Ergodic to non-ergodic transition in low concentration Laponite

At variance with previous determinations, recent investigations on watersuspension of a synthetic clay (Laponite) have shown the presence of an arrestedphase also at very low clay concentrations (down to Cw = 0.003). Thissurprising behaviour has been studied in a wide clay concentration range. As aresult the existence of two different routes towards the arrested phase, applyingto low and high Laponite concentrations, has been found. We can speculatethat at high clay concentration the system would form a Wigner glass whoseelementary units are single Laponite platelets, as already indicated in previousworks. At low clay concentrations, in contrast, theWigner glass is supposed tobe composed of clusters of Laponite platelets; in this case the clusters would bestabilized by the competition of long-range electrostatic repulsion and shortrangeattractive interactions. A similar behaviour has been recently found in asimulation work (Sciortino et al 2003 Preprint cond-mat/0312161).