Thursday, December 15, 2011

Thurs-Demo: The One that Rises

Whew! Another semester is almost finished. Just a couple of final exams and #gradingjail to go now. Here's a demo that isn't exactly quick - to really see the results you need to let the thing go for about 1000 hours...

The height above some free water surface (such as the water table) that surface tension can "pull" a column of water is proportional to the size of the pores between the soil grains. Like "capillary tubing" where the narrower the diameter of the tube, the higher water can move above the free water surface, the smaller the space between granular particles, the higher the capillary rise. This demo works well in medium and fine sands, although the highest capillary rise (perhaps up to a few meters) occurs in silts. There is minimal capillary rise between tiny clay particles due to their shape - they pack together closely enough to inhibit water movement.

The graph above shows the results from several iterations of the experiment. The colored dots represent the experiment that is ongoing (and shown in the video). What's really cool is that by using video tracking, I can refine my time measurements down to the first few seconds of the experiment. Expanded to 1000 hours, that gives me a data set that covers over seven orders of magnitude. I'm struck by how consistent this movement is between experimental runs and different grain sizes (the higher the mesh number, the smaller the particles).

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