Hydrodynamic orienting of asymmetric microobjects under gravity

TL;DR

This paper investigates how asymmetric microobjects, modeled as chains of spheres, orient themselves under gravity in viscous fluids, revealing that spinning beads enhance orientation and that chains tend to form a stable V-shape.

Contribution

It introduces a simple chain model to analyze hydrodynamic orientation of asymmetric microobjects and shows the significant role of bead spinning in this process.

Findings

Spinning beads slightly accelerate sedimentation.

Chains tend to orient towards a V-shaped stable configuration.

Non-symmetric microobjects exhibit hydrodynamic orienting behavior.

Abstract

It is shown that nonsymmetric microobjects orient while settling under gravity in a viscous fluid. To analyze this process, a simple shape is chosen: a non-deformable `chain'. The chain consists of two straight arms, made of touching solid spheres. In the absence of external torques, the spheres are free to spin along the arms. The motion of the chain is evaluated by solving the Stokes equations with the use of the multipole method. It is demonstrated that the spinning beads speed up sedimentation by a small amount, and increase the orientation rate significantly in comparison to the corresponding rigid chain. It is shown that chains orient towards the V-shaped stable stationary configuration. In contrast, rods and star-shaped microobjects do not rotate. The hydrodynamic orienting is relevant for efficient swimming of non-symmetric microobjects, and for sedimenting suspensions.