Translation-rotation decoupling of colloidal clusters of various symmetries

Single-particle tracking was used to measure the diffusion in aqueous suspension of dilute colloidal clusters, 2-9 mu m in size, fabricated by joining 1.57 mu m spheres into planar arrangements of various particle numbers and symmetries (doublet, trimer, square, pentamer, hexamer, and so forth). They were allowed to sediment close to a glass surface and their Brownian motion parallel to the surface, effectively in a two dimensional (2D) geometry, was imaged in a microscope in the presence of 3 mM monovalent salt to essentially screen electrostatic interactions. Geometric asymmetry produced systematically increasing discrepancy between the equivalent hydrodynamic radius of translation and rotation-tabulated in this paper. Our observations include cases where the effective hydrodynamic radius changes more rapidly for translation than rotation, the converse, and also cases where the effective hydrodynamic radius for translation changes significantly, while that of rotation is effectively constant. The significance is to document the connection between translational and rotational 2D mobilities for geometrical shapes not described by the Stokes-Einstein-Debye equations for spherical particles.