3D Brownian diffusion of submicron-sized particle clusters.

We report on the translation and rotation of particle clusters made through the combination of spherical building blocks. These clusters present ideal model systems to study the motion of objects with complex shape. Since they could be separated into fractions of well-defined configurations on a sufficient scale and because their overall dimensions were below 300 nm, the translational and rotational diffusion coefficients of particle doublets, triplets, and tetrahedrons could be determined by a combination of polarized dynamic light scattering (DLS) and depolarized dynamic light scattering (DDLS). The use of colloidal clusters for DDLS experiments overcomes the limitation of earlier experiments on the diffusion of complex objects near surfaces because the true 3D diffusion can be studied. When the exact geometry of the complex assemblies is known, different hydrodynamic models for calculating the diffusion coefficients for objects with complex shapes could be applied. Because hydrodynamic friction must be restricted to the cluster surface, the so-called shell model, in which the surface is represented as a shell of small friction elements, was most suitable to describe the dynamics. A quantitative comparison of the predictions from theoretical modeling with the results obtained by DDLS showed an excellent agreement between experiment and theory.