The viscosity of the motor oil doesn’t matter here. As long as it’s motor oil, it will work! Petri dishes are ideal, because they’re thin enough for a magnet to penetrate. Some people have also had success using vegetable oil instead of motor oil. However, vegetable oil is very thin, and may not combine with the iron oxide as well.
You may have heard about harvesting iron oxide from cassette tapes using acetone. If you’d like to try this method, break apart cassette tapes and soak the tape in acetone for 1 hour outside. Then, use a knife to gently scrape the iron oxide off the tape into a dish. Just keep in mind that many people have tried this method with little success, so it’s a better idea just to buy iron oxide outright. You can also make ferrofluid with magnetic ink developer or ferric chloride, but these are very expensive. Iron oxide is much more affordable.
You won’t notice any immediate changes right away, and that’s okay. Just wait until you grab a magnet in the next step!
When the iron oxide mixes with the oil, the iron particles stay suspended in the fluid rather than mixing entirely. When you introduce a magnet to the ferrofluid, the magnet pulls the particles apart, forcing the fluid into interesting shapes, peaks, and valleys. Try moving your magnet to the side and the top of the dish, too. Just be careful not to dip it in the ferrofluid, or things could get messy!
Ferrofluid only starts to break down when it’s exposed to air. If you keep it sealed in a glass jar, it has a shelf life of up to 10 years.
Do not pour ferrofluid down the drain, as it will clog your pipes.
