The effect of curvature on the diffusion of colloidal bananas

TL;DR

This study explores how the curvature of colloidal banana-shaped particles influences their diffusion behavior, revealing non-monotonic anisotropic diffusion patterns and the significance of local drag effects.

Contribution

It provides the first detailed experimental analysis of how curvature affects the translational and rotational diffusion of colloidal rods and rings, supported by theoretical consistency.

Findings

Diffusion varies non-monotonically with opening angle.

Fastest diffusion axis switches at 180°.

Nearly closed rings have higher rotational diffusion.

Abstract

Anisotropic colloidal particles exhibit complex dynamics which play a crucial role in their functionality, transport and phase behaviour. In this work, we investigate the two-dimensional diffusion of smoothly curved colloidal rods -- also known as colloidal bananas -- as a function of their opening angle, {\alpha}. We measure the translational and rotational diffusion coefficients of the particles with opening angles ranging from 0{\deg} (straight rods) to nearly 360{\deg}(closed rings). In particular, we find that the anisotropic diffusion of the particles varies non-monotonically with their opening angle and that the axis of fastest diffusion switches from the long to the short axis of the particles when {\alpha}>180{\deg}. We also find that the rotational diffusion coefficient of nearly closed rings is approximately an order of magnitude higher than that of straight rods of the same length. Finally, we show that the experimental results are consistent with Slender Body Theory, indicating that the dynamical behavior of the particles arises primarily from their local drag anisotropy. These results highlight the impact of curvature on the Brownian Motion of elongated colloidal particles, which must be taken into account when seeking to understand the behaviour of curved colloidal particles.