Swimming and Viscosity, Part 2

Yesterday's post on "Swimming in Jello" stirred up a real bee's nest over at reddit/physics, so I thought I would reply to the most common complaints about my post.

First, I apologize to all that I didn't get a chance to catch the firestorm earlier when I could have done more to control it. I now realize that my original post is lacking a key piece of information - adding Jello (gelatin) to ~ room temperature water will not turn it into the thick solid gel that we all have eaten. I made some up this morning, but even after 2 hours, the water is only slightly thicker but still extremely runny. The water is also very cloudy from all the undissolved gelatin. In order to get the solid gel, the gelatin needs to be "cooked" to about 180 F or so and then cooled, something that would not be possible in a swim pool.

Having worked with numerous gelling agents over the decades, I knew this, but I should have been clearer about it upfront. Again, I apologize. So when I saw the cartoon characters dumping Jello into the pool, I falsely assumed that no one would think that it would turn to a solid, but rather just to a slightly thicker liquid, and that further that the cartoonist was making the assumption that the increase in viscosity would slow Michael Phelps down.

The article that I linked to (yes, I have fixed the link) about swimming in guar gum takes advantage of the fact that it will gel water without needing to be heated. Jello is protein based, but guam gum and other gums are carbohydrates. There are also synthetic thickeners such as carbopols, pluronics and crosslinked polyvinyl alcohol. These latter actually work best if you first disperse them in ice water and let the ice melt, certainly a viable option for gelling a swim pool.

By the way, if anyone is interested in repeating the guar gum pool, you better be very rich or willing to wait a few years. Guar gum is in extremely short supply right now as it is the preferred gelling agent for fracking, and there's a lot of extremely rich farmers in India as a result. In a few years, the crop will be larger and prices will fall to normal.

Gelled water is non-Netwtonian, a very broad term that I can tell from more than a few of the comments is also not well understood. Gelled water will be not only shear-thinning (the viscosity will drop as the shear rate increase) but also thixotropic (the viscosity at a constant shear rate will drop over time, but will then increase after the shearing has stopped). All of these characteristics will lower the viscosity as someone swims through it, so if you want to use viscosity to slow down a swimmer, you need the opposite approach, something that makes water dilatant and rheopectic. by making into Oobleck - just add cornstarch.

Sadly, the cartoonist missed that simple and fun approach as the real way to slow down Michel Phelps.