An automated single-particle tracker: application to characterization of non-azimuthal motion in Couette flows at low Reynolds number

We describe an experiment that allows us to record 3-dimensional trajectories of single particles in Couette shear flows, at low Reynolds number. The core of the apparatus is a Couette cell with transparent contrarotating cylinders. Fluorescent spherical particles are used as tracers. A single tracer is imaged onto a webcam, equipped with a home-made autofocus system. For a given average shear rate, tracking of an individual tracer is performed automatically by driving the amount of contrarotation between both cylinders and the position of the webcam. The performance of the tracker is illustrated through examples of trajectories of neutrally buoyant tracers in a Newtonian fluid. The setup is mostly aimed at characterizing complex flows in non-colloidal concentrated suspensions and wet granular materials. We show examples of 3d trajectories in a dense suspension of 200 lm spherical grains, revealing details of the short-scale diffusive- like particle motion, together with flow localization and large-scale non-azimuthal flow patterns