are turned into hydrophobic by chemical treatment, which results superhydrophobicity because of combination with highly porous surface morphology. Main advantage of this method is, superhydrophobic surface can be achieved easily, while conventional superhydrophobic surfaces usually require complex fabrication steps. Furthermore, this method can be coated on various 3D surfaces.