Nano in Nature

How can we be covered in water and not get wet?

It was noticed that the striking white flowers of the lotus plant stayed dirt free and dry even after being submerged in muddy river water. This helps the lotus in two ways: Stops bacteria sticking to the surface. Keeps the lotus cool.

The Lotus leaf has two interesting features: The surface of the leaf is covered in a waxy material which makes it water fearing or hydrophobic. How does the lotus keep clean?

To understand the second feature we have to take a closer look… And closer…And closer still… The surface of the lotus leaf is actually quite rough. The projections increase the surface area reducing the amount of contact water droplets have with the surface.

When water falls on the surface of a leaf it clumps together to form large droplets. The combination of the waxy coating and rough surface allow the droplets to roll right off taking dirt and micro-organisms that sit on the surface with them.

We can create surfaces that stay dry and clean in the same way as the lotus leaf. Self cleaning windows Waterproof clothes How does this benefit us?

But some animals are doing the opposite! Instead of shedding water the Namib Desert Beetle uses nanostructures to capture it. A combination of hydrophilic ridges and hydrophobic furrows allows collection of moisture from fog. By applying this idea to buildings we could trap moisture and use as a water source.

What do you need to walk upside down on the ceiling?

Geckos have the amazing ability to cling to any surface at any orientation. This is due to the structure of their feet which maximises surface area.

The soles of geckos’ feet are made up of overlapping lamella. These lamella are covered with tiny hairs called setae. Each setae branches out into hundreds of spatula shaped structures. A closer look

Most surfaces when you look at them appear smooth but at a molecular level they can be quite rough. Objects may look like they’re touching all the surface but they are not. Geckos’ feet are flexible enough to fit into the nooks and crannies of any surface. At such close contact, forces of attraction called ‘Van der Waals’ forces arise between the setae and the surface creating grip. How does a gecko stick?

Scientists are taking inspiration from the nanofibres in geckos’ foot hairs to develop adhesives that will bind to wet and dry surfaces. Adhesives made from carbon nanotubes which imitate the setae on geckos’ feet have been developed. Gecko glue

Can we make fibres stronger than steel?

Very strong! It is the material with the highest known strength, about 5 times that of steel of the same weight. It is also elastic; spider silk can stretch up to 10 times its own length. How strong is spider silk?

Spider silk is produced from about six silk glands beneath the spider’s abdomen. The silk consists of protein molecules, long chains comprising thousands of amino-acid elements. The protein is formed as a liquid by silk glands and squeezed out -the liquid thread hardens as it leaves. More about spider silk

Reports that goats have been genetically modified to secrete silk proteins in their milk. The milk from the goats was made into a super- strong silk strand. The thread could be weaved to make strong materials. Spider goat?

The fibres in caterpillar cocoons are almost as strong as steel. In CRANN, researchers are taking inspiration from these cocoons. They are synthesising fibres that are stronger than steel but 5 times lighter - by mixing plastic (PVA) with carbon nanotubes. Caterpillar cocoons

Ultra-tough bullet-proof vests. Sports equipment. Aeronautics industry. Car parts. Household goods. What type of materials?

How is light manipulated at the nano-scale?

Colours of some materials are determined by a chemical pigment that absorbs some light and reflects the rest (“chemical colour”). For example, chlorophyll in plants absorbs light in the blue and red part of the spectrum but reflects green. This is why leaves appear green. But first colours…

An object can change colour when the light interacts with the physical structures (“physical colour”). When light waves strike a transparent surface some of the light is reflected. A few light waves penetrate the material and reflect off the bottom of the surface. These waves pass upwards and re-join the original waves. When the crests of the reflected waves line up they are ‘in phase’ and constructive interference is observed. The waves combine and the colours appear brighter. When the crests of the reflected waves do not line up they are ‘out of phase’ and destructive interference is observed. The waves cancel each other out and colours appear dimmer. Physical colour…

The extraordinary colours on the butterfly come from the interaction of light on the nanometre size structures on their wings. The same effect can be seen on a CD when you tilt it. The Morpho butterfly

The blue morpho butterfly is native of central and south America, the male is known for its iridescent blue colour. The iridescent colours on its wings are not created by pigments, but by the way light interacts with the nano structures on the wings’ scales. The true colour of the wings can be seen when light is passed through the wing but not reflected. Here is a clearer image of the scales taken using an electron microscope. The scales on the wings are arranged like tiles on a roof. Each scale is approx. 70 x 200 µm. If we take a closer look at the scales, we can see ridges. Each ridge is approx. 800 nm wide and contains structures that reflect the light. A cross section of these ridges show the nanostructures that reflect the light. The microribs are shaped like evergreen trees (short branches at the top long branches at the base) allowing for multiple reflections. Let’s take a closer look at the butterfly

Scientists are using their understanding of the structure of butterfly wings to develop new fabrics, dye-free paints, and anti-counterfeit technologies for currency. Applications

Natural nanomaterials are inspiring scientists! Lotus effect: –Self-cleaning windows. –Waterproof clothing. Gecko feet: –Adhesives. Spider silk and caterpillar cocoons: –Stronger, lighter materials. How many more natural nanomaterials can you remember? Summary