Friction coefficient of rod-like chains of spheres at very low Reynolds numbers. I. Experiment

Abstract

We present a new method to measure hydrodynamic friction forces on colloidal particles at very low (≃10-5) Reynolds numbers with high accuracy. Exploiting magnetic field induced aggregation of microscopic paramagnetic spheres into straight chains of calibrated length, we determine the anisotropic friction coefficient of the chains, with a range from one to 100 particles, by video-microscopic observation of sedimentation speed. The results are found to agree well with the Slender-body theory, even for small chain lengths. The effects of the proximity of a wall are also explored.