Viscosity

I have just started reading Chapter 5 of the book Biological Physics by Philip Nelson, which is called Life in the Slow Lane: The Low Reynolds-Number World. The book is an undergraduate introduction to biophysics which is extremely well written and very pedagogic. The undergraduate word however just means that the mathematics of the book is not very advanced, for there are a lot of physical insights that are extremely interesting and valuable for any physicist.

In this chapter, Nelson is writing about the difference in the relative viscosity for macroscopic and microscopic objects and the effect of it to the world of cells. In the very beginning, he explains the experiment in the video above, the only difference being that in the book you only have one coloured drop.

In the experiment, the container is composed by two concentric cylinders with corn syrup, a very viscous fluid, filling the space between them. As you then can see, drops of coloured syrup are put in this space. Note how the fluid is viscous by the fact that the drops don't even move once they are there. Then, the handle is turned and the internal cylinder is rotated a number of times. The fluid is dragged by the rotation and the drops apparently mix. The magic happens when the cylinder is rotated in the opposite direction and, miraculously, the drops unmix and reappears almost intact.

The explanation of how this can happen is quite interesting and is given in Nelson's book. What happens is that the drops never really get mixed, because the fluid is so viscous that there is no turbulence. Without turbulence, there is only a very organised laminar movement of the fluids and not the disordered wandering of molecules that causes mixing. The molecules actually stop moving (at least almost) when the rotation stops. When the rotation is realised in the opposite direction, the molecules simply retrace their previous steps and come back to the place where they were in the beginning. Of course, that's not perfect and you can see the drops had fuzzy boundaries where some diffusion and mixing did happen, but that is negligible.

The most interesting part of the discussion in Nelson's book comes afterwards where he explains that water is very viscous from the point of view of bacteria and this kind of effect happen in the microworld. This brings problem for them to move as, if they just swing upwards and backwards some kind of structure, they will never move because the fluid will just trace back the previous movement. I stopped somewhere around there.  If you are interested, I highly recommend Nelson's book.