NATURAL NANOMATERIALS

What is a natural nanomaterial?

Belonging to natural world

No human modification or

processing

Remarkable properties due

to inherent nanostructures

BIOMIMICRY“The examination of nature, its models, systems,

processes, and elements to emulate or take inspiration from in order to solve human

problems.”◦ Basically scientists see something really cool in

nature and try and copy it.

◦ Lets get to some examples!!!!

Biomimetic material

Adhesives

High tensile strength fibre

Dirt and water resistant paint

IMOD Display technology

Reduced drag suits for athletes

Structural elements

Aerospace and automotive applications

Inspired from

Gecko’s feet

Spider silk

Lotus leaf

Butterfly wings

Shark skin

Wood, ligaments and bone

Toucan’s beak

Lotus leaf

• Clean leaves in muddy habitat

• Superhydrophobicity

• Water droplets roll off dragging dirt along with it

• ‘Self Cleaning effect’

How is this ‘Nano’?

• “Lotus effect”

• Self-cleaning properties of the lotus plant are the combination of the micro-structure of the leaves and of the epidermal cells on its rough surface, which are covered with wax crystals

Self Cleaning effect ?

• Large contact angle due to epicuticula wax and micrometer-scale bumps on the leaf

• The epicuticula wax provides the low surface free energy, and the micrometer-scale bumps brings a large extent of air trapping when contacting with water, which is essential for superhydrophobicity.

(a) SEM micrographs (shown at three magnifications) of lotus (N. nucifera) leaf surface, which

consists of microstructure formed by papillose epidermal cells covered with epicuticular wax

tubules on the surface, which create nanostructure (Bhushan et al. in...

Bhushan B Phil. Trans. R. Soc. A 2009;367:1445-1486

Diagram summarizing the connection between roughening and self cleaning. While on smooth surfaces the particles are mainly redistributed by water (bottom), they adhere to the droplet surfaces and are removed from the leaves when the droplets roll off

Technical Applications• Treatments, coatings, paints, roof tiles, fabrics and other

surfaces that can stay dry and clean themselves in the same way as the lotus leaf.

• Super-hydrophobic coatings comprising Teflon microparticles have been used on medical diagnostic slides.

• Self-cleaning glasses installed in the sensors of traffic control units.

• Lotus effect superhydrophobic coatings applied to microwave antennas can significantly reduce rain fade and the buildup of ice and snow.

Gecko’s Sticky Feet• Geckos are lizards belonging to the

infraorder gekkota.

• Can cling to any surface at any orientation

• Can walk on smooth and rough surfaces

• Upside down on a glass surface

• Walk on a dirty or wet surface maintaining full contact

What makes it possible?• At the initial study people thought that

friction and glue are the reasons.

• Geckos lack glandular tissue on their toes, so sticky secretions were ruled out early in the study of gecko adhesion.

• The friction hypothesis was also dismissed quickly because, by definition, friction only acts in shear; therefore, it cannot in itself explain the adhesive capabilities of geckos on inverted surfaces.

• Anton Haase first suggested that geckos stick by intermolecular forces.

• Remarkable toes.

• Gecko toe is crossed by ridges covered with hair like stalks called Setae , which branch into hundreds of tiny endings called spatulae.

Small ridges called ‘Scansors’

◦ Numerous ‘Setae’ projections◦ Setae - 100 µm long, 5 µm diameter

◦ Spatulae (200nm wide projections)

◦ These structures provide essential shear force and adhesive force.

◦ So, In order to study scientists used a newly developed microsensor to measure the adhesive force (which resists pulling) and shear force (which resists sliding) of an isolated gecko seta.

• When they first tried to measure these forces, the resistance to sliding was not more than what they expected from plain friction.

• It wasn't until they oriented the seta correctly that they discovered the importance of specific motions in getting the seta to stick.

• Slightly pressing the seta against the surface yielded a shear force of about 40 micro newtons. Combining the preload with 5 micrometers of displacement (drag) gave an even larger shear force.

• Around 6.5 million setae on a gecko feet could generate 1,300 newtons of shear force enough to support the weight of two medium-sized people based on measurements from single setae. These numbers suggest that a gecko is only attaching 3 percent of its setae in generating the strongest force (20 newtons) measured in whole-animal experiments.

• But gecko is only attaching 3 percent of its setae in generating force.

• Geckos could use it to withstand tropical storms, resist predator attack or recover their grip after a drop.

• A few years ago, they observed that simply increasing the angle between the seta shaft and the substrate to 30 degrees causes detachment. As this angle increases , the increased stress at the trailing edge of the seta causes the bonds between seta and substrate to break.

• In this way attachment and detachment process occurs and it moves fastly.

• Using this technology scientists are trying to create artificial, gecko like adhesives.

• They made good progress toward fabricating synthetic spatulae.But if we compare it with gecko standards still they are primitive.

Water StridersWater striders able to walk on water for a number reasons like

• Surface area

• Gravitational forces

• Surface forces(van der waals forces)

• A waxy(hydrophobic) surface on their legs

• Most importantly micro hairs on their feet are ‘nano-groovy’.

• A large water-repellent force was produced by nanostructures on the water strider’s leg.

Butterfly WingsNature uses light on Nano scale

What makes colour ?Three possible reasons for colour :

1. Pigment : If colour is due to pigment , it never changes. For eg . a plant leaf.

2. Scattering of light.

3. Iridescence.

Chemical Colour and structural colour.

Case of ButterflyThe wings of Butterflies often display extra ordinary colours which

are a consequence of the wings’ surface and its interaction with light.

It also exhibits the phenomenon of IRIDESCENCE.

IRIDESCENCE

• Physical Colour

• Interaction of light with physical structure of the surface.

• Structures must be nanosized . ( Visible light : between 300-700 nm.)

• This interaction of light with nano-rough surface can lead to constructive or destructive interference.

IRIDESCENCE

• Colour , intensity and angles of iridescence depends on thickness and refractive index of the substrate, and on the incident angle and frequency of incident light.

• Opals : Natural iridescence : packed silica spheres in the nm range, uniform in size and arranged in layers ( appropriate conditions for interference.)

Phenomenon of iridescence in butterflies

• Wings : rows of scales arranged like tiles in a roof.

• Each scale is about 70*200 micrometer wide

• Smaller structure on its surface : very intricate and highly ordered nm organisation of ridges

• Each ridge is about 800 nm wide

• The spaces between them form natural photonic crystal that can generate cons/dest interference.

SEM Analysis of Wings

• Shows even more intricate structure called

SETAE : looks like fir trees

• About 400 nm long, responsible for producing

constructive interference in blue wavelengths

which generates strong blue colour.

Super hydrophobicity and Self cleaning property

• The morpho butterfly’s wing structure also exhibits another“remarkable” nano-phenomenon— it is waterproof.

• super-hydrophobic - minimization of contact rendered by the apices of the triangular ridges and that the chitin material is hydrophobic.

• Roughened hydrophobic surfaces have air filling the interstitial spaces thereby reducing the liquid-to-solid contact area.

• Water then tends to attract each other better forming larger and larger droplets

• Roughened hydrophilic surfaces act in an opposite way: they enhance water adsorption.

Applications• Production of photonic crystals.

• Wrapping foils, decoration paper, hair sprays and, nail polish as well as thin films for light emitting diodes and photonic crystal lasers .

• Textile Industry : to create a new pale blue iridescent fabric. The fabric is a multi-layer construction containing similar refractive indices: 61 layers of nylon 6 (n=1.60) and polyester (n=1.55), each with a thickness of about 70-90 nm).

• These applications may even find that “photonic crystal diodes and transistors will eventually enable the construction of an all-optical computer”

Toucan’s Beak: Strong aand light

• Despite its large size (a third of the length of the bird) and considerable strength, the toucan beak comprises only one twentieth the bird’s mass.

• While the large strong beak is useful in foraging, defense and attracting mates, its low density is essential for the toucan to retain its ability to fly.

The exterior of the beak is made up of overlapping tiles of keratin, the sulfur-containing fibrous protein that makes up hair, fingernails, and horn.

The interior of the beak is constructed of a rigid foam made of a network of calcium-rich bony fibers connected by membranes. The membranes are similar in composition to keratin

Applications

• Automotive panels that could protect passengers in crashes. (as the beak's sandwich structure also behaves as a high energy impact-absorption system.)

• Construction of ultra light aircraft components.