Many-Body Force and Mobility Measurements in Colloidal Systems

We demonstrate a technique for simultaneously measuring each component of the force vectors and mobility tensor of a small collection of colloidal particles based on observing a set of particle trajectories. For a few-body system of micron-sized polymer beads in oil separated by several particle radii, we find that the mobility tensor is well-described by a pairwise Stokeslet model. This stands in contrast to the electrostatic interactions, which were found to deviate significantly from a pairwise model. The measurement technique presented here should be simple to extend to systems of heterogeneous, non-spherical particles arranged in arbitrary 3D geometries.