Hydrodynamic orienting of asymmetric microobjects under gravity

Abstract

It is shown that non-symmetric microobjects orient while settling under gravity in aviscous 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 ofexternal torques, the spheres are free to spin along the arms. The motion of the chain isevaluated by solving the Stokes equations with the use of the multipole method. It isdemonstrated that the spinning beads speed up sedimentation by a small amount andincrease the orientation rate significantly in comparison to the corresponding rigid chain. Itis shown that chains orient towards the V-shaped stable stationary configuration. Incontrast, rods and star-shaped microobjects do not rotate. The hydrodynamic orienting isrelevant for efficient swimming of non-symmetric microobjects and for sedimentingsuspensions.