Experimental and numerical study of rigid particles with two planes of symmetry approaching a stable, stationary orientation while sedimenting

This work investigates experimentally and numerically the dynamics of rigid particles with two orthogonal symmetry planes settling under gravity in a highly viscous fluid at a Reynolds number much smaller than one. Joshi & Govindarajan (2025 Phys. Rev. Lett . 134(1), 014002), showed theoretically that for such shapes, the dynamics are qualitatively different for different signs of the product of two rotational–translational mobility coefficients, evaluated with respect to the particle centre of mass in a symmetric reference frame. However, upon examining a particle’s shape, it is not immediately evident if this product is negative, positive or zero. In this paper, we demonstrate how to estimate these coefficients and the sign of their product from experiments, using special initial orientations, and also numerically, based on the Stokes equations. Especially interesting are the ‘settlers’ – such particles that reorient and approach a stationary stable orientation, and we focus our study on this class of shapes. We show experimentally that cones, crescent moons, arrowheads and open flat rings are the settlers, and we evaluate from the experiments their rotational–translational mobility coefficients. Then, we reconstruct each experimental shape as a rigid conglomerate of many touching beads, and use the precise Hydromultipole code to calculate the mobility coefficients for the conglomerate. The numerical and experimental values are close enough to determine that the particles are the settlers, and to estimate the characteristic reorientation time scales. Our findings apply to non-Brownian micro-objects in water-based solutions – experimentally by the similarity principle and theoretically based on the Stokes equations. The reorientation of sedimenting rigid particles to a stationary stable configuration in a relatively short time might be used for environmental, biological, medical or industrial applications.