Doings in an Evaporation Pot

In the Northern hemisphere, it is winter, and temperatures are low. Meteorologists tell us that colder air cannot hold as much moisture as warmer air, so when you heat up the air (to heat your house), the air becomes much drier. This can cause problems with skin and respiratory system, due to the low humidity in the warmer air.

Those of us that have problems with low humidity are encouraged to place a large pot of water on the stove and turn the burner on to its lowest setting. This setting is not enough to boil the water, but heating up the water makes it more likely to evaporate small amounts of water into the air.

I am currently on a contract, away from my home in north central Texas, in an apartment in a house in southwest Ohio. The temperatures here are much lower than in Texas, and the air is drier to begin with, so putting approximately 48 fluid ounces of moisture into the air helps me not to have chapped hands and a raw throat.

While I do not leave it on when I leave for more than just a few minutes, it is always on while I am at home. Once, while stepping into the kitchen for something, I glanced into the pot to check for the water level. The water level was okay, but I also glimpsed a spot on the bottom of the pot. When I looked more closely to see what it was, it appeared to be a large number of tiny flecks that had settled to the bottom of the water.

But why would all of those flecks gather in one place? It seemed to me that the flecks should have just settled in whatever location they happened to be located, which would result in no concentration, and, therefore, be more or less invisible, due to there not being any aggregation to attract my attention.

And yet, there it was.

By this time, the issue had my attention, so every time I went into the kitchen, I would peer in to see if I could see what was going on. At times, it looked as if things had moved around in there, but I could not be certain, since the pattern was basically random.

Once when I peered in, however, I saw some of those flecks moving! They would spiral up maybe an inch, before starting to fall back to the bottom again. That’s when I realized that, when the heating element received current, the heat it produced would set up currents in the water, and the flecks were so light that they were borne on the currents. They tended to settle, and remain, in places where the current was not strong enough to move the tiny flecks. Generally all in one spot, although sometimes two or more loci are revealed.

Of course, when I pour in more water to replace that lost to evaporation, that sets up additional currents, stronger that the heat-related currents, and the flecks again disappear as being to small to locate individually. But as those current dissipate and the heating element starts moving those flecks again, they reform into one or more islands. As time passes (and more water is poured into the pot), it becomes evident that new flakes are settling out of each new addition of water, to add to the end result.

You are seeing two things:

  1. Convection cells. When a body of fluid is heated from below and/or cooled from above, it becomes unstable: because of the different densities, the hot and cold zones of fluid want to swap places, and the flow tends to self-organize into convection cells of a characteristic size. You see this in all sorts of places, ranging from a heated pot of water, to clouds, to the magma plumes that fuel volcanoes in some places. The characteristic size of the convection cells that develop is a function of a whole bunch of parameters including the temperature difference, fluid viscosity/density, depth, and others.

  2. Saltation. Particles at the boundary of a moving fluid tend to get entrained in the flow; once they fall into a stationary volume of fluid, they tend to stay put for good. This also explains why sand and other crud in the road (e.g. the “marbles” of tread material that gather on race tracks) tends to accumulate on the shoulders: if it’s in the driving lane, it keeps getting kicked around by passing cars until it finally lands in a place where it doesn’t get kicked around any more.