  The rotational Brownian motion of colloidal spheres in dense suspensions
reflects local hydrodynamics and friction, both key to non-linear rheological
phenomena such as shear-thickening and jamming, and transport in crowded
environments, including intracellular migration and blood flow. To fully
elucidate the role of rotational dynamics experimentally, it is crucial to
measure the translational and rotational motion of all spheres simultaneously.
Here, we develop compositionally uniform colloidal spheres with an off-centre,
fully embedded core with a different fluorophore to the particle body, allowing
access to rotational motion for all particles at the single-particle level. We
reveal interparticle hydrodynamic rotational coupling in charged colloidal
crystals. We also find that higher local crystallinity in denser crystals
enhances rotational diffusivity, and that nearly arrested particles exhibit a
stick-slip rotational motion due to frictional coupling. Our method sheds new
light on the largely-unexplored local rotational dynamics of spherical
particles in dense colloidal materials.
