Coatings xperience: a different view of natural.

Natural has become an important word these days as emphasis on green technologies and sustainability increases throughout the coatings value chain. "Natural" in this context refers to the source of the raw materials used to manufacture the resins or additives incorporated into a coating formulation. There are some researchers, though, who are looking at natural phenomena, rather than substances, to develop novel coatings with advanced performance properties.

SUPERHYDROPHOBIC COATINGS--PART A: THE LOTUS EFFECT 

The lotus leaf has been known for some time to exhibit ultra-hydrophobic characteristics. This quality is attributed to the presence of nano-scale three dimensional patterns on its surface that are covered with a nanoscopic water-repellant coating. Several companies have developed coating products based on the idea of the lotus effect.

Recently, researchers at the National Institute for Interdisciplinary Science and Technology in India have more closely mimicked the surface of lotus leaves through the use of carbon nanotubes with a nanometer thick paraffin coating. (1)

With the lotus leaf, water can only come in contact with minute structures on the surface that protrude about 3-10 [micro]m. The water repellant coating prevents the water from getting into the spaces between the structures. As a result, the water "beads" and rolls off the leaf, taking dirt with it.

[ILLUSTRATION OMITTED]

Ayyappanpillai Ajayaghosh and his colleagues attached para-phenylenevinylene (PPV) to multi-walled carbon nanotubes (MWCNT) using a self-assembly process. The aromatic rings of the PPV attach themselves to the honeycomb structure of the nanotubes, while the long hydrocarbon chains of the PPV act as a paraffin-like coaling.

When a dispersion of these modified nanotubes is applied to glass, metal, or mica surfaces and then allowed to dry, a water-repellant coating forms that behaves in the same way as the lotus leaf when exposed to water. While the microscopic structure is not regular like that found on the surface of the lotus leaf, the roughness of the coating at the nanoscale is similar.

The coating has shown similar behavior with various water-based acidic and basic solutions and imparts superhydrophobic characteristics to surfaces. The researchers have patented this nano-composite coating.

SUPERHYDROPHOBIC COATINGS--PART B: LEARNING FROM WATER-SKATING BUGS

The water strider is an insect that skates across water at high speed. It is able to do this because it has tiny hairs on its legs that trap pockets of air between its legs and the water. These air pockets provide a cushioned layer that will not break the water's surface tension, and thus keep the bug afloat.

Researchers Qinmin Pan and Min Wang in the Department of Applied Chemistry at the Harbin Institute of Technology in China used the water-strider concept to construct very small boats (postage-stamp sized) made of copper mesh with millimeter-sized pores. (2) A rough surface mimicking the microstructure found on the legs of the water strider was created by depositing tiny amounts of silver nitrate. The mesh was then dipped into a bath containing a water-repellant substance. This technology was initially developed by Iain Larmour and colleagues at Queen's University in Belfast, Ireland.

The combined coating on the copper mesh resulted in formation of a layer of air between a test boat and water. The presence of this cushion of air made it possible for the boat to displace significantly more water than an untreated one. As a result, the boat with this coating is able to carry much larger loads than would ordinarily be expected.

Interestingly, the coated boat sinks when placed in organic solvents.

Coated boats were found to stay afloat carrying nearly five times the weight supported by uncoated mesh boats. In addition, coated mesh boats were able to remain afloat even when submerged slightly under water.

The scientists are investigating how changing the pore size of the mesh and varying the materials used to create the uneven coating on the surface can improve the buoyancy of boats.

While the technology is not suitable for scale-up to large vessels--the entire surface must be coated for the effect to be achieved--it does have potential for use in aquatic robots and environmental monitoring devices.

[ILLUSTRATION OMITTED]

ANTIREFLECTIVE COATINGS: LOOKING THROUGH THE EYES OF A MOTH

The dramatic rise in energy costs over recent years has led to greater interest in alternative energy sources. Silicon-based solar cells are a leading candidate. But there are problems associated with solar power, a major one being the inability to utilize all of the solar energy to which cells are exposed.

While silicon is an excellent substance for storing and transmitting electrical energy, its high refractive index causes nearly one-third of the incident light that strikes solar cells to be reflected back rather than be converted to usable energy.

Antireflection coatings (ARCs) have been developed to reduce the amount of light reflected. Typically, these coatings are comprised of silicon-nitride thin films. These coatings are quite expensive, however, and are most efficient only for wavelengths around 600 nm.

[ILLUSTRATION OMITTED]

Again, we look to nature for an answer. It turns out that moths have antireflective "coatings" to reduce reflectivity in their compound eyes. And these coatings minimize reflection over a wide range of wavelengths and angles of incidence. Specifically, the outer corneal lenses of moths are made up of non-close-packed conical-shaped protuberances that are typically less than 300 nm in height and spacing. It is these "corneal nipples" that provide the desired antireflective effect.

A research team in the Department of Chemical Engineering at the University of Florida (UF) has developed a spin-coating technique for producing colloidal crystals that mimic the structure of the antireflective coating feature found in the eyes of moths. (3) The non-close-packed colloidal monolayers are comprised of hexagonally ordered silica particles on silicon wafers.

According to UF student Wei-Lun Min, these colloidal ARCs are intrinsically more stable and durable than multilayer ARCs. In addition, the coatings have a high aspect ratio and nonwetting properties on top of their broadband antireflection capabilities.

The process is also much simpler than that used for current commercial antireflective coatings. Existing manufacturing systems can be employed to create the wafer-scale silicon pillar arrays. In addition to being valuable for improving the efficiency of solar cells, these new coatings may have applications for other optical systems.

UF Professor Pen Jiang's group is also developing coatings that mimic the self-cleaning effect of the wings of the cicada. Like the eyes of moths, legs of water striders, and leaves of the lotus plant, the cicada wings have natural microstructures. They are in fact very similar to those in moth eyes--non-close-packed, hexagonally ordered nipple-shaped protrusions with sub-300-nm scale dimensions.

Natural phenomena are phenomenal indeed.

References

(1) Ajayaghosh, A., "Bioinspired Super-hydrophobic Coatings of Carbon Nanotubes and Linear [PI] Systems Based on the 'Bottom-up' Self-Assembly Approach," Ang. Chem. Intl. Ed., 47, No. 51, 5750-5754, 2008.

(2) Pan, Q. and Wang, M., "Miniature Boats with Striking Loading Capacity Fabricated from Superhydrophobic Copper Meshes," ACS Appl. Mater. Interfaces, 1 (2), p. 420-423, 2009.

(3) Min, W.-L., Jiang, B., and Jiang P., "Bioinspired Self-Cleaning Antireflection Coatings," Adv. Materials, 20 (20), p. 3914-3918, 2008.