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Secrets of Citrus Micro-Jets

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The Hidden Power of Squirting Fruit

High-speed video reveals how bending the skin of an orange or similar fruit releases tiny bursts of aromatic oil at 30 feet per second from microjets. But what is their use?

There’s nothing like the physics of fruit. Specifically, citrus fruit. What you’re seeing is the skin of a fruit being squeezed by pliers to produce dozens of ultra-fast microjets. The tiny streams rocketing out of the skin have been slowed down 266 times. In real time, these jets travel 30 feet per second — about the speed of a Champagne cork. Other plants also produce microjets. This squirting cucumber does it to spread its seeds. In the animal kingdom a similar process is used for swift attacks, like that of the ferocious spitting cobra. Scientists who study the mechanism of citrus jets found that sacs of oil lie on the soft layer beneath a more rigid skin. As the fruit is squeezed, the skin hits a sudden failure point. The skin breaks, and the fluid erupts under pressure. Here’s a ruptured sac afterward. Scientists don’t know why citrus fruits have this ability, but they’re keen to harness its power. It could help with commercial printing, medicine delivery, and even bridge safety. Imagine a sensor that changes color or gives off a signal when a bridge has hit its maximum load. But for now, it’s a reminder that some of the most mundane aspects of daily life can hold beautiful secrets.

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High-speed video reveals how bending the skin of an orange or similar fruit releases tiny bursts of aromatic oil at 30 feet per second from microjets. But what is their use?CreditCredit...Smith and Dickerson, University of Central Florida

Among the many sub-disciplines of physics are quantum mechanics, cosmology and the physics of fruit.

It’s not a field that universities or learned societies recognize, but it fits the investigation of how reservoirs in the skin of citrus fruit burst and shoot out micro-jets of aromatic oil at more than 30 feet per second.

Andrew Dickerson and his colleagues at the University of Central Florida in Orlando investigated the phenomenon purely out of curiosity. Anyone who has handled a lemon or a navel orange may have noticed that when the skin is bent, a little bit of oil comes out in a tiny spritz. These are to be distinguished from the wayward squirts of juice that can hit a dinner companion when you are trying to add a dash of lemon to your sole.

The scientists used high-speed videography to track how the process works and found sacs of oil in the relatively soft part of the skin just below the more rigid outer layer. They used pliers to bend skin of several citrus fruits and found that at a certain point, the stress on the skin causes a break and the oil reservoir empties in a burst.

Micro-jets are found in other plants and in animals as well, such as spitting termites and spiders. Why citrus plants show this action when the skin is bent in an extreme way isn’t known, although the oils are toxic to some insects, plants and microbes. The micro-jet phenomenon is the reason that bartenders twist an orange or lemon peel to release the flavor, but that is not likely to have played a part in their evolution.

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The experiments, reported in the Proceedings of the National Academy of Sciences, may be helpful in fields like medicine or commercial printing or even in devising monitors for stress in bridges. For instance, a micro-jet sensor might indicate when too much stress is put on part of a bridge, bursting and spraying a dye.

That’s speculation. Dr. Dickerson said that he had undertaken the investigation because of conversations eight years ago with David Hu of Georgia Tech when Dr. Dickerson was an undergraduate working in Dr. Hu’s lab.

And one of the things he takes away from the findings, after viewing the high-speed videos, is that everyday life is filled with hidden marvels, “ubiquitous examples of natural beauty.”

“How many times have you peeled an orange?” he asked. “How many other things in life pass us by and we don’t appreciate them?

James Gorman is a science writer at large and the host and writer of the video series “ScienceTake.” He joined The Times in 1993 and is the author of several books, including “How to Build a Dinosaur,” written with the paleontologist Jack Horner. More about James Gorman

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