Smart materials can radically change their shape, texture, color, or other properties ý often with the flip of a switch. Some smart fluids, for example, can be transformed from the consistency of syrup to toffee to rock, and back again, in a fraction of a second.
Magnetic smart fluids, which scientists sometimes call magnetorheological fluids, change state depending on the strength of a magnetic field. Here's why.
If you've ever played with a magnet and a pile of iron filings, you know that the filings will line up along the magnet's fields like this.
If you stir iron filings into a fluid, such as vegetable oil, the filings will drift about randomly. When you place a magnet close to the oil/iron filings mixture, the iron filings try to line up along the magnetic fields, just as they do when they are not in oil. The oil fills up the spaces between the lined-up filings. Attraction between the oil molecules and the iron particles pulls the rows of filings in tight, which makes the smart fluid stiff, more like a solid than a liquid. The stronger the magnetic field, the more solid the smart fluid becomes.
Other smart materials respond to changing electrical fields, temperature, or acidity. Piezo-electric crystals are smart solids that change shape when they are placed in an electric field; miniature microphones and speakers are often built of piezo-electric crystals. Even the liquid crystals in laptop computer displays are a type of smart material -- changing the electric field on a liquid crystal can turn the crystal from transparent to opaque, in turn changing what you see on the screen.
You can make your own smart fluid!
Buy iron filings from a scientific supply house, or make your own with a file and a chunk of iron pipe.
Fill a paper cup about a third of the way up with filings, then pour in enough corn oil to make an liquid you can stir easily.
Put a strong magnet next to the cup. Your smart fluid will be attracted to the magnet, and some oil will ooze out. Pour out the excess oil, and what remains is a smart fluid that you control with the magnet.
For more information, see scitoys.com.