Kinetics of the glass transition of fragile soft colloidal suspensions

Abstract

Microscopic relaxation timescales are estimated from the autocorrelation functions obtained by dynamic light scattering experiments for Laponite suspensions with different concentrations (C L ), added salt concentrations (C S ) and temperatures (T ). It has been shown in an earlier work [Soft Matter 10, 3292 (2014)] that the evolutions of relaxation timescales of colloidal glasses can be compared with molecular glass formers by mapping the waiting time (t w ) of the former with the inverse of thermodynamic temperature (1/T ) of the latter. In this work, the fragility parameter D, which signifies the deviation from Arrhenius behavior, is obtained from fits to the time evolutions of the structural relaxation timescales. For the Laponite suspensions studied in this work, D is seen to be independent of C L and C S , but is weakly dependent on T . Interestingly, the behavior of D corroborates the behavior of fragility in molecular glass formers with respect to equivalent variables. Furthermore, the stretching exponent β, which quantifies the width w of the spectrum of structural relaxation timescales is seen to depend on t w . A hypothetical Kauzmann time t k , analogous to the Kauzmann temperature for molecular glasses, is defined as the timescale at which w diverges.Corresponding to the Vogel temperature defined for molecular glasses, a hypothetical Vogel time t ∞ α is also defined as the time at which the structural relaxation time diverges. Interestingly, a correlation is observed between t k and t ∞ α , which is remarkably similar to that known for fragile molecular glass formers. A coupling model that accounts for the t w -dependence of the stretching exponent is used to analyse and explain the observed correlation between t k and t ∞ α .

To make a glass—avoid the crystal

“…These systems do not form  Thomas Palberg palberg@uni-mainz.de crystals. Rather, they assume an amorphous structure in the solid phase through different paths already at low volume fractions [51]. In analogy to the Wigner-crystals formed by spheres through electrostatic repulsion, such states have been termed Wigner-glass by some authors [41].…”

Study of dynamical heterogeneities in colloidal nanoclay suspensions approaching dynamical arrest

“…As t w increases, the decay of F s ( q , t ) slows down considerably. Furthermore, the decay of F s ( q , t ) can be described as a two-step process 34 comprising a fast decaying exponential part and a comparatively slower stretched exponential part 31 , 35 , 36 , 44 and can be expressed as Here, τ 1 is the fast or secondary relaxation time (corresponding to the diffusion of a Laponite particle inside the cage formed by the neighbors), τ ww is the structural or primary α -relaxation time (representing its cooperative diffusion to a neighbouring position), β is a stretching exponent, and a is the weight factor for the faster secondary relaxation process 34 . Fits of the experimental data to Eq.…”

“…Fragility 4 , 5 of the colloidal glasses is studied by the modified Vogel-Fultcher-Tammann (VFT) equation in which 1/ T is replaced by ϕ (the relevant control parameter for colloidal glasses) 47 . Since Laponite suspensions transform to a non-ergodic state as t w increases, therefore t w has been used as the control parameter to study the Laponite glassy dynamics 31 , 33 – 36 , 44 , 48 . The fragile behavior of Laponite suspensions is studied by employing the modified VFT equation in which ϕ is replaced with t w 34 , 36 such that the modified VFT equation can be written as follows 33 , 34 , 36 :

Figure 1 ( a ) Self intermediate intensity scattering functions ( F s ( q , t ) decay curves) vs. delay times recorded at various waiting times ( t w ) at θ = 90° ( q = 2.2 × 10 −2 nm −1 ).

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“…The growth of the α– relaxation time in a super-Arrhenius manner as t w increases has been established experimentally 28 , 34 , 35 in Laponite suspensions. Recent studies in our group have established that the slowdown in the dynamics of Laponite suspensions resembles the slowdown reported in fragile glass-forming molecular liquids if t w of the former system is mapped to (1/ T ) of the latter in the Arrhenius and the Vogel-Fulcher-Tammann equations, which represent, respectively, the temperature dependences of the secondary and primary relaxation modes 31 , 33 , 34 , 36 . In an earlier study, S. Jabbari-Farouji et al .…”

Fragile-to-strong crossover, growing length scales, and dynamic heterogeneity in Wigner glasses

“…First, whether the softness does really contribute to the drastic change in the fragility remains controversial. Indeed, in several experiments [36,37,45,46], it was found that the fragility in soft colloids is insensitive to the softness of the interaction potentials. Previous simulations with model soft colloids showed that the drastic variation in m k cannot be reproduced by merely modifying the softness of the potential [37,[47][48][49].…”