FRICTION COEFFICIENT OF ROD-LIKE CHAINS OF SPHERES AT VERY-LOW REYNOLDS-NUMBERS .1. EXPERIMENT

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