Two-dimensional Brownian motion of anisotropic dimers

TL;DR

This study investigates the anisotropic diffusion of colloidal dimers in two dimensions through experiments and theory, revealing translational-rotational coupling and diffusion coefficient splitting that decay over time.

Contribution

It provides a quantitative comparison between experimental measurements and theoretical models for anisotropic diffusion of dimers, including rotational dynamics and scattering functions.

Findings

Measured diffusion coefficients parallel and perpendicular to dimer axis show splitting.

Translational-rotational coupling is significant and depends on initial orientation.

Experimental results agree quantitatively with theoretical predictions.

Abstract

We study the 2D motion of colloidal dimers by single-particle tracking and compare the experimental observations obtained by bright-field microscopy to theoretical predictions for anisotropic diffusion. The comparison is based on the mean-square displacements in the laboratory and particle frame as well as generalizations of the self-intermediate scattering functions, which provide insights into the rotational dynamics of the dimer. The diffusional anisotropy leads to a measurable translational-rotational coupling that becomes most prominent by aligning the coordinate system with the initial orientation of the particles. In particular, we find a splitting of the time-dependent diffusion coefficients parallel and perpendicular to the long axis of the dimer which decays over the orientational relaxation time. Deviations of the self-intermediate scattering functions from pure exponential relaxation are small but can be resolved experimentally. The theoretical predictions and experimental results agree quantitatively.