Thursday, March 07, 2013

Slow-Mo Sedimentation: When Stokes' Law Doesn't Apply

Thanks to the ever helpful folks at LRRD, the colored sediment for my lab's Em2 arrived last week. I shot some high speed video of a scoop falling through water. Based on the results, I tried again with the help of my colleague Todd Zimmerman. We tried a few different colored gels on the translucent backdrop, but the first one that I used, a nice deep blue, works the best. I think it's because of the red-orange hues in all of the sediment particle sizes contrast well with the blue.

Colored sediment: when Stokes Law does not apply from Matt Kuchta on Vimeo.

Contrast this video with the footage we captured a few years ago of ball bearings falling through corn syrup:
What a Drag! Falling Through Syrup from Matt Kuchta on Vimeo.

The single ball bearing is a good example of how Stokes' Law works. I blogged about it before, too. But the first video shows many particles. These particles are banging into each other, the combined mass of the particles is also pushing the water around in turbulent eddies. Stokes' Law does not apply because the settling of each particle is hindered by interactions with other particles and the surrounding fluid. In these cases, we're often without a simple, elegant equation to describe what's happening. Instead, we have to rely on empirical observations. Such as the bedforms left behind in the sediments after the particles are deposited. In the end, however, many of the smallest particles are left behind to drape over the entire pile of material. So even in these chaotic, turbulent systems, Stokes' observations can still help inform us of these processes.

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