Sedimenting rigid particles of certain shapes approach a stationary orientation

TL;DR

This study combines experiments and simulations to analyze how specific shaped rigid particles reorient and settle into stable configurations in viscous fluids, revealing insights into their mobility and stability.

Contribution

It provides new experimental and numerical data on the reorientation dynamics and stability of sedimenting particles with specific shapes, extending theoretical models.

Findings

Certain shapes reorient to stable stationary configurations

Two mobility coefficients have opposite signs

Reorientation occurs in a relatively short time

Abstract

This work investigates experimentally and numerically the dynamics of rigid particles settling under gravity in a highly viscous fluid. We demonstrate that certain shapes: cones, crescent moons, arrowheads, and open flat rings reorient and approach a stationary configuration. We determine the mobility coefficients and the characteristic reorientation times. We find out that the two rotational-translational mobility coefficients have opposite signs. Therefore, based on the equations of motion for rigid bodies with two orthogonal planes of symmetry, theoretically derived by Joshi and Govindarajan, Phys. Rev. Lett., 134, 2025, 014002 and Ekiel-Jezewska and Wajnryb, J. Phys. Condens. Matter, 21, 2009, 204102, we conclude that the approached stationary configurations are stable. Owing to the similarity principle, our experimental findings apply to micro-objects in water-based solutions. The reorientation of sedimenting rigid particles of certain shapes to a stationary stable configuration in a relatively short time might be used for biological, medical, or industrial applications.