Ergodic to non-ergodic transition in low concentration Laponite

At variance with previous determinations, recent investigations on water suspension of a synthetic clay (Laponite) have shown the presence of an arrested phase also at very low clay concentrations (down to Cw = 0.003). This surprising behaviour has been studied in a wide clay concentration range. As a result the existence of two different routes towards the arrested phase, applying to low and high Laponite concentrations, has been found. We can speculate that at high clay concentration the system would form a Wigner glass whose elementary units are single Laponite platelets, as already indicated in previous works. At low clay concentrations, in contrast, theWigner glass is supposed to be composed of clusters of Laponite platelets; in this case the clusters would be stabilized by the competition of long-range electrostatic repulsion and shortrange attractive interactions. A similar behaviour has been recently found in a simulation work (Sciortino et al 2003 Preprint cond-mat/0312161).