The Ekman layer and why tea leaves go to the center of the cup
Consider a transparent, cylindrical container filled with water and sitting in the center of a record player turntable. When the turntable is started suddenly, the container rotates with the turntable, but the bulk of the fluid initially remains at rest. A thin (~1 mm) viscous boundary layer (Ekman layer) forms almost immediately at the bottom and top (if there is a lid) of the fluid. Here we describe a laboratory or demonstration exercise in which we use dye crystals1 and fine particles2 as tracers to study the flow in the Ekman layer. We also give a general method for using fine particles to measure the time it takes for the bulk of the fluid to rotate with the turntable (spin-up time) for different rates of rotation and fluid depths.
Heavers, R., & Dapp, R. (2010). The Ekman layer and why tea leaves go to the center of the cup. Physics Teacher, 48 (2), 96-100. https://doi.org/10.1119/1.3293655