Discovering a new fundamental underwater force

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.”


From EurekAlert!

69 thoughts on “Discovering a new fundamental underwater force

    • Why do clouds appear to form in distinct clumps? several explanations

      At some level, the same process that leads to the clumping of plastic microbeads could work for water droplets. It seems to me that water droplets in clouds are a more complex problem though.

      • Cb,

        Thanks for the link. I noticed that upwelling LWIR from solar heated surface (ground temps could be as high as 120 to 150F) were not considered by the interviewed professors. At condensation the previously saturated (invisible) water vapor is a ‘greenhouse’ gas and upon condensation (visible) would act/thermalize as a blackbody. Couple that with the characteristics of phase change and we are looking at a significant factor.

      • Suggest one take a trip in/experience a glider flight, on a nice, sun-shiny day?

        (The word we are looking for here in this practical demo is: “thermals”.)

    • I suspect the unusual bonding characteristics of the water molecule is at least partly responsible here.
      It was not that long ago that we had full realization of how water molecules clump to form super molecules.
      It would be informative to do the experiment with a much less polar liquid.

      • re: “It was not that long ago that we had full realization of how water molecules clump to form super molecules.”

        Yea verily, I give you “the rain drop”.

  1. “It’s almost like we discovered an effective new force”.

    Almost. Close, but no cigar. This is not a “force” like the electromagnetic force until a lot more scientific rigor is applied, and that does mran computer modeling.

    • Consider viewing from above two airplanes taking off simultaneously from an airport on the equator, headed for a destination also on the equator, but 1/4th the way around the globe. Each follows a Great Circle route, one starting off to the northeast, and the other to the southeast…. At first they seem to be affected by a repulsive force, but then, after reaching the midpoint of the flight, they seem to be affected by an attractive force….Is it a “Force” or are they each just flying straight lines in non-Euclidian space?

      • Both planes would follow the equator, the great-circle route to another destination on the equator.

        Are you a bot?

        • Dave, you are correct for distances less than 1/2 the length of the equator. If the start and stop points are on opposite sides of the Earth, every other great circle route would be shorter than an equatorial route, with circumpolar routes being shortest of all. (The rotation of the Earth causes equatorial radii to be shorter than polar radii.)


    • I suspect that this is a surface tension related effect. Surface tension creates the meniscus to curve up at the wall of a container.

      When mixing things it takes a fair amount of work to break up particle clumps. Seems reasonable that when two particles floating in water touch that small bit of energy would be shifted to pulling the particles together. The forces could be van der Waals or hydrogen bonding, at least for small particles.

      • One professor to another, upon viewing a very shapely young lady:

        “Remember, she’s 90% water!” [Note: I don’t know the actual percentage.]

        Other professor, after another look:

        “Oh, yes, but check out that surface tension!”

    • Hmm, yes… why is the stratified density significant? In what way is this different to clumping at the water/air interface? Clearly we’ve missed something here..

      • re: “In what way is this different to clumping at the water/air interface?”

        Take note of how the ‘gasoline’ in movies behaves versus real gasoline; water has surface tension the belies the substitution of water for most ‘gasoline’ scenes in movies.

        PS. Gasoline exhibits no surface tension, so ‘drops’, per se, do not form as with water.

    • I’m not familiar with any of this, so don’t take this too seriously, but … when I read the article I thought that as the particles bounce around the water moving between them would be moving fastest at the points where the particles are closest to each other. Counter-intuitively, the laws of hydrodynamics say that a faster-moving liquid has lower pressure, therefore the particles would be drawn towards each other.

    • re: “Why isn’t this ‘fundamental underwater force’ simply the Van der Waals force, proposed in 1873 by Johannes Diderik van der Waals, which accounts for the clumping together of otherwise neutral particles in a fluid suspension? ”

      Because, per the article. mathematicians, not physicists, were conducting the research?

  2. I love the way science advances by the mistake of a graduate student. It happens more often than less..

    • Yes, it’s the sort of thing that is much more likely in a cloistered environment like a university than out in the wide world.

      Think of Fleming’s dripping snot into a petri dish before leaving it to fester for a couple of weeks. In the tidy outside world it would have been cleared up before anyone noticed that the culture failed to grow round the snot.

      The real world is not tidy!

  3. This is interesting, as one type (s) of spherical particle, bubbles are important features in the ocean, one of the problems with gas transfer coefficients. Some have electrical properties, organic skins, cluster. Fish, among others, produce all sorts. Difficult to research. They did look at a little old literature (Open access).
    “….for this geometry there is an exact steady solution10,11 to the Navier-Stokes equations coupled to an advection-diffusion equation for salt concentration. This solution shows a boundary layer region in which there is a density anomaly and a shear flow up the top side of the sloped wall. We remark that experimental work of Allshouse and Peacock, using a freely suspended wedge-shaped object, demonstrated that such flows are in fact sufficient to self transport a single object12,13.”

    Some do this.
    Blanchard, D. C. 1963. The electrification of the atmosphere by particles from bubbles in the sea. pp. 71-202, In, Sears, M. (Ed.) Progress in Oceanography I. Macmillan, NY. And this. Fox, F. E. and K. F. Herzfeld. 1954. Gas bubbles with organic skin as cavitation nuclei. Journal Acoustical Society of America. 26(6):984-989.

  4. I already discovered this force as a kid when I observed how the last few cheerios in the bowl always seem to cling together for survival.

    • I discovered fire when playing with a box of matches. I also discovered that whiskers on a stuffed animal don’t grow back when you cut them.

    • Excellent observation! But further research is needed. What happens if you mix cheerios with rice-chex? Is there integration or segregation? And what does this mean in terms of so-called societal science? Send more money (and milk.)

  5. A good example of serendipity in science, rather like Fleming and penicillin.
    A good job McLaughlin saw something interesting and didn’t just chuck the experiment into the bin.
    How much potentially good science goes into the bin because the results of an experiment don’t fit the narrative?
    Maybe Feynman would have had something to say about that

  6. Interpretation and re-interpretation are important and mandatory steps of the scientific process!

  7. May be a variant of the Casimir effect. Between two near clumps or particles the thermal oscillations frequencies are less then outside, leading to an apparent attraction due to the imbalance of wave pressure.

    • ChrisB– We were at Crater Lake when the ranger told us the minor amount of scum was probably pollen. There does not seem to be a lot of oceanographic interest in surface phenomena, maybe partly because it is complicated and not easy to study. Not as much time to look and think as there was. Besides horizontal eddies there are ups and downs, even mini-Langmuir streaks– Woodcock, A. H. 1941. Surface cooling and streaming in shallow fresh and salt waters. Journal of Marine Research. 4(2):153-160. Biologists haven’t used this much to figure out how larvae interact. Recruitment still mostly a mystery.

      For ScottyP below– Schmidt, W. 1936. Cause of ‘Oil Patches’ on water. Nature. 137:777.

    • re: “If this is the explanation then one should observe a similar clumping of beads floating on the surface.”

      The forming of a ‘meniscus’ (and the small forces present) in the likes of a test tube, but as it applies to ‘beads’ in this case …

  8. This is a wonderful springboard for imagination. If the “rain” of nutrients pauses on its way to the bottom of the sea it would create a layer of nutrients, and critters would congregate to exploit it, and bigger creatures would arrive to eat them. Maybe there are ecosystems a half mile down we don’t know about.

  9. Besides forces that cause things to stratify and clump there are forces that lead to abrupt up-welling and overturning.

    One very cool thing to witness is the sudden overturning of a still lake on a calm and sunny spring day. Very cold water has the same quality as ice and floats on top of warmer water, but once is warms a degree or two it behaves more normally and sinks. Therefore as the final ice melts and surface water warms in a freshwater lake, all the water on top abruptly heads to the bottom and stuff from the bottom, including dead bits of leaves and straw, come up to the top. Fish that were in a sort of torpor abruptly become active (as do fishermen).

    The more I learn about “systems” the more wonderfully complex they appear. My sympathies to those who attempt to model such things; if you leave out one variable, you’re screwed.

    • “My sympathies to those who attempt to model such things; if you leave out one variable, you’re screwed.”

      Not in Settled Science. Then it is mandatory to leave out the (((sun)))

    • There can be very minor circumstances that prevent things from happening that you expect to happen. For example I had a hard time understanding how a thin invisible oil sheen prevents waves, it just didnt make sense, then one morning after a cold night I went to the Lake Michigan shore and there was a 15 mph onshore wind and I expected waves (I’m sure the forecast called for 3 footer) but no waves .. flat calm. The previous night was calm and cold and froze the surface to about about 1/8″ thinkness. The stiff wind was pushing this thin ice sheet to shore at a decent velocity and it was making a heckuva racket as it piled up on shore but there were just no waves even though the ice was broken in the open water here and there. Some might say sure dummy when there is ice there are no waves but that’s how finally accepted how an thin oil sheen can prevent the wind from grabbing the surface and making waves. I think when water is slushy with surface ice crystals it also retards waves.

  10. It’s just cause by the back radiation energy of the water droplets, and the CO2 they have acquired from the atmosphere. That’s why CO2 causes rain and snow. 🙂

  11. This is the same cause as the Coanda effect.

    The “forces” are differential only because one side is shielded (slightly) from Brownian motion that effects all surfaces of the spheres. Therefore, they tend to collect together.

    Details at 11:00


  12. Just a thought. Could this be one of the critical mechanisms that started life on this planet 3.x billion years ago?

  13. I want a government grant to study why when children play outside boys get dirtier than girls. I need the grant money every year for the 50 years to do the study.

    • No, no. Multiple grants are required so not to discriminate against alternative outcomes: do girls who identify as boys get as dirty as boys when playing outside? Do boys identifying as girls stay as clean? If a girl gets as dirty as a boy, should transgendering be scheduled? etc.

      Including these considerations will GUARANTEE that you will get all the grants you desire.

  14. This is an interesting discovery that could lead to tailings ponds (or settling ponds of any type) being operated more efficiently.

  15. Colloid science always struck me as something of a dark art, despite the intensely theoretical rigorous treatments I occasionally came across as a grad student.

    Sadly, these days, if I see anything being reported by people somehow connected with studying climate I make the working assumption that they are simply over-funded novices or climate-naïfs, rediscovering things well known to the real experts in the field for decades.

    • Colleague John Elliston devoted years to intellectual effort to properly place colloidal properties of rock and mineral particles into the broader context of petrogenesis and orogenesis.
      His book is now available, see Google.
      As scientists might anticipate, previous wisdom about (for example) molten phases was laid open when mechanisms using mobile gel-like matter fitted observations better than molten.
      John did not just notice a laboratory oddity. He noticed geological oddities on every field trip and wove them into different concepts. Well worth getting the book and learning from it if you are a scientist with an open mind willing to learn more than “settled science”. Geoff S

  16. “It is like we discovered an effective new force” New? The movement of colloidal particles in a fluid mixture or solution with concentration gradients is called “diffusiophoresis”, and is well known.Try Google.

  17. 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.

  18. 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.

  19. 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.

  20. 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’.

  21. These wizards have just re-discovered sucrose gradient ultracentrifugation. They are obviously not cell biologists.

  22. 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.

  23. 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?

  24. The larger the cluster, the stronger the attractive force –> The larger the cluster, the stronger the pressure

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