Equilibrium structure and fluctuations of suspensions of colloidal dumbbells

TL;DR

This study uses Brownian Dynamics simulations to explore how the structure and dynamics of dense suspensions of colloidal dumbbells change with particle elongation, revealing significant effects on shear viscosity and collective behaviors.

Contribution

It provides new insights into the equilibrium structure and linear-response dynamics of colloidal dumbbells with varying aspect ratios, highlighting the impact on shear viscosity and collective motions.

Findings

Structural changes with increased elongation

Weak orientational correlations mostly with neighbors

Dramatic increase in shear viscosity at high packing fractions

Abstract

We investigate the structure and equilibrium linear-response dynamics of suspensions of hard colloidal dumbbells using Brownian Dynamics computer simulations. The focus lies on the dense fluid and plastic crystal states of the colloids with investigated aspect (elongation-to-diameter) ratios varying from the hard sphere limit up to 0.39, which is roughly the stability limit of the plastic crystal phase. We find expected structural changes with larger elongation with respect to the hard sphere reference case and very localized orientational correlations, typically just involving next-neighbor couplings. These relatively weak correlations are also reflected in only minor effects on the translational and rotational diffusion coefficients for most of the investigated elongations. However, the linear response shear viscosity exhibits a dramatic increase at high packing fractions ($\phi\gtrsim 0.5$) beyond a critical anisotropy factor of about $L^* \simeq 0.15$ which is surprising in view of the relatively weak changes found before on the level of colloidal self-dynamics. We suspect that even for the small investigated anisotropies, newly occurring, collective rotational-translational couplings must be made responsible for the slow time scales appearing in the plastic crystal.