nanotechnology makes "super" surfaces


by Dr Chiara Neto

 

Nature has been making surfaces that have "super"- properties for millions of years. Through nanotechnology we can now try to mimic nature and reproduce some of these superlative characteristics, to solve some of the world's most pressing problems. In this article I will describe two such examples, the super-hydrophobic lotus leaf and the water harvesting surface of a beetle's back.

 

1. The superhydrophobic lotus leaf

In the Buddhist religion the lotus plant is a symbol of purity and regeneration, because its leaves always remain clean and dry, even when emerging from muddy ponds. We define the surface of the lotus leaves as "super-hydrophobic", i.e. super water-hating. For the lotus, superhydrophobicity means more sunlight and fewer bacteria. For us, inspiration.

The surface of the lotus leaf is structured in a very sophisticated and advanced manner, as depicted in the scanning electron micrograph in Figure 1. The leaf presents a double scale of textures: hydrophobic waxy microscopic bumps (micrometers in size) covered in nano-scale hairs. This particular double scale roughness is especially engineered to trap pockets of air, so when the leaf is exposed to the pond's muddy waters, hardly any water actually comes into contact with the surface, and droplets roll straight off, taking dirt particles with them. The result is a surface on which water flows easily and readily, leaving no residue behind, in other words a self-cleaning surface.

 

Figure 1. Scanning electron micrograph of the surface of a lotus leaf.
Image courtesy of Ms Liwen Zhu, 2011.



How do you mimic the superhydrophobic lotus effect in the lab?

My group at the School of Chemistry is interested in devising new simple ways to imitate the lotus effect, and produce self-cleaning surfaces. One simple method relies on repeatedly immersing a substrate into a solution of stearic acid, a hydrophobic surfactant; with every dip, a thin layer of hydrophobic crystals is left behind. The deposited crystals create a very rough surface, made up of a dense forest of thin platelets, oriented side-on, producing a combination of micro-scale and nano-scale features. This combination of surface chemistry and special surface roughness produces the desired effect, a surface which is completely repellent to water, and that maintains this effect even after long exposure to water. In surface science terms, the contact angle formed by water on these synthetic superhydrophobic surfaces is very high [2].

What's a high contact angle?

The measurement of contact angle qualitatively identifies how wettable a surface is. The contact angle is the angle formed at the interface between liquid, solid and air when a droplet is deposited on a surface, and is measured within the droplet. A very high contact angle means the surface is highly water-repellent: on a superhydrophobic surface, water droplets look like perfect spherical marbles, as shown in the optical micrograph in Figure 2, and roll off very easily, carrying with them dirt particles. Our surfaces, with a contact angle of 172 degrees, are as water-repellent and self-cleaning as the lotus leaf. We have also developed other, more high tech approaches to achieve superhydrophobicity, which could find applications in microfluidic devices.



How is this useful?
There are many commercial and industrial applications that would benefit from surfaces on which the contact with liquids is minimised: for example, superhydrophobic coatings have the potential to reduce rusting of metal, fouling of ship hulls by barnacles, and hydrodynamic drag on objects that move through liquids. Superhydrophobic glass, paint and textiles could be made to be stain-repellent, so we wouldn't need to wash windowpanes nor our clothes so often, and we wouldn't need to repaint the walls of our homes as often. Great efforts are being conducted in academia and industry to translate the concept of superhydrophobicity into real world applications, with the outstanding challenge being the fabrication of superhydrophobic coatings that are robust and easy to apply on a large scale. My group has a collaboration with a major paint company to devise a new generation of paint coatings that will be stain- and water- repellent. Preliminary results are promising, but we still need to crack the secret that nature holds to make nanotechnology that can really work for us.

Figure 2. High contact angle of water on a superhydrophobic surface. Adapted from [1].

2. Harvesting water from thin air

Another excellent example of nature leading the way in terms of designing "super" surfaces is the Stenocara beetle from Africa's Namib Desert, shown in Figure 3, which harvests water from humid winds at dawn. The beetle's mostly waxy exoskeleton is patterned with microscopic wax-free bumps that are hydrophilic, or water-loving. Condensation of water from the atmosphere can only occur on these hydrophilic bumps, so when the drops grow larger, they detach, run along the hydrophobic part of the exoskeleton and into the beetle's mouth. This sophisticated mechanism allows the beetle to survive in one of the most arid regions of the planet. My group has devised an artificial mimic of the Stenocara beetle exoskeleton, which is based on thin polymer film dewetting.

 

Figure 3. The Stenocara beetle of the Namib desert collects water from the atmosphere.
Photo courtesy of Hans Hillewaert.



How does thin polymer film dewetting work?

Everyone has experienced the dewetting of liquid films in their everyday life, for example the break-up into droplets of a layer of water on the windscreen of a car. The process we use in the lab is just as simple. Two polymer films (the top one hydrophilic, the bottom one hydrophobic) are spin-coated onto a substrate to create a double layer. When heated, the top unstable polymer breaks apart, transforming into micro-droplets, like those shown in the optical micrograph in Figure 4. This effectively creates a pattern of hydrophilic bumps on a hydrophobic background, a microscopic mimic of the Stenocara beetle exoskeleton. When humid air is passed over this surface held in a vertical position, water droplets start to condense, first on the hydrophilic bumps, and then grow into larger drops, until they are too heavy and they detach from the surface. This water can then be collected and used. The novelty of our approach is that it is economic and truly up-scalable.

 

Figure 4. A dewetted polymer film bilayer produces hydrophilic droplets on a hydrophobic background. Adapted from [3].



How much does your surface improve condensation?

We can collect forty to fifty per cent more water on our micro-patterned surfaces than on a corresponding flat film. We calculate that if we scaled up our lab-scale samples to a one square meter prototype, we could collect two to three litres of water per square metre per hour, which is of the same order of magnitude as the water needs of a typical household. We are in the final stages of securing collaboration with an Industry partner that will help us expand our research and eventually translate this idea into practical application. Ultimately we envisage producing these micropatterned surfaces on large plastic sheets, which could be mounted on the side of buildings to collect atmospheric water under the typical humidity conditions found in Eastern Australia (70% humidity). These surfaces could help us tackle drought problems in countries like Australia, by providing a new way to collect water, which is truly sustainable and inexpensive. As this is a delocalised way to collect water and does not rely on being connected to the city's main water supply, we have also received many offers to apply the idea to the collection of water in emergency shelters or in remote locations.

 

References
  1. Neto, C; Joseph, KR and Brant, WR. On the superhydrophobic properties of nickel nanocarpets. Phys. Chem. Chem. Phys., 11 (41), 9537-9544, 2009. DOI: 10.1039/b909899b
  2. Joseph, KR and Neto, C. On the superhydrophobic properties of crystallized stearic acid. Aust. J. Chem., 63 (3), 525-528, 2010. DOI: 10.1071/CH09292
  3. Thickett, SC; Neto, C and Harris, AT. Biomimetic surface coatings for atmospheric water capture prepared by dewetting of polymer films. Advanced Materials, 23 (32), 3718-3722, 2011. DOI: 10.1002/adma.201100290

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BIOgraphy: dr chiara neto

Chiara was born and educated in Florence, Italy. After classical secondary studies, which included studying the language, culture and history of ancient Greece and Rome, the origins and development of philosophy, and arts history of the Renaissance, she moved to a scientific education by enrolling in a BSc and Masters degree (Laurea) in Chemistry at the University of Florence. After receiving her first degree cum laude, she continued her PhD at the same University, but keeping a clear agenda to study and live overseas. Her dreams came true through collaboration with the Department of Applied Mathematics at the Australian National University, an attractive destination as the group had pioneered research on her PhD topic, the direct measurement of surface forces. Chiara's PhD thesis was completed under the joint supervision of Prof. Piero Baglioni in Florence and Dr. Vince Craig at the ANU. Her results on slip of simple liquids at the interface with solid surfaces, relevant for the micro-control of liquid flow and to confined biological systems, were published in a paper that has become a seminal contribution in the field.

From then onwards, Chiara embarked in other scientific adventures around the world, first in 2002-2003 as a postdoctoral fellow in the Department of Applied Physics at the University of Ulm, Germany, in one of the world-leading groups in the field of thin film stability and dewetting (S. Herminghaus, K. Jacobs). Here she contributed significantly to this field with the study of a novel polymer pattern, dubbed "satellite holes", that develops upon dewetting of thin polystyrene films.

In 2003, after being awarded an Australian Postdoctoral (APD) fellowship, Chiara moved back to the Australian National University, to work on developing nanorheology, a new technique to measure liquid boundary slip based on a custom-modified atomic force microscope.

Since April 2007, Chiara has been working in the School of Chemistry at the University of Sydney and leads a research group in the field of liquid/solid interfaces, including wetting and dewetting, interfacial liquid flow, and development of new functional surfaces. Chiara is now an Australian citizen, but maintains strong ties with Europe, and through her contacts she has been able to attract several excellent postgraduate students to Sydney, a new generation of scientists who have embraced the challenge of international scientific exchange and collaboration.

 

Dr Stuart Thickett, Dr Chiara Neto and collaborator
Associate Professor Andrew Harris (Chemical Engineering
)

 

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