Holographic Measurements of Anisotropic Three-Dimensional Diffusion of Colloidal Clusters

TL;DR

This study uses digital holographic microscopy to precisely measure the anisotropic 3D diffusion tensor of colloidal clusters, revealing asymmetries and subtle symmetry breaking effects in their diffusion behavior.

Contribution

It provides the first experimental measurements of the full 3D diffusion tensor for colloidal clusters without existing analytical solutions, demonstrating the method's accuracy and relevance.

Findings

Measured all nonzero elements of the 3D diffusion tensor with 1% precision.

Detected anisotropic rotational and translational diffusion in colloidal clusters.

Observed a small symmetry-breaking effect in the triangular cluster's rotational diffusion.

Abstract

We measure all nonzero elements of the three-dimensional (3D) diffusion tensor D for clusters of colloidal spheres to a precision of 1% or better using digital holographic microscopy. We study both dimers and triangular trimers of spheres, for which no analytical calculations of the diffusion tensor exist. We observe anisotropic rotational and translational diffusion arising from the asymmetries of the clusters. In the case of the three-particle triangular cluster, we also detect a small but statistically significant difference in the rotational diffusion about the two in-plane axes. We attribute this difference to weak breaking of threefold rotational symmetry due to a small amount of particle polydispersity. Our experimental measurements agree well with numerical calculations and show how diffusion constants can be measured under conditions relevant to colloidal self-assembly, where theoretical and even numerical prediction is difficult.