The Importance of Being Polycrystalline

• food
• macro

Whether or not it's got anything to do with global warming, it's gloriously sunny here in the United (snigger implicit) Kingdom. And since it's impossible to buy coconut icecream, I am attempting to make it With Science.

The naïve way to make icecream is to take cream and freeze it. However, this gives you a solid block of milky ice. You can try and catch it when it's only partially frozen, but that relies on precise timing and will usually just produce slush. The key to making tasty icecream lies in how the molecules inside it are arranged. When a liquid freezes, the molecules slowly lose their kinetic energy and form bonds with each other, holding them together in a regular fashion. This regimented pattern spreads out to the other molecules, and hey presto, you have a state change from liquid to solid. If the liquid's left undisturbed while this happens, you end up with just one big crystal of ice — structurally sound and impossible to break into.

To make proper icecream, you need to keep stirring the mixture while it freezes so that each individual well-ordered region doesn't have time to align up with the others. This gives a collection of small crystals with no overall arrangement — a polycrystalline structure.

Also, despite the name, icecream also contains sugar and egg yolks, as well as any other flavourings you choose to put in. Useful things to remember:

• Alcohol goes down easier because it produces less vapours to trigger your palate and make you gag. But an icecream mix with more than 30% alcohol will not freeze.
• Some fruit tastes sweeter. The expansion of the freezing ice in the cell walls makes them rupture, making the sugars inside more accessible. Bananas and grapes become very sweet when frozen.
• Some fruit doesn't taste of anything much at all when frozen — sour or more complex flavours need a sense of smell as well as taste to make it to your neurons, and it takes warmth to evaporate odour molecules.
• If frozen, yoghurt with artificial sweeteners tastes much sweeter, but I have no idea why. Possibly because there's no other flavours to counteract it?

Low-tech icecream making

You will need:

• An icecream recipe
• A container (not porcelain! it will shatter!)
• A fork or spoon
• A freezer

Put ice cream mix in container, place in freezer, stir every 20 minutes or however often you can be bothered. Eat when it looks like the sort of texture you want to eat (no, it's not very complicated. But do not leave it all day in the freezer with the spoon/fork embedded in the middle all the same).

Higher-tech icecream making

You will need:

• An icecream making ball
• An icecream recipe, as above
• Lots of ice
• Salt

I have one of these. It is a neat idea: a two-container system. The outside one contains ice, which when you put the salt in melts. As I mentioned above, freezing involves making bonds while losing heat — and since melting is freezing in reverse, it makes sense that melting involves breaking bonds and needs extra heat. The energy to break these bonds is nicked from the inner container, which has the icecream mixture in it. The whole thing is encased in a plastic ball which you roll around languidly since it can't stand up to hardcore playing. Ingredients get agitated and stirred by the rolling and shaking, and chilled by the ice/salt mixture.

However, there are a couple of problems: the icecubes that come out of my trays are too big, so I have to smack them with a hammer to get them to go in (I did have a ball-peen hammer to hand in the kitchen, yes). Also it requires an insane amount of ice. It suggests using this device when camping, but under what circumstance would you have available industrial quantities of ice for the icecream-maker, but not a freezer to make ice-cream the low-tech way above?

Highest-tech

Buy an icecream maker. Insert ingredients. Turn on. Eat icecream.

But a polycrystalline structure is useful in a lot of places. Blacksmiths don't just plunge their hot forged metal into water to cool it off — the important thing is to cool it quickly so the metallic crystals don't have time to align in the same direction. A monocrystalline sword/plough/gate/suit of armour would be very brittle and very useless.

In a more modern context, polycrystalline polymers are used in the next-gen of solar cells. Because the individual crystals can be arranged in a jumbled configuration, the semiconducting1 plastics that make up organic2 photovoltaics can be applied in a varied of novel ways — evaporation, spraying, and printing. Even in very small amounts, they make a solar cell about half as efficient as a traditional, expensive, heavy silicon cell. Now companies (helped by sane governments like that of Germany) can roll out organic solar cells that compensate for their lower efficiency by being extraordinarily cheap to make in large quantities. Hopefully, more governments and private individuals will choose to own and use sources of alternative energy as it becomes more available, and we wean ourselves off fossil fuels.

Let's hear it for the polycrystalline arrangement of matter — helping us through summer with icecream, and solar electricity for our fridges and radios.

1. Semiconductors are midway between a conductor and an insulator. Their electrons can loosened from their bond enough to move around and carry current, but only if you put energy in to get them out in the first place. You can put this energy in with photons, ie light — this is how solar cells work. ←

2. Organic in a chemistry sense = has carbon, hydrogen and oxygen in it. Examples: fossil fuels, DNA, melanin. It does not mean poncy food hand-washed in spring-water by virgins. ←