Ahmed Helal uncaps a cardboard cylinder that reminds me of a cheap kaleidoscope and pulls out a pair of plastic syringes, connected tip-to-tip by what looks like a small black rubber tube. Each syringe is about a third full with a dull silvery goo. He pushes the syringe plungers back and forth between his thumbs, and I watch the leaden-colored gel duly shift easily between the two syringes. Then he hands me the syringes to try.
“It’s a little dried out, but it still works,” he assures me, in case his demonstration hasn’t been enough. I imitate his motions and find the material between the syringes quite pliable. Nothing magical so far about my first conscious contact with a smart fluid, the lay term for the type of material represented by the dark, glinting gray in the syringes.
But he’s about to show me what’s so smart about a smart fluid, and why engineers like Ahmed, a graduate student in Peko Hosoi’s laboratory at MIT, are interested in building practical devices with these materials. He places a small magnet on the rubber tube connecting the two syringes.
To my surprise, the syringe plungers now won’t budge. Ahmed takes the syringes back and moves the magnet a little further away from the bridge between the syringes, and with some effort is able to shove one of the plungers in enough to make the silvery liquid shift again from one syringe to the other. But it takes a lot of pressure to do so.
So what is a smart fluid? There are two kinds of smart fluid: those controlled by magnetic force, like the one Ahmed has shown me, and those that are instead controlled by electrical current. (Ahmed tells me he could equally well show me that kind of smart fluid, but he usually doesn’t demonstrate it because of the pretty high voltage running through them it, which, while not dangerous, would give an unpleasant electric shock.) In each case, the application of a magnetic field or an electric current changes the properties of the fluid such that it can go from flowing liquid to a solid capable of withstanding significant pressure and back again.
While a little different from the syringes I got to try, the setup in this video gives you a good sense of how quickly the properties of a smart fluid can change when subjected to (in this case) an electric current.
Smart fluids are actually made up of two different components mixed together. The “fluid” part is usually a type of oil, often made of silicone, whose properties aren’t changed by electricity or magnetism. The “smart” part is the small globe-like particles, no bigger than a speck of dust, swimming in that fluid, which are made of metal compounds coated with insulating materials. These particles, when subjected to an electric current, will rearrange their molecules to have a positively charged side and negatively charged side (or a north pole and south pole when in a magnetic field). When no electric or magnetic field is present, the particles move freely and individually in the oil.
Once an electric current or magnet is applied, though, the particles will undergo their molecular rearrangements, as opposite charges or magnetic fields are drawn to each other. In the process, each particle aligns with the field and the other particles, forming long chains of particles that can block off an area like the narrow rubber tube between the syringes (see the below pictures from Wikipedia).
In order to explain to me how smart fluids work, Ahmed shows me a video taken under a microscope of these particles in motion. Ordinarily, they flow smoothly through the tube. But as soon as an electrical charge starts running across the tube, the particles clump into long chains which gum up the thin passage, a transition from moving leopard spots to static tiger stripes. With enough pressure (just as Ahmed showed me with the syringes), these clots can be broken and the particles can flow again in the oil.
The ability to control smart fluids so quickly at the flick of an electric or magnetic switch has been appealing to engineers for a while now, and the change in property from seemingly strong and solid to liquid and flowing (and back again) has been used to industrial advantage, particularly in shock absorbing systems for cars and even an earthquake-proof building in Japan. And magnetic smart fluids with slightly larger particles could power satellites the size of a deck of cards or cure liver cancer. Maybe, one day, smart fluids will be as ubiquitous in our lives as smart phones.