A miscalculated Fluids Lab demonstration leads to a new understanding of how particles accumulate in lakes and oceans
University of North Carolina at Chapel Hill
VIDEO: Ocean particle accumulation has long been understood as the result of chance collisions and adhesion. But an entirely different and unexpected phenomenon is at work in the water column. Like so many discoveries, this one began accidentally. A graduate student intended to show a favorite parlor trick — how spheres dumped into a tank of salt water will “bounce” on their way to the bottom, as long as the fluid is uniformly stratified by density. But the student in charge of the experiment made an error in setting up the density of the lower fluid. The spheres bounced and then hung there, submerged but not sinking to the bottom.
Credit: Robert Hunt/UNC-Chapel Hill
A team of mathematicians from the University of North Carolina at Chapel Hill and Brown University has discovered a new phenomenon that generates a fluidic force capable of moving and binding particles immersed in density-layered fluids. The breakthrough offers an alternative to previously held assumptions about how particles accumulate in lakes and oceans and could lead to applications in locating biological hotspots, cleaning up the environment and even in sorting and packing.
How matter settles and aggregates under gravitation in fluid systems, such as lakes and oceans, is a broad and important area of scientific study, one that greatly impacts humanity and the planet. Consider “marine snow,” the shower of organic matter constantly falling from upper waters to the deep ocean. Not only is nutrient-rich marine snow essential to the global food chain, but its accumulations in the briny deep represent the Earth’s largest carbon sink and one of the least-understood components of the planet’s carbon cycle. There is also the growing concern over microplastics swirling in ocean gyres.
Ocean particle accumulation has long been understood as the result of chance collisions and adhesion. But an entirely different and unexpected phenomenon is at work in the water column, according to a paper published Dec. 20 in Nature Communications by a team led by professors Richard McLaughlin and Roberto Camassa of the Carolina Center for Interdisciplinary Applied Mathematics in the College of Arts & Sciences, along with their UNC-Chapel Hill graduate student Robert Hunt and Dan Harris of the School of Engineering at Brown University.
In the paper, the researchers demonstrate that particles suspended in fluids of different densities, such as seawater of varying layers of salinity, exhibit two previously undiscovered behaviors. First, the particles self-assemble without electrostatic or magnetic attraction or, in the case of micro-organisms, without propulsion devices such as beating flagella or cilia. Second, they clump together without any need for adhesive or other bonding forces. The larger the cluster, the stronger the attractive force.
Like so many discoveries, this one began accidentally, a couple years ago, during a demonstration for VIPs visiting the Joint Applied Mathematics and Marine Sciences Fluids Lab that Camassa and McLaughlin run. The pair, long fascinated with stratified fluids, intended to show a favorite parlor trick — how spheres dumped into a tank of salt water will “bounce” on their way to the bottom, as long as the fluid is uniformly stratified by density. But the graduate student in charge of the experiment made an error in setting up the density of the lower fluid. The spheres bounced and then hung there, submerged but not sinking to the bottom.
“And then I made what was a good decision,” said McLaughlin, “to not clean up the mess.” Go home, he told the grad student. We’ll, deal with it later. The next morning, the balls were still suspended, but they had begun to cluster together — to self-assemble for no apparent reason.
The researchers eventually discovered the reason, though it took more than two years of benchmark experimental studies and lots of math.
You can see the phenomenon at work in a video the researchers produced. Plastic microbeads dropped into a container of salt water topped with less dense fresh water are pulled down by the force of gravity and thrust upward by buoyancy. As they hang suspended, the interplay between buoyancy and diffusion — acting to balance out the concentration gradient of salt — creates flows around the microbeads, causing them to slowly move. Rather than moving randomly, however, they clump together, solving their own jigsaw-like puzzles. As the clusters grow, the fluid force increases.
“It’s almost like we discovered an effective new force,” Camassa said.
The discovery of this previously unknown first-principle mechanism opens the doors of understanding for how matter organizes in the environment. In highly stratified bodies of water, such as estuaries and the deep ocean, being able to mathematically understand the phenomenon may allow scientists to model and predict the location of biological hotspots, including feeding grounds for commercial fish or endangered species. Harnessing the power of the phenomenon might also lead to better ways to locate ocean microplastics or even petroleum from deep-sea oil spills. Or, in an industrial-sized version of the Fluids Lab experiment, the mechanism might be used to sort materials of different densities, for example different colors of crushed recyclable glass.
“We’ve been working for years with stratified systems, typically looking at how stuff falls through them,” McLaughlin said. “This is one of the most exciting things I’ve encountered in my career.”
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The story told by these alleged researchers/scientists remind me of the various engineers and scientists I worked with in the 1970’s in coal gasification process development. Time and again I would witness questions and musings of those who decided to step out of the box of scientific reasoning due to their encountering repeated failures in their attempts to achieve their goals or intended outcome, be discarded and ridiculed with the blind arrogance of educated intelligence.
When I read: ” First, the particles self-assemble without electrostatic or magnetic attraction or, in the case of micro-organisms, without propulsion devices such as beating flagella or cilia. Second, they clump together without any need for adhesive or other bonding forces. The larger the cluster, the stronger the attractive force.”___ i was reminded of the tendency for people who believe that they know everything about their field of expertise, to dismiss possible solutions and observations. These same highly educated people often spoke in disparaging tones and dramatically acted as if they had honestly explored other possible explanations for the observations of others and experimental failures.
Never once did I witness such individuals create anything new or discover anything previously unknown. So I am inclined to suspect that they really only see the world in terms of gravity or “settling science”, 😉 (OK,.. that was not much of a pun. I could not resist lightening up things. Recalling these experiences from the past was depressing and put a wet blanket on me as an enthusiastic young Chemical Engineer. The first time I witnessed such dismissive behavior depressed me for two weeks.)
Then there is this new research:
Water can form double layers around particles that create powerful barriers and boundaries. Biological systems depend on water’s abilities in order to function. For an introduction to this wondrous world read “The Fourth Phase of Water: Beyond Solid, Liquid, and Vapor”, by Gerald Pollack, or his other book, “Cells, Gels and the Engines of Life.” Prepare for delight and awe.
I wonder what the temperature of the water was and if it has an effect. As water approaches the freezing mark, the water molecules line up and create rope-like strings of water. As the temperature continues to drop they become more common. This is a fourth phase of water – between liquid and solid.
Maybe there are more.
Regards
Crispin
This is one example of a system reaching equilibrium. It is hard to predict the ways a system gets to equilibrium. I would not be surprised if additional co2 added to atmosphere causes the atmosphere to reach equilibrium in a yet unknown way.
Yes. I’ve often mused on the idea that CO2 must strongly affect the hydrological cycle by allowing (super)saturated water vapor to radiate (cool) more effectively. Yet all we seem to be bombarded with is how CO2 affects the whole planet, as if cloud formation was nothing of any great significance.
Water has a powerful property: surface tension based on hydrogen bonding. The surface need not be in contact with air. What is the nature of the particles added to the water? Are they relatively hydrophobic or hydrophilic? If you dropped a dense oil into water, you would expect the globules to merge into larger spheres, lacking other forces that might distort the ideal spherical shape. Why a sphere? Because the surface to volume ratio is minimized with a sphere, and the surface tension tends to drive the oil to a sphere. The larger the volume, the greater the surface:volume effect.
Other forces mentioned above may be involved, but I doubt the strengths of these factors approach the forces of surface tension. Consider how different water is to methane, having about the same mass. Is methane a liquid at RT? Not even close. Hydrogen bonding is powerful. Liquid water has about 3.5 bonds per molecule, higher in ice. Water must expand to form all four bonds, hence water ice floats on liquid water.
Happens to be my old field, studied using sediment traps, measuring marine snow, zooplankton pelletization and fluff-layer resuspension. Actually micro-modeling of flockulation processes never found much use and probably still isn’t. This above looks more like media ‘storytelling’.
These wizards have just re-discovered sucrose gradient ultracentrifugation. They are obviously not cell biologists.
“being able to mathematically understand the phenomenon”
Rather, being able to physically understand the phenomenon..
Mathematics itself offers no insights into physical phenomena. It is the language of physical description, but provides none of the physical content.
It seems there are some scientists too poorly trained in the meaning of science to realize the fundamental uniqueness of physical knowledge.
Word to the wise: Vide van der Waal’s forces.
Wait. Frame of reference question:
Are the mathematicians intentionally using the term “fluid” here? Is this a phenomenon of physical “fluids,” or of “liquids?”
In other words, the experiment showed the effect in water. Is it also possible in air, which is a fluid, but not a liquid?
The larger the cluster, the stronger the attractive force –> The larger the cluster, the stronger the pressure