What Did I Just Watch? Ferrofluids Explained

Gregory F. Maxwell via Wikimedia Commons // GFDL 1.2

The black fluid looks like common oil or ink. But suddenly, it leaps from a shallow pool to form a phalanx of rotating spikes. It's somehow soft and sharp at the same time. What on Earth is going on here?

This is a ferrofluid: a liquid that becomes strongly magnetized in the presence of a magnetic field. Ferrofluid is a portmanteau of ferromagnetic—the mechanism that draws certain materials to magnets—and fluid (for obvious reasons). A ferrofluid contains nanoscale particles like magnetite, hematite, or another compound containing iron—all of which are attracted to magnets. The particles have to be small enough to randomly distribute throughout the fluid.

A key feature of a ferrofluid is that it's a colloidal suspension. That means the insoluble particles are suspended in the fluid, so it has two states of matter in one solution. (A more common colloidal is milk, which is butterfat globules suspended in a water solution.)

That dual state is important, because a ferrofluid acts like any other liquid until the particles get near a magnet. Then they force the fluid to behave in ways that are stunning to watch:

Those peaks and valleys reflect the magnetic field, as well as the effects of surface tension and gravity.

The ferrofluid was invented by NASA scientist Steve Papell in 1963. He had hoped to transform rocket fuel into a ferrofluid so it could be magnetically drawn toward a pump inlet in a weightless environment, imposing a sort of artificial gravity. That didn't come to fruition, but today ferrofluids are used in a wide variety of applications, including electronics, engineering, medicine, and art. You can even make your own ferrofluid at home! Mix powdered iron fillings into corn oil, grab a magnet, and let the fun begin. Pro-tip: don't get the ferrofluid too close to the magnet or it will leap out of the container—and splatter everywhere.

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'Lime Disease' Could Give You a Nasty Rash This Summer
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A cold Corona or virgin margarita is best enjoyed by the pool, but watch where you’re squeezing those limes. As Slate illustrates in a new video, there’s a lesser-known “lime disease,” and it can give you a nasty skin rash if you’re not careful.

When lime juice comes into contact with your skin and is then exposed to UV rays, it can cause a chemical reaction that results in phytophotodermatitis. It looks a little like a poison ivy reaction or sun poisoning, and some of the symptoms include redness, blistering, and inflammation. It’s the same reaction caused by a corrosive sap on the giant hogweed, an invasive weed that’s spreading throughout the U.S.

"Lime disease" may sound random, but it’s a lot more common than you might think. Dermatologist Barry D. Goldman tells Slate he sees cases of the skin condition almost daily in the summer. Some people have even reported receiving second-degree burns as a result of the citric acid from lime juice. According to the Mayo Clinic, the chemical that causes phytophotodermatitis can also be found in wild parsnip, wild dill, wild parsley, buttercups, and other citrus fruits.

To play it safe, keep your limes confined to the great indoors or wash your hands with soap after handling the fruit. You can learn more about phytophotodermatitis by checking out Slate’s video below.

[h/t Slate]

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Why Eating From a Smaller Plate Might Not Be an Effective Dieting Trick 
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It might be time to rewrite the diet books. Israeli psychologists have cast doubt on the widespread belief that eating from smaller plates helps you control food portions and feel fuller, Scientific American reports.

Past studies have shown that this mind trick, called the Delboeuf illusion, influences the amount of food that people eat. In one 2012 study, participants who were given larger bowls ended up eating more soup overall than those given smaller bowls.

However, researchers from Ben-Gurion University in Negev, Israel, concluded in a study published in the journal Appetite that the effectiveness of the illusion depends on how empty your stomach is. The team of scientists studied two groups of participants: one that ate three hours before the experiment, and another that ate one hour prior. When participants were shown images of pizzas on serving trays of varying sizes, the group that hadn’t eaten in several hours was more accurate in assessing the size of pizzas. In other words, the hungrier they were, the less likely they were to be fooled by the different trays.

However, both groups were equally tricked by the illusion when they were asked to estimate the size of non-food objects, such as black circles inside of white circles and hubcaps within tires. Researchers say this demonstrates that motivational factors, like appetite, affects how we perceive food. The findings also dovetail with the results of an earlier study, which concluded that overweight people are less likely to fall for the illusion than people of a normal weight.

So go ahead and get a large plate every now and then. At the very least, it may save you a second trip to the buffet table.

[h/t Scientific American]

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