Ergodic to non-ergodic transition in low concentration Laponite

Abstract

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, the Wigner 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 short-range attractive interactions. A similar behaviour has been recently found in a simulation work (Sciortino et al 2003 Preprint cond-mat/0312161).

More on the Phase Diagram of Laponite

“…These results confirm the previous ones about the arrested phase found at very low clay concentrations in pure water [15,16]. The time necessary for the sample to evolve in an arrested state depends on clay and salt concentrations, as we will see later.…”

“…Directly from the raw spectra reported in Fig 2 it is already possible to observe as the evolution with waiting time is qualitatively different for low and high concentration samples, as also observed in free salt samples [15,16]. …”

“…In particular low (C w = 0.6% - to these measurements the same data analysis used previously [15,16]. The two different decaying, the fast and the slow ones, can be described quantitatively assuming for f (q, t) a time dependance given by the weighted sum of two contributions, a simple exponential to represent the fast decay (weight a) and a stretched one [e −(t/τ 2 ) β ] for the slow decay (weight (1 − a)).…”

“…A large number of Laponite dispersions at several salt concentrations have been prepared: for C s = 1 × 10 −3 M, C s = 2 × 10 −3 M, and C s = 5 × 10 −3 M. The samples investigated are indicated by dots in the C s − C w phase diagram reported in Figure 1, also indicated are the samples previously studied [15,16] in pure deionized water (C s ≃ 1 × 10 −4 M).…”

“…larger than C w = 1.5 % is too fast to be followed with photoncorrelation measurements. Data at C s = 1 × 10 −4 M are from our previous works [15,16].…”