Brownian motion of snowman-shaped colloidal particles

We investigate the Brownian motion of isolated snowman-shaped particles consisting of pairs of large and small spheres by video microscopy. Our observations reveal that the particle exhibits varying degrees of anisotropic translational diffusion depending on the reference point used for tracking. In particular, when tracking the snowman's geometrical center, the diffusion coefficient along the particle's long axis (Da) is greater than that along the short axis (Db). When tracking the large sphere's center, Da and Db are identical, while tracking the small sphere's center results in Da being smaller than Db. Since Da remains constant across these geometrical centers, the higher Db thus leads to the fastest diffusion for the small sphere's center. These differences in diffusion arise from the varying coupling between translational and rotational motions, determined by the tracking points relative to the center of hydrodynamic stress (CoH). The CoH has been experimentally confirmed to be the geometrical center of the snowman-shaped particle. Our findings are consistent with the Langevin theory for the Brownian motion of anisotropic particles.