Does This Rheometer Make Me Look Fat?
I didn’t manage to get myself scraped off onto this month’s Accretionary Wedge – oh, noes! While I am tragically subducted into the mantle, though, the rest of you can read about the many open questions currently puzzling the geoblogosphere.
Perhaps I can make it into the volcanic arc or something with a late entry about what’s making me go “hmm” this afternoon. It’s related to volcanoes, too: the behavior of glass beads suspended in a zinc iodide solution, spun between two cylinders.
The picture adorning this post is from an article by Völtz et al. in Physical Review E. They spun a suspension of tiny glass beads between two cylinders, and found that under the right conditions, the beads will arrange themselves into sheets of neat hexagons. This arrangement governs the large-scale behavior of the bead-fluid mixture.
Suspensions of perfectly spherical, neutrally buoyant particles are not found in nature, but if you want to find a dense suspension of funny-shaped, not-so-neutrally-buoyant particles, you just need to visit your nearest slush puddle, mud puddle, beach, or crystal-rich magma chamber.
What has me going hmm, though, is not the difference between real mud and glass beads. I’m going hmm about the difference between experimental setups, where you are typically working with a thin layer of fluid (in this case, the gap between cylinders was large enough for perhaps 10-30 glass beads), and the real world, where things are thick and have weird 3D structures. There are good reasons to use thin layers for experiments – it means you don’t have to worry so much about turbulence, and you can get a large region in the middle of your apparatus where conditions are nicely uniform – but I have a hard time imagining neat hexagons appearing in a big wet sandbox. Are our experiments so thin that we’re missing some important 3D structures?
Voltz C, Nitschke M, Heymann L, and Rehberg I, 2002, Thixotropy in macroscopic suspensions of spheres, Phys. Rev. E 65:5, pp. 1539-3755. DOI: 10.1103/PhysRevE.65.051402 .