Decoupling of translational and rotational diffusion in quasi-2D colloidal fluids

Abstract: We observe the translational and rotational diffusion of dimer tracer particles in quasi-2D colloidal samples. The dimers are in dense samples of two different sizes of spherical colloidal particles, with the area fraction ${\phi}$ of the particles varying from dilute to nearly glassy. At low ${\phi}$ rotational and translational diffusion have a ratio set by the dimer size, as expected. At higher ${\phi}$ dimers become caged by their neighboring particles, and both rotational and translational diffusion slow. For short dimers we observe rapid reorientations so that the rotational diffusion is faster than translational diffusion: the two modes of diffusion are decoupled and have different ${\phi}$ dependence. Longer dimers do not exhibit fast rotations, and we find translational and rotational diffusion stay coupled for all ${\phi}$. Our results bridge prior results that used spheres (very fast rotation) and long ellipsoids (very slow rotation).