Decoupling of translational and rotational diffusion in quasi-2D colloidal fluids

TL;DR

This study investigates how translational and rotational diffusion of dimer particles in quasi-2D colloidal fluids become decoupled or coupled depending on particle size and density, revealing complex dynamic behaviors.

Contribution

It demonstrates the size-dependent decoupling of translational and rotational diffusion in dense colloidal systems, bridging previous findings with spherical and elongated particles.

Findings

Short dimers exhibit rapid reorientations and decoupled diffusion.

Long dimers show coupled translational and rotational diffusion.

Diffusion behaviors vary with particle size and area fraction.

Abstract

We observe the translational and rotational diffusion of dimer tracer particles in quasi-2D colloidal samples. The dimers are in dense samples of two different sizes of spherical colloidal particles, with the area fraction ${\phi}$ of the particles varying from dilute to nearly glassy. At low ${\phi}$ rotational and translational diffusion have a ratio set by the dimer size, as expected. At higher ${\phi}$ dimers become caged by their neighboring particles, and both rotational and translational diffusion slow. For short dimers we observe rapid reorientations so that the rotational diffusion is faster than translational diffusion: the two modes of diffusion are decoupled and have different ${\phi}$ dependence. Longer dimers do not exhibit fast rotations, and we find translational and rotational diffusion stay coupled for all ${\phi}$. Our results bridge prior results that used spheres (very fast rotation) and long ellipsoids (very slow rotation).