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TRANSCRIPT
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
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