Spinning Eggs in Milk: New Study Unveils Secrets of Fluid Dynamics
Utah State University's assistant professor of mechanical engineering Tadd Truscott is a top expert in the field of fluid mechanics.
A vibrant image of a hard-boiled egg spinning in a bowl of blue milk is unveiling new insight into the complex realm of fluid dynamics.
The slow-motion shot was captured by USU assistant professor of mechanical engineering Tadd Truscott, a top expert in the field of fluid mechanics.
Truscott’s research is unveiling new clues into how spinning spheres can draw fluid from a bath like a pump. His latest study landed on the March cover of the journal Physics of Fluids where he and his team describe how a rotating sphere can pull fluid up its sides.
“When a hard-boiled egg spins through a pool of milk on the kitchen counter, the milk rises up the sides of the egg and droplets are ejected,” said Truscott. “This phenomenon occurs when any partially submerged object whose radius increases upward from beneath the fluid surface, such as spheres, inverted cones and rings, spins in a liquid bath.”
As seen in the photo, fluid is ejected from the surface near the maximum radius in one of three ejection modes: jets, sheets or sheet breakup. Additionally, a surprisingly large flow rate is induced by the spinning object.
Truscott’s team focused its study on spheres to characterize the modes of ejection and measure the sheet breakup distance through the use of high-speed cameras.
To the untrained eye, it may appear that the fluid is ejected from around the egg due to centrifugal forces overcoming the forces of surface tension. But Truscott says that isn’t the reason.
“If this were the case, then at higher spin rates, ejection would occur at lower angles of theta — meaning at a lower position on the egg,” he said. “However, our experimentation shows that at higher spin rates, the fluid remains attached near the egg’s equator.”
What this means is that the flow phenomena is an inertia-dominated boundary layer or thin film that is driven by the centrifugal force induced by the rotation of the sphere similar to the spinning disk of a Von Karmán viscous pump.
With additional research, Truscott says the spinning sphere could be incorporated into several practical applications. For example, it could be used as a scavenging pump because it is capable of high flow rates with high viscosities.
The sphere can rest on the bottom of the container that is being emptied and continue to pump fluid. And it only has a single moving part making it easy to clean because the pumping mechanism is exposed.
Truscott is the principle investigator for the study. The article was co-authored by Kenneth R. Langley and Daniel Maynes.
Related links:
USU Mechanical and Aerospace Engineering Department
Contact: Tadd Truscott, 435-797-8246, taddtruscott@gmail.com
Writer: Matt Jensen, 435-797-8170, matthew.jensen@usu.edu
This screenshot from a high-speed camera shows droplets of milk ejecting from the equator of an egg. A new study reveals how spheres can act like a pump when spinning in a bath of fluid.
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