Origin of Two-Step Glass Transition in 2D Colloidal Suspension of Rods

We report an experimental study of monolayers of colloidal rods which exhibits a two-step glass transition with no appearance of nematic domains. Instead, it was found that rods tend to align with each other randomly in two preferred configurations, parallel, or perpendicular. We argue that the bi-modal distribution of the relative angles of the rods in the glassy phase is due to the existence of an activation barrier separating two local free energy minima in the configurational space. This barrier increases significantly when the first transition is approached, thereby freezing out the rotational motion while the translational motion remains diffusive. The results suggest that the activation barrier for rods rotating relative to each other is the key to the two-step glass transition.