Watch this awesome fluid dynamics lab demo, and then stick around for the science:
How fluids flow
Liquid flows are described in two regimes, laminar and turbulent. Laminar flow is a smooth, constant fluid motion, as if the fluid molecules were marching in-pace, single file. Turbulent flow is dominated by chaos, producing eddies and unstable vortexes in the fluid.
In a flow that is turbulent, eddies and vortexes form, and internal forces rise in the liquid. This aggressive flow is important to understand because the more turbulent the flow, the more friction is produced. So much friction is generated that engineers dealing with long distances, such as in the mile-long pipes that deliver water to your house, must account for it or the water would slow to a halt before it got to your faucet.
Turbulent flow is how we typically imagine the movement of fluids. Consider a bridge post in a river. I’m sure you have seen the result of the interaction: eddies and swirling waters trail behind the post.
Laminar flow is a much less chaotic case. Sometimes called “streamline” flow, fluids moving in a laminar fashion move like sheets or layers. There is no mixing between the sheets; no eddies, no vortexes. Laminar flow is like watching playing cards slide past each other.
Engineers describe the transition between laminar and turbulent flow with what is called the Reynolds number. This number is a ratio of the forces acting inside the fluid, namely inertia and viscosity. The higher the velocity, the higher the Reynolds number, and the greater the turbulence. The inertia that comes along with water speeding along a river, for example, overpowers the viscosity of the water (think “thickness” or internal friction), and characterizes what happens in chaotic river rapids. Conversely, the higher the viscosity, the lower the Reynolds number, and the lower the turbulence. For example, a thick substance like molasses tends to flow in sheets without much turbulence.
Below a Reynolds number of around 2000 the flow is laminar; any higher than that and the streamline flow gives way to turbulence.
What is happening in the video?
Like molasses, the corn syrup in the video above has a very high viscosity or “thickness.” Combine this with slow velocity imparted by the rotating handle, and you get a very low Reynolds number. So, the corn syrup in the video is flowing laminar, but how do the colored drops come back together when the flow is reversed?
When a fluid flows in sheets, encountering an obstacle can still create turbulence. Here is a look at a laminar fluid moving over a sphere (note the “streamlines”):
Even with laminar flow, we might expect the rotating handle in the video to mix up the drops beyond recognition. But it turns out that the video above is demonstrating a special case.
If a fluid is very viscous and is moving slowly enough, creeping flow can occur. With a Reynolds number of less than one, creeping flow indicates that the viscous forces far outweigh the inertial forces of the fluid’s motion. Like a marching band encountering an obstacle, in creeping flow the laminar sheets of fluid move around obstructions without breaking rank.
The corn syrup in the video above is in a creeping flow; the rotating handle creates no turbulence in it.
The bottom line
So, how do the colored drops in the video reform? Because we have a creeping flow situation, the corn syrup moves in (nearly) perfect parallel sheets. The handle rotates, the friction between the syrup and the handle moves the closest sheet, and that movement causes friction between the first sheet and the next closest sheet to the handle, which moves the next sheet, and so on.
Even after five turns of the handle, the creeping flow produces no mixing or turbulence. The amount of friction that was produced by turning the handle five times moves the syrup a certain amount, in separate sheets. So turning the handle back the opposite way in theory should produce the same amount of friction, this time moving the sheets back to their original positions. The demo realigns these sheets, restoring the drops to their original shape with hardly any distortion.
Think of it this way: each drop in the corn syrup is like a fully solved Rubik’s cube. When you begin, each side is a whole color (a whole drop). If you then turn each vertical third of the cube a certain amount, like mixing the corn syrup with the handle and moving the “sheets” around, you will end up with a jumbled cube. But if you turn the vertical thirds of the cube back the exact opposite way, you will end up where you started with a fulled solved cube. The drops in the demo above illustrate this beautifully.