Let’s grow a solid

How solids growPut your own name here!

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What is special about a solid?

It keeps its shape.

A natural object

A man-made object

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Liquids Have no shape

so atoms can move round freely.

When solids get hot they melt to form a liquid

- like chocolate

on a hot day.

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Solids remember how they grew !

We look at 2 ways that solids grow. and the types of solid formed.

Water can form:

1. frost and snowflakes – formed out of the air

2. ice in icicles or on a pond – formed from water

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1. Snowflakes and frost

are very irregular,

have many points or edges,

have lots of space inside them.

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2. Ice in icicles

Ice on a pondIt is transparent with very flat surfaces

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Water from the tap

and water from melted ice

Which is which?

Liquids do not remember.

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How do solids grow?

Why is it interesting and useful to know?

We make modern devices by growing a solid atom-by-atomto give us the form that we want.

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1

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Snowflakes grow from vapour We have air with very

few water molecules in it.

When the water molecule hits a cold surface it will stick just where it lands

Molecule – a new word?

A molecule of water is the tiniest particle of water that can exist and still be water.

Cold surface

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A computer can show this

We use a simulation from Hong Kong

http://apricot.polyu.edu.hk/~lam/dla/dla.html

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http://apricot.ap.polyu.edu.hk/dla/dla.html

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Growing a snowflake

100 molecules

11 000 molecules

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Ice from a puddle

Why is it perfectly flat?

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Crystals

When solids grow the atoms add on row by row to form a layer.

Then another layer grows on top.

Then another -A tiny natural crystal in a stone

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How many layers of atoms in this chunk of quartz crystal?

One layer is very, very thin.

About 10000 layers make the thickness of paper.

The crystal has about 100 million layers of atoms!How big can a crystal grow? See

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

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Growing a solid from liquid or a solution

An atom will join the solid only if the conditions are exactly right.

If the solid is near to its melting point the atoms can move around so as to get a more perfect pattern.

We call this annealing.

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Growing a solid atom by atom Send a beam of atoms through a vacuum and on to a flat surface.

Keep the surface hot so that the atoms can move around and find the best place to settle.

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What is the arrangement of atoms?

Each atom wants to be close to other atoms.

What pattern do we get?

We see lines crystals !

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More atoms

Can you see a pattern?

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Lines

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More lines

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- in 3 directions

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Bubble rafts show this-

We will make some soon.

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How to make a bubble raft

Bowl of water

Add one spoonful of detergent very carefully - stir gently.

Put in the yoghurt 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.

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A bubble raft is a ‘solid’ layer on the surface of the water.

A bubble raft grown quickly.

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What do we see ? The bubbles attract each other.

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

The bubbles are attracted to the edge of the container and to your fingers!

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

this is called annealing.

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A bubble raft grown very slowly

Can you see the rows?

University of Cambridge

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The bubbles are in rows

In 3 directions

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Who are the experts at creating this order?

Bees honeycomb

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What else do we learn ?

Bubbles are made from air and soapy water

BUT

They do not look like air, or water.

They do not behave like air, or water. A foam

dance!

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What is special about bubbles?

The soap films are very thin.

A thin film of water is very different from water in a bucket or glass.

We have a very thin sandwich

air- water –air

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Now back to solids

All crystalline solids have atoms in layers like lots of bubble rafts.

We can grow crystals from a solution of salt in water.

Special ‘sandwich’ crystals can be grown, which have different atoms in different layers.

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Sandwich crystals can now be grown layer by layer

MBE (Molecular Beam Epitaxy) 1 layer per second 2,000,000 seconds for 1 mm

= 23 days for 1 mm

These special crystals are used in-LEDsand-----

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You use manufactured crystals in –Comput

er ‘chips’

Chips are in:

mobile phones

games consoles

MP3 players--

laptops

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What have we learned ?•A solid remembers the way it is formed.

•We learned that modern devices depend on atomic layer sandwiches.

•We saw that soap bubbles look very different from water and air and behave differently too!

•We grew a bubble raft on water and watched it anneal.

•We get perfect solids from liquids and if the solid is free to readjust, anneal.

•A solid formed from vapour may be like a snowflake.

Scientists do amazing things!

There is lots to find out!Keep asking questions!