I read this article Discovering a new fundamental underwater force which describes the discovery of the mechanism that causes particles in stratified fluids, e.g. the oceans, to move towards others particles and bind together.

The article describes an well known experiment that failed and lead to the initial discovery and ignited the following research:

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.

Where can I find the original experiment that they intended to perform?

What is the experiment's name?

Can you share a link to a video demo of it?

  • 2
    $\begingroup$ The paper is available on the Nature web site and it includes references to earlier papers describing suspension at a salinity discontinuity. Are such references what you want, or are you looking for a popular science level description of the phenomenon? $\endgroup$ Dec 21, 2019 at 9:42

1 Answer 1


These videos can be found behind the link provided in comments; Videos form Nature.com
Here is a Link to youtube video

My first idea about the reason was that it's the Bernoulli causing pressure differences between particles and this would cause some flow.

But if you look this video on the end from 0:15- to end, there is on lower-left corner a single "particle" which travels in a way which makes this explanation impossible.

As I believe to have some insights to fluid dynamics, I continued to think this further and indeed, if you go in the details this turns out to be an ordinary "secondary flow". To notice this, you need to look this best video about the experiment and on the very first (half-)second of it!

You will see a vortex. This Vortex might be caused by some surface evaporation etc, but this vortex is definitely present and it transfers heat from the lower fluid layer to the evaporating surface. This causes the top fluid layer to circulate like a hurricane.

But the "particles" are rotating only at the beginning? so why is that? First of all, as the temperature difference becomes small enough, there is no energy to maintain this rotation. The "secondary flow" still remains and keeps up the convection. This convection is away from center in top of the fluid and towards center in the bottom of the fluid, so as there is two layers of fluid these flows are going opposite directions. This provides us the information, that the majority of these "particles" are closer to the top layer of the fluid.

But there are few particles which has a density closer to the bottom layer of the fluid, and they are thus travelling in opposite direction. You can see them on the linked best video at 12 sec forward. A picture below shows the one's to look;

particulate aggregation and self-assembly in stratified fluids

Good old educational video about secondary flows to those with more interest.


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