Reorientation of rigid objects settling under gravity in a very viscous fluid

Sedimentation of rigid objects with two perpendicular symmetry planes is

investigated experimentally, theoretically, and numerically for the

Reynolds number much smaller than unity. We select shapes that approach

a stationary orientation: cones, crescent moons, arrowheads, and open

rings. Shapes approaching a stationary inclination were theoretically

classified as settlers by Joshi and Govindarajan, Phys. Rev. Lett. 134,

014002, 2025; the dynamics of an example of the settler was determined

theoretically and numerically by Ekiel-Je\.zewska and Wajnryb, J. Phys.

Condens. Matter 21, 204102 (2009).

In this work, the settling velocity and the rotational-translational

mobility coefficients are determined. Their values are used to show the

stability of the final orientation and to evaluate the characteristic

reorientation times that are short enough for potential applications.

Owing to the similarity principle, our experimental findings can be

applied to microobjects in water-based solutions. Reorientation of

particles of certain shapes described in this work is expected to lead

to the formation of ordered dilute suspensions.