Bursting Citrus Peel Oil Glands Inspire New Approach for Microjetting Fluids

Newswise — WASHINGTON, D.C., November 21, 2017 -- When was the last time you took a close look at an orange, lime or lemon peel? Outer citrus peels, more formally called “exocarp,” are covered with tiny oil glands or cavities that can explode in an outward direction, often spraying toward you, if bent in an attempt to gain access to the inner fruit. 

This makes it difficult to peel an orange without getting your hands wet, no matter how careful you are to avoid damaging the inner fruit. So, a group of researchers at the University of Central Florida set out to explore how the material properties during this phenomenon enable the production of these high-speed jets that often go unnoticed or unappreciated, even by avid citrus consumers. 

During the 70th annual meeting of the American Physical Society’s Division of Fluid Dynamics, being held Nov. 19-21, 2017, in Denver, Colorado, Nicholas Smith, a doctoral student working with Andrew Dickerson, an assistant professor of mechanical and aerospace engineering, will present their work comparing the “jetting behavior” of five citrus fruits and running tests to determine the material properties of their peels. 

The researchers are zeroing in on the material properties of citrus peels via tensile tests, using similar tensile testing mechanisms that you’d use to find the stiffness and yield stress of metals and plastics. They use a high-speed camera with a macro lens to capture videos, tracking droplets and obtaining other salient mechanical measurements. This property data is then put into finite element simulations to calculate fluid pressures at gland bursting. 

“The explosion of citrus jets, when slowed down by our cameras, are akin to watching a hundred fireworks fired in random sequence at the finale of a show,” Dickerson said. 

They discovered that bending citrus peels compresses the soft material around the tiny oil glands, which increases the fluid pressure. Eventually the fluid pressure exceeds the failure strength of the outermost membrane. The resulting explosion of oil -- often at speeds greater than 10 meters per second -- and the emptying of oil glands offers a new way of jetting small quantities of oil. 

Until now, creating fluid jets on the microscale has primarily been tackled via the more traditional route of using precision pumps and small nozzles. These significant findings may offer applications a new avenue. 

“Citrus fruits have shown us another method, which was previously uncharacterized, to disperse small amounts of fluid,” Dickerson said. “We show the creation of brief jets -- the diameter of a human hair -- formed from the rupture of small fluid pockets housed in a soft, composite medium. The composite nature of the medium is the key to the explosive emptying of the miniscule oil pockets.” 

The group’s work may “enable new methods of precise and rapid fluid dispersal, perhaps within the realm of drug delivery, like a one-time use inhaler, or in chemical processing,” Dickerson said. They also envision embedding these types of pockets into material surfaces as an early warning system for failures -- embedded pockets would burst and release a visible liquid under excessive deformation. 

“Next, we’ll turn our attention to unexplored splashing phenomena as it pertains to both biological and synthetic applications,” Dickerson said. 

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Abstract: M5.00010: “Microjets of citrus fruit," by Nicholas Smith and Andrew Dickerson, is at 8:00-8:13 a.m. MST, Nov. 21, 2017, in Room 405 of the Colorado Convention Center. http://meetings.aps.org/Meeting/DFD17/Session/M5.1

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The Division of Fluid Dynamics of the American Physical Society exists for the advancement and diffusion of knowledge of the physics of fluids with special emphasis on the dynamical theories of the liquid, plastic and gaseous states of matter under all conditions of temperature and pressure. https://www.aps.org/units/dfd/ 

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