Ergodicity breaking and aging dynamics in Laponite-Montmorillonite mixed clay dispersions

Ergodicity breaking and aging dynamics in a mixture of colloidal clays, Laponite (L) and Montmorillonite (MMT), were investigated. The intensity of the light scattered off L-MMT dispersions scaled with solid volume fraction (phi) as, I similar to phi(alpha), with alpha = 1.49, for Laponite alpha = 0.95, for 2 x 10(-4) < phi < 8 x 10(-3), and it remained invariant above phi = 8 x 10(-3). For MMT, alpha = 1.38 and no such invariance was noticed. Ergodic to non-ergodic transitions were observed in aged samples with a characteristic ergodicity breaking time, tau(EB) approximate to 10(2) - 10(5) s. Dynamic structure factor, g(1)(q,t) exhibited two-mode relaxation, g(1)(q,t) = a exp(-t/tau(1)) + (1 - a) exp-(t/tau(2))(beta). The stretching parameter, beta was found to decrease linearly with aging time, t(w) whereas slow mode relaxation time increased as tau(2) similar to exp (bt(w)) and tau(1) remained invariant of t(w). Spatial dependence of tau(1) and tau(2) showed, tau(1),tau(2) similar to q(-2) in the ergodic phase, considered arising from the diffusive motion. After the ergodicity breaking time (t > tau(EB)), the slow mode relaxation was no more diffusive and revealed tau(2) similar to q(-1), indicating dynamic arrest of the platelets in the glass phase. The temporal evolution of the slow mode relaxation time tau(2) in full aging regime i.e. above tau(EB), followed a power law growth. The tau(EB) was found to decrease with concentration as tau(EB) similar to exp(-phi/phi(0)). A sharp rise in anisotropy, D-p in the L-MMT glass system occurring precisely at the ergodicity breaking time was observed with the overall aging dependence given by D-p similar to t(w)(0.46). Heterogeneity in these samples was probed from the Cole-Cole plot of rheology data. It is concluded that the aging dynamics was governed by the Laponite platelets with the MMT providing a suitable matrix for appropriate interparticle interactions.