Dynamics of inertialess sedimentation of conically deformed rigid disks.

When thinking of everyday examples of sedimenting objects, we often imagine a periodic motion, be that a fluttering motion of a leaf falling in the air, or a coin falling in water. By contrast, sedimentation in the limit of vanishing inertia, known to be linked to particle shape, is typically more predictable - achiral particles such as flat disk, rods and ellipsoids, follow oblique paths set by their initial orientation, while chiral particles, such propellers and helices, have a screw motion.

In this talk we study inertialess sedimentation of an achiral particle with one plane of symmetry. It is created by wrapping a circular disk around cones, so that one side the disk is more tightly curved than the other. We find that such particles tend to well-defined equilibrium orientations (with the tightly curved end pointing downwards), but sediment either in a straight line or a spiral depending on how curved they are. They also exhibits transient dynamics of increasing complexity as their asymmetry tends to zero. In the limiting case of two identical curvatures, when the disk has two planes of symmetries, i. e. it is cylindrically deformed, we recover a complex sedimentation motion that involves periodic reorientation of the disk in ever-repeating sequence of pitching and rolling.