Diffusion of flexible colloidal clusters

Abstract

Colloidal particles exhibit Brownian motion when suspended in a liquid and they are large enough to visualize with optical microscopy. These characteristics make colloidal systems suitable models for studying thermal motion. The diffusion of colloids of various shapes has been studied and Brownian motion of colloids has been found to be influenced by their shape. In this work, we investigated the influence of a constantly changing shape of the colloid on the colloid's Brownian motion. This can give insight in the diffusion of many flexible compounds in nature and industry, such as molecules, polymers, proteins and bacterial flagellae. We studied and compared the diffusion of rigid- and freely jointed trimers as well as hydrodynamic correlations between the trimers' time-dependent position, orientation and bond angle. We find two differences in the Brownian motion of rigid and freely jointed trimers. The first is that trimers with a changing internal angle have lower translational and rotational diffusion coefficients than their rigid counterparts. The second difference pertains to the hydrodynamic correlations in the diffusion tensor of the clusters. We find a negative correlation between diffusion along the short axis of the freely jointed trimers and their internal angle. This observation shows that the translation of these flexible particles is coupled to a conformational change of the cluster. Lastly, we show that flexible chains can be made with the same procedure. Analysis of those can give insight into diffusion of flexible structures like polymers and proteins.