National Science and Engineering Week

Stripes

Sand: clip artMackerel sky: Dreamstime

Sand: clip art

Zebra: KA GehringChipmonks: KA Gehring Tiger: clip art

Beetle: Linda Offord

Spots

Dalmation : Clip art

Spotted deer: KA Gehring

Leopard : clip art

Giraffe: KA Gehring

Pores in steel

F. Martin et al Electr. Acta 54,2009

Rings

Agate : LH Conklin

Fairy ring

Mineral rings

dhensley@utk.edu

Water rings: clip art

Rings on a peacock

tail: Dreamstime

Crystals: Regular patterns

Quartz Crystal dreamstime

High-resolution lattice images of thin Si crystals.Atoms are black

Bubble raft: Dept of Materials Science:

University of Cambridge Honey comb: clip art

Atoms are black

T Walther University of Sheffield

Plan of TalkPlan of Talk

1. How crystals grow into regular shapes

2. How nature makes stripes, spots, patterns

3. How scientists and engineers use materials that 3. How scientists and engineers use materials that

build themselves into useful shapes.

Crystalline Order

We look at how a real crystal forms

by looking at order at a surface.

• We represent an atom by a circle• Each circle is attracted towards the nearest circles

• Each atom moves to be close to other atoms.

Do you see lines ?

More linesWe have a

triangular

lattice and

each circle is

sounded by

10

sounded by

six circles

This is the same pattern as we get if we pack regular hexagons

as in a pavementas in a honeycomb

KA Gehring

Clip art

Bubble rafts show this order

This one was grown

very slowly This one was grown fast

Dept of Materials Science: Cambridge

KA Gehring

How to make a bubble raftBowl of water

Yoghurt pot with a small

Add one spoonful of detergent very carefully - stir gently.

Put in the yoghurt pot - you will get

13

small hole

pot - you will get a stream of bubbles.

The yoghurt pot is below the water surface and above the bottom of the bowl. Move your hand slowly from side to side to spread the bubbles.

The bubbles attract each other.

They all move a little so that a new bubble goes in the best place.

If a bubble bursts or the raft is disturbed it will

What did we see ?

If a bubble bursts or the raft is disturbed it will repair itself – annealing.

The edges of the raft were relatively smooth.

We saw lines extending across the raft.

‘Self Organisation’

‘Patterns extending over long distances can be generated by small entities following simple rules’

We have two examples so far:

Bubble rafts – each bubble is attracted to its Bubble rafts – each bubble is attracted to its neighbours.

Each bee that builds a new cell in a honeycomb fits it into the cells that are already there.

In both cases we see that a lattice is generated that extends over large distances.

What shape are the rafts?The raft with the most bonds is a triangle – but this is

the arrangement of the bubbles!

6 neighbours5 neighbours4 neighbours3 neighbours2 neighbours1 neighbour

Shapes of crystals The shape of a crystal mirrors the arrangement

of the atoms within the crystal.

Faces can be very flat as for this ice off a pond: KA Gehring

Dr Geetha Balakrishnan University of Warwick: watch a crystal grow in the laboratory from

a melt: http://www.youtube.com/watch?v=t7WiGTQwpFY

and from a solution occurring naturally over millions of years from BBC website.

Quartz: dreamstime

http://news.bbc.co.uk/1/hi/sci/tech/8466493.stm

Flat and curved

An array of hexagons is flat

Wasps use this too

If you add some pentagons

you get a rounded surface(20 hexagons, 12 pentagons)

Wasps use this too

when they build their

nests

Dreamstime

What was special about soap/detergent?

The molecules of soap are special:

A head that will mix with

water – like vinegar

A tail that hates water – like A tail that hates water – like

olive oil

A soap film on water

Heads in the water tails in the air.

This is what lowers

the surface tension

enough so that

bubbles can persist

over time.

Two layers of soap-like molecules

(lipids) form a membrane between

two liquids!

This is how all the cell membranes form separating one

living cell from another

So far we have generated simple

structures: rafts, crystals and

membranes

What about stripes and spots?

For this we need a new idea!

OSCILLATIONS

Oscillations??

We need the idea of ‘feedback’

A swing door is a simple example of ‘negative

feedback’ .

Push it open – it will close gently

Oscillations??

We need the idea of ‘feedback’

A swing door is a simple example of ‘negative

feedback’ .

Oscillations??

We need the idea of ‘feedback’

A swing door is a simple example of ‘negative

feedback’ .

Oscillations??

We need the idea of ‘feedback’

A swing door is a simple example of ‘negative

feedback’ .

Oscillations??

We need the idea of ‘feedback’

A swing door is a simple example of ‘negative

feedback’ .

Push it open – it may swingPush it open – it may swing

An example from the natural worldPredator –prey Lotka-Volterra equations

There is an island with lots and lots of grass,

There are rabbits

There are more rabbits.....

This is an example of ‘positive feedback’

But there are also foxes....

and foxes live by catching rabbits

Rabbits eat grass and multiply

Rabbits are eaten by foxes

Foxes live longer and have more baby Foxes live longer and have more baby foxes when they catch plenty of rabbits

More foxes will catch more rabbits: negative feedback on rabbit population

The number of rabbits increase

The number of foxes increase

The number of rabbits decreases

The number of foxes decreasesThe number of foxes decreases

RabbitsFoxes

Lynxes caught in Canada

This is an example of positive /negative feedback giving oscillations

Belousov and Zhabotinsky were very surprised when they first saw a mixture

of chemicals oscillating.of chemicals oscillating.(B-Z reaction)

A mixture of many chemicals is required.We saw an example of a stirred reaction.

http://www.youtube.com/watch?v=bH6bRt4XJcw&feature=related

If it is not stirred then we see evolving stripes.http://www.youtube.com/watch?v=Ch93AKJm9os&feature=related

Rabbits and FoxesInput: Grass This is consumed throughout

Output: Dead foxes All foxes die naturally in the

end. The number of dead foxes increases

throughout.

There are two intermediate steps: live rabbits and There are two intermediate steps: live rabbits and

live foxes it is the numbers of live animals that

oscillate.

B-Z Reaction

The reaction starts with an input and ends when it

is all in the final state.

An intermediate chemical is coloured and oscillates.

But – this was predicted independently by a British mathematician Alan Turing

He showed how STATIC stripes and spots can form on animals by and spots can form on animals by an oscillating chemical reaction.

But actually he was interested in a much more fundamental

problem...

A fertilised embryo is SPHERICALit develops into an animal that has a

back and a front and a top and a bottom

This is the most dramatic case of self organisation that we know.

Clip art

Stripes are formed in the developing foetus.

The pattern of stripes are fixed at the time they f orm and grow with the animal.

Two types of zebra, Burchill’s zebra, widely spaced stripes, and Imperial zebra, more narrow stripes .

Animals have a top and a bottom which are very different and two sides which are very nearly the same - why ?

Clip art

Clip art

Clip art

Spots occur where there is a chance that an area can become dark which then spreads.Spots can also grow into patches:

Leopard : clip art Giraffe: KA Gehring

Leavesanother example of chemical stripes giving the pattern of veins or thorns.

Dreamstime Clip art

Scientists use ‘self organisation’

Nano-technology requires organisation on the nano-scale

~ size of a human hair.

One nanometer ~3 atoms

1

10,000

One nanometer ~3 atoms

1. Grow a semiconductor device from layers grown atom by atom

2. Assemble spheres together to control light3. Make molecules of this size that will set into the shapes we want.

Soap bubbles and foam are made from air, water with a

very little soap.

But the are quite unlike air, water or soap.

A thin film of water behaves differently from water in a

cup.

It is a thin sandwich:

Air | Soapy Water| Air

Thin films of semiconductors

are very different too… Dreamstime

Sandwich crystals are grown layer by layer

• MBE (Molecular Beam Epitaxy)

– 1 layer per second

– 2,000,000 seconds for 1 mm

• = 23 days for 1 mm

41

• = 23 days for 1 mm

• These special crystals

are used in

computers/LEDs/lasers……..

SiGe/Si thin filmT Walther University of Sheffield Sheffield University has a

national centre for growing these semiconductor sandwiches.

InAs/GaAs thin films grown by MBE

This is used to produce ‘quantum dots’

Overview of sample with 3 different InAs layers

Lattice image of InGaAs quantum dot just formed

T Walther Shef.

These are used to make Quantum Dot LasersThese are used to make Quantum Dot Lasers

AFM image of dots

M Skolnick

Structures to guide light

Visible light has a wavelength λ~ 500nm

Light can be controlled

by structures of this size.

This can be done by a pattern

made by drilling holes in

a film.

Or can use self organisation

Gold coated glass Assembly of spheres Electrodeposition Etch spheres

Nano-Templating

simple, robust, low cost: vary diameter,

thickness, material

JJ Baumberg

Cambridge

Silica spheres forming a

lattice T Volotinen

Use Chemistry: Block co-polymers

Do you remember soap? Water loving

head and water hating tail.

Block co-polymers: Block co-polymers: Long Blue and short RED Long RED and short Blue

Equal lengths

Special Arrangements: Red and Blue separateLong Blue and short RED Equal lengths

With Three blocks: you get a tangle. Used in soles of sports shoes

Hot off the press- DNA OrigamiPWK Rothemund Nature 440 297, 2006

Two strands: 4 bases that pair G-C and T-A wound in a helix.Can choose any shape (in two dimensions) and force one long strand of DNA to adopt this shape by adding

suitably chosen short strands.

AT

C G

G C

A T

This is just the beginning:

What effort? 3 months to write basic design computer programEach structure one week to design and one week to synthesise the special DNA (commercially), a few hours to mix and grow the structure. Making the images took about 2 days of careful work .

Many, many more examples of the Many, many more examples of the

use of self organisation use of self organisation

But it is time for me to stop!But it is time for me to stop!

I hope you

• Will look at the natural world with a new

interestinterest

• Will appreciate more of the ways in which

scientists are producing new materials and

devices.

Many thanks to

colleagues who listened to me patiently

to T Walther, G Balakrishnan, M Skolnick,

T Volotinen and J Baumberg who provided pictures

and