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Why Are Dried Coffee Stains Darker At The Edges Than In The Center?

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If you spill coffee anywhere near as often as I do, you may have noticed something peculiar: When a puddle of coffee is allowed to dry undisturbed, the brown sediment collects almost entirely on the edge of the spill rather than being spread out evenly. It was not until recently that we could explain why a drop of coffee (or wine, or ink) dries this way. The theory of coffee ring formation was published in 1997 by a group of University of Chicago physicists. Lest you think it entirely academic, this curiosity of fluid dynamics is a problem in the world of inkjet printers, and there is serious technological interest in overcoming it.

The actual mechanism is somewhat like this: Water evaporates faster from the exposed edges of a coffee drop than from the interior. For this reason, you might expect that the drop would shrink in area as it dries. In practice, however, the drop edge gets pinned by bits of solid material or by the texture of the table, and this prevents the edge from receding. In order to accommodate the slightly faster evaporation at the edges, the liquid at the center flows outwards. The molecules and solid bits floating in the water get swept along in this outward flow, and as the drop continues to dry, they pile up at the edge like windblown snow against a fence.

Researchers trying to turn off the coffee ring effect have largely done so by counterbalancing it with surface tension effects that recirculate the particles during drying. Recent work from the University of Pennsylvania has shown that irregularly-shaped particles are able to resist ring formation by linking up into big floating rafts. Not everyone is trying to squelch the coffee ring effect, though. Several research articles have shown that it can be employed to gently lay out arrays of particles on surfaces, making it a possible tool for micro-manufacturing.

If you set out to watch the coffee ring form on your desk, you'll find that the whole process is about as fast as, well, watching coffee dry. Fortunately for us, our friends at Penn captured the process in time-lapse in a video explaining their recent work:

Andrew Koltonow is a graduate student in Materials Science and Engineering at Northwestern University.

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Words
Why Is 'Colonel' Spelled That Way?
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English spelling is bizarre. We know that. From the moment we learn about silent “e” in school, our innocent expectations that sound and spelling should neatly match up begin to fade away, and soon we accept that “eight” rhymes with “ate,” “of” rhymes with “love,” and “to” sounds like “too” sounds like “two.” If we do sometimes briefly pause to wonder at these eccentricities, we quickly resign ourselves to the fact that there must be reasons—stuff about history and etymology and sound changing over time. Whatever. English. LOL. Right? It is what it is.

But sometimes English takes it a step too far, does something so brazen and shameless we can’t just let it slide. That’s when we have to throw our shoulders back, put our hands on our hips and ask, point blank, what is the deal with the word “colonel”?

“Colonel” is pronounced just like “kernel.” How did this happen? From borrowing the same word from two different places. In the 1500s, English borrowed a bunch of military vocabulary from French, words like cavalerie, infanterie, citadelle, canon, and also, coronel. The French had borrowed them from the Italians, then the reigning experts in the art of war, but in doing so, had changed colonello to coronel.

Why did they do that? A common process called dissimilation—when two instances of the same sound occur close to each other in a word, people tend to change one of the instances to something else. Here, the first “l” was changed to “r.” The opposite process happened with the Latin word peregrinus (pilgrim), when the first “r” was changed to an “l” (now it’s peregrino in Spanish and Pellegrino in Italian. English inherited the “l” version in pilgrim.)

After the dissimilated French coronel made its way into English, late 16th century scholars started producing English translations of Italian military treatises. Under the influence of the originals, people started spelling it “colonel.” By the middle of the 17th century, the spelling had standardized to the “l” version, but the “r” pronunciation was still popular (it later lost a syllable, turning kor-o-nel to ker-nel). Both pronunciations were in play for a while, and adding to the confusion was the mistaken idea that “coronel” was etymologically related to “crown”—a colonel was sometimes translated as “crowner” in English. In fact, the root is colonna, Italian for column.

Meanwhile, French switched back to “colonel,” in both spelling and pronunciation. English throws its shoulders back, puts its hands on its hips and asks, how boring is that?

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Big Questions
Why Do Cats Love Scratching Furniture?
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Allergy suffering aside, cat ownership has proven health benefits. A feline friend can aid in the grieving process, reduce anxiety, and offer companionship.

The con in the cat column? They have no reservations about turning your furniture into shredded pleather. No matter how expensive your living room set, these furry troublemakers will treat it with the respect accorded to a college futon. Do cats do this out of some kind of spite? Are they conspiring with Raymour & Flanigan to get you to keep updating home decor?

Neither. According to cat behaviorists, cats gravitate toward scratching furniture mostly because that love seat is in a really conspicuous area [PDF]. As a result, cats want to send a message to any other animal that may happen by: namely, that this plush seating belongs to the cat who marked it. Scratching provides both visual evidence (claw marks) as well as a scent marker. Cat paws have scent glands that can leave smells that are detectable to other cats and animals.

But it’s not just territorial: Cats also scratch to remove sloughed-off nail tips, allowing fresh nail growth to occur. And they can work out their knotted back muscles—cramped from sleeping 16 hours a day, no doubt—by kneading the soft foam of a sectional.

If you want to dissuade your cat from such behavior, purchasing a scratching post is a good start. Make sure it’s non-carpeted—their nails can get caught on the fibers—and tall enough to allow for a good stretch. Most importantly, put it near furniture so cats can mark their hangout in high-traffic areas. A good post might be a little more expensive, but will likely result in fewer trips to Ethan Allen.

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