Dynamics and rheology of colloidal rod suspensions measured with differential dynamic microscopy*

Colloidal rod suspensions can form a variety of structures and can form arrested phases at relatively low volume fractions. These properties make them excellent rheological modifiers and useful as model systems to understand complex systems like networks of biopolymer filaments. Both macro- and micro-rheology methods have characterized rod suspensions and gels. Here, we use a relatively new microrheology technique, differential dynamic microscopy (DDM), which combines elements of video microscopy and dynamic light scattering. With DDM we explore how the time scale of density fluctuations, form of the intermediate scattering function, and non-ergodicity parameter scale with both rod volume fraction and aspect ratio. Because DDM works with real space images, we can verify with direct visualization the length scales we pull out from the light-scattering-like analysis framework of DDM. Further, we compare our DDM microrheology with macroscopic bulk rheology in order to make connections between microscale dynamics and bulk mechanical properties, connections which have been built for colloidal gels of spherical particles but which are more challenging to make with anisotropic particles.