Few-body hydrodynamic interactions probed by optical trap pulling experiment.

We study the hydrodynamic coupling of neighboring micro-beads placed in a multiple optical trap setup allowing us to precisely control the degree of coupling and directly measure time-dependent trajectories of entrained beads. We performed measurements on configurations with increasing complexity starting with a pair of entrained beads moving in one dimension, then in two dimensions, and finally a triplet of beads moving in two dimensions. The average experimental trajectories of a probe bead compare well with the theoretical computation, illustrating the role of viscous coupling and setting timescales for probe bead relaxation. The findings also provide direct experimental corroborations of hydrodynamic coupling at large, micrometer spatial scales and long, millisecond timescales, of relevance to, e.g., microfluidic device design and hydrodynamic-assisted colloidal assembly, improving the capability of optical tweezers, and understanding the coupling between micrometer-scale objects within a living cell.