A lesson from nature - biomimetics

Materials World magazine
1 Oct 2009

Zaki Ahmad and Faheemuddin Patel from the Mechanical Engineering Department at King Fahd University of Petroleum and Minerals in Dhahran, Saudi Arabia, describe possible uses of hydrophobicity found in nature.

The lotus flower (Nelumbo Nucifera) is a symbol of purity and divine wisdom and has played an important role in the cultures of India, China and Japan. Seeding in muddy waters it rises above the mud and its leaves are always spotless. Rainwater rolls off the surface as beads and removes dirt from the surface of the leaves. This property of keeping water away from surfaces is called hydrophobicity and is also found in insects, such as the water strider and Namib desert beetle. The back of the beetle is bumpy, waxy and superhydrophobic, which allows it to harvest water during foggy periods. The water condenses on the bumps and slides down the back of the beetle through the waxy microfluidic channels into the mouth.

This phenomenon has enabled researchers to control the flow of water at the micro- and nanoscale and has allowed the mimicking of nature to harvest liquid from fog in arid regions. With the growth in nano technology, the art of mimicking the desert beetle at the micro- and nanoscale on artificial surfaces is advancing. Roses grip water droplets in place even when turned upside down. This is due to water repelling epicuticular wax crystals.

Degrees of success

Hydrophobicity is determined by a contact angle – the angle between water droplets and the surfaces. If the contact angle is less than 90º, the surface is referred to as hydrophilic. It is hydrophobic if the angle is greater than 90º, and superhydrophobic if the angle is greater than 160º. Superhydrophobic surfaces have a minimum hysteresis between the advancing and reducing contact angle, which is given by ∆Q = (Øadv - Ørec).

The secret of hydrophobicity lies in its hierarchical surface roughness, which comprises a hybrid micro- and nanostructure. The surface shows microscale mounds and nanoscale bumps containing waxy epicuticular crystals. This structure provides a roughness that repels water and dust. This is an example of naturopathy, where the surface keeps itself clean from dust and water without involving human effort.

The efforts to mimic the lotus effect on man-made engineering surfaces may be classified in two ways – making a rough surface from low surface energy materials or modifying a rough surface with a material of low surface energy. The low energy materials include fluorocarbons, silicones, organic materials like polyamides and polycarbonates, and inorganic materials like ZnO and TiO2. Methods to make a rough substrate include etching and lithography, sol-gel processing, colloidal assemblies, electrochemical deposition, electro spinning, wet chemicals and routes for physical vapor deposition.

Recent developments in the production of superhydrophobic surfaces include plasma fluorination of polybutadiene films. This,followed by crosslinking of polybutadiene films,produces superhydrophobic surfaces. Such films have already been used in strain-proof textiles, medical implants, marine coatings and microelectronics. Studies on superhydrophobicity show the possibility of separating oil from water by creating stainless mesh films with both the micro and nanostructural characteristics of a lotus flower. A hard coating stainless mesh film with both superhydrophobic and superoleophobic properties has been produced by spraying with a low energy compound of polytetrafluoroethylene.

It has long been believed that hydrophobic surfaces cannot be prepared from amphiphilic polymers. This has been belied by the formation of foam such as PVA (polyvinyl alcohol) nanofibres. A water contact angle of 171.2º±1.6º was shown for these nanofibres. This shows that amphiphilic polymers can be used to prepare superhydrophobic surfaces.

Super repellent Al2O3 coating films with high transparency have been prepared by sol-gel techniques in boiling water with a contact angle of 165º. The contact angle increased with higher immersion time and super water repellency was shown by immersion for 30 seconds. The transmission of visible light was recorded at 90%. On treatment with fluoroalkyl-trimethoxysilane, the films became highly transparent.

A new theory of switching between superhydrophobicity and superhydrophilicity is being developed using ZnO nanorod films. The ZnO nanorods are prepared by the sol-gel technique for engineering surfaces where both hydrophobic and hydrophilic characteristics might be needed.

Cleaning up

An easy-to-clean surface is the aim of many architectural, industrial and domestic coatings. An attempt has been made to produce a nanocoating with self-cleaning and water repellent properties using OH¯ functional acrylate additives, which are shown to provide self-cleaning properties. Common binders such as polyurethane, when combined with specific hydrophobic additives, provide friendly self-cleaning surfaces. The polar component of the surface is reduced by 97% when using crosslinkable acrylate additives. A commercially developed silicone modified polyacrylate gives natural replacement of epicuticular nanowax and can be used to remove contaminated particles.

Titanium dioxide has also been commonly used to convert harmful self-cleaning paints containing nitrous oxide gas into harmless by-products. Titanium dioxide particles have been commonly embedded in polysiloxanes. These nanoparticles breakdown foul NOx to HNO3, which is washed away by rain or neutralised by calcium carbonate particles in paints.

Surface hydrophobicity is increasingly being incorporated in thermal spray nanostructured coatings and notable advances have been made. The work undertaken at the King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, is a dual approach in which TiO2 particles are plasma sprayed onto a pre-designed rough surface (lotus surface) on a steel substrate. In recent years, the two-level surface roughness has been induced by shot penning, cavitation and cavitation shotless penning (CSP) and carbon nanotubes are being used to create nanobumps.

The phenomenon of hydrophobicity is being used in thermal spray technology as shown by the development of nanostructured TiO2 coatings for harsh environment. As the self cleaning effect is based on physico-chemical principles, it can be transferred to man-made surfaces. Hydrophobicity of surfaces is governed by their chemical properties and dependent on an appropriate microstructure. In thermal spray nanostructured coatings, the advantage of hydrophobicity would be lost if the surface homogeneity is not controlled.

The application spectrum of hydrophobic surfaces is dramatically expanding, giving impetus to its marketing potential because of its self-cleaning properties.

Superhydrophobic surfaces have a large market potential, particularly in the gulf region. Water is detrimental to metals and huge losses are caused by wear, surface degradation and corrosion. Hydrophobicity is potentially naturopathy to prolong the life of materials.

Further information: Professor Zaki Ahmad