Abstract
The dynamics of aqueous Laponite clay suspensions slow down with increasing sample waiting time (tw). This behavior, and the material fragility that results, closely resemble the dynamical slowdown in fragile supercooled liquids with decreasing temperature, and are typically ascribed to the increasing sizes of distinct dynamical heterogeneities in the sample. In this article, we characterize the dynamical heterogeneities in Laponite suspensions by invoking the three-point dynamic susceptibility formalism. The average time-dependent two-point intensity autocorrelation and its sensitivity to tw are probed in dynamic light scattering experiments. Distributions of relaxation time scales, deduced from the Kohlrausch-Williams-Watts equation, are seen to widen with increasing tw. The calculated three-point dynamic susceptibility of Laponite suspensions exhibits a peak, with the peak height increasing with evolving tw at fixed volume fraction or with increasing volume fraction at fixed tw, thereby signifying the slowdown of the sample dynamics. The number of dynamically correlated particles, calculated from the peak-height, is seen to initially increase rapidly with increasing tw, before eventually slowing down close to the non-ergodic transition point. This observation is in agreement with published reports on supercooled liquids and hard sphere colloidal suspensions and offers a unique insight into the colloidal glass transition of Laponite suspensions.
Highlights
Glasses are characterized by amorphous order and solid-like rigidities and have a wide range of applications in our day-to-day lives[1, 2]
We believe that our study, which demonstrates the existence of remarkable similarities between the kinetic arrest pheonemena in Laponite clay suspensions and in supercooled liquids, provides valuable insight into the colloidal glass transition of Laponite suspensions
Dynamical heterogeneities in aging Laponite colloidal suspensions are investigated by the three-point dynamic susceptibility formalism with respect to the control parameters, tw and φ
Summary
The procedure outlined above is implemented to obtain Ncorr for aging Laponite colloidal suspensions at concentrations where they are expected to form a repulsive glass In this regime, tw is expected to behave as the control parameter for slowdown of the dynamics and is analogous to φ for hard sphere colloids[34, 35, 44]. The decrease of β (or increase in the width of distribution of relaxation time scales, Gww, as shown in Fig. 2(b)) as Laponite suspensions approach non-ergodicity indicates that the dynamics turn progressively heterogeneous.This implies growing correlations in the system, i.e. the appearance of a large number of regions of correlated groups of particles with distinct average relaxation times as the waiting time of the suspension increases. The increasing values of Ncorr with tw, estimated from the three-point dynamic susceptibility in the present study provides a quantitative measure of the fragile supercooled liquid-like heterogeneous dynamics in soft glassy colloidal Laponite suspensions
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