Brownian motion of flexibly-linked colloidal rings

TL;DR

This study investigates the conformations and diffusive behavior of flexible colloidal rings with 4 to 8 segments, revealing their freely-jointed nature, unique diffusion properties, and size-dependent flexibility, with implications for synthetic and biological ring polymers.

Contribution

It introduces an experimental model system of colloidal rings, characterizes their conformations and dynamics, and compares findings with hydrodynamic simulations, highlighting size-dependent flexibility.

Findings

Colloidal rings are freely-jointed up to steric constraints.

Flexible colloidal rings exhibit higher diffusion coefficients than chains.

Internal deformation fluctuations decrease with ring size and saturate for larger rings.

Abstract

Ring, or cyclic, polymers have unique properties compared to linear polymers, due to their topologically closed structure that has no beginning or end. Experimental measurements on molecular ring polymers are challenging due to their polydispersity in molecular weight and the presence of undesired side products such as chains. Here, we study an experimental model system for cyclic polymers, that consists of rings of flexibly-linked micron-sized colloids with $n$=4..8 segments. We characterize the conformations of these flexible colloidal rings and find that they are freely-jointed up to steric restrictions. We measure their diffusive behavior and compare it to hydrodynamic simulations. Interestingly, flexible colloidal rings have a larger translational and rotational diffusion coefficient compared to colloidal chains. In contrast to chains, their internal deformation mode shows slower fluctuations for $n\lesssim 8$ and saturates for higher values of $n$. We show that constraints stemming from the ring structure cause this decrease in flexibility for small $n$ and infer the expected scaling of the flexibility as function of ring size. Our findings could have implications for the behavior of both synthetic and biological ring polymers, as well as for the dynamic modes of floppy colloidal materials.