Glass transition of soft colloids

Abstract

We explore the glassy dynamics of soft colloids using microgels and charged particles interacting by steric and screened Coulomb interactions, respectively. In the supercooled regime, the structural relaxation time τ_{α} of both systems grows steeply with volume fraction, reminiscent of the behavior of colloidal hard spheres. Computer simulations confirm that the growth of τ_{α} on approaching the glass transition is independent of particle softness. By contrast, softness becomes relevant at very large packing fractions when the system falls out of equilibrium. In this nonequilibrium regime, τ_{α} depends surprisingly weakly on packing fraction, and time correlation functions exhibit a compressed exponential decay consistent with stress-driven relaxation. The transition to this novel regime coincides with the onset of an anomalous decrease in local order with increasing density typical of ultrasoft systems. We propose that these peculiar dynamics results from the combination of the nonequilibrium aging dynamics expected in the glassy state and the tendency of colloids interacting through soft potentials to refluidize at high packing fractions.

Anomalous behavior and structure of a liquid of particles interacting through the harmonic-repulsive pair potential near the crystallization transition

“…The states at lower temperatures were obtained by cooling the higher temperature states at constant pressure. The points on the curves correspond to the scaled distances at which the proper integrals of the radial density lead to the coordination numbers 1,2,3,4,5,6,7,8,9,10,15,20,25,30,40,50,60,70,80,90,100,120,140,160,180,200. Note that in the lower temperature glass states, i.e., at T ≤ 0.002 there develops an additional small peak at (r/a) ≈ 2.67.…”

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

“…Thus, some have argued that the softness does not dominate the fragility of colloidal glasses. Instead, it was suggested that other mechanisms relying on the the microscopic details of the soft colloids are important for the drastic change in the fragility in the previous experiments [37,50,51]. Here, we elucidate the effect of the softness on the fragility of soft colloids in order to resolve the conflicting interpretations.…”

“…This finding was used to suggest that softer potentials should result in strong glasses [35]. However, the drastic variations in m k have not been reproduced in previous simulations by modification of the softness of the potential alone [37,[47][48][49]. For instance, Philippe et al, considered the soft particles interacting via the Hertz potential V H (r) = H (1 − r/σ) 2 Θ(r − σ), where Θ(x) is the Heaviside step function, and σ is the diameter of the particles [37].…”

“…Typically, φ g is obtained from experimental data using τ α (φ g ) = 100s. For colloidal systems, when τ α = 100s, τ α /τ V F T 10 5 , thus φ g can be obtained using τ α (φ g )/τ V F T = 10 5 [36,37]. In order to calculate φ g , therefore, one should consider the range of φ where τ α 100s or τ α /τ V F T 10 5 , which, unfortunately, is not practical using computer simulations.…”

“…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].…”