Irreversible Rearrangements, Correlated Domains, and Local Structure in Aging Glasses

Bidisperse colloidal suspensions of temperature-sensitive microgel spheres were quenched from liquid to glass states by a rapid temperature drop, and then the glass was permitted to age. Irreversible rearrangements, events that dramatically change a particle’s local environment, were observed to be closely related to dynamic heterogeneity. The rate of these irreversible events decreased during aging and the the number of particles required to move as part of these irreversible rearrangements increased. Thus, the slowing dynamics of aging were governed by growing, correlated domains of particles. Additionally, short-range order developed, and a spatial decay length scale associated with orientational order was found to grow during aging.

Figure 1

Aging dynamics. Mean-square displacement at different times after the quench ($t_w$): the liquid state before the quench (solid diamonds), $t_w=0.25\mathrm{s}$ (solid circles), 1 (open triangles), 3.33 (open squares), 36 (solid triangles), 86 (solid triangles), 143 (open circles), and 10 000 sec (open diamonds). Inset: $N_{\mathrm{MSD}}$, the fraction of particles with a given mean-square displacement for $0\le t_w\le 3\sec$ and $\Delta t=0.33\sec$. Particles that irreversibly rearrange have mean-square displacements that fall in the shaded region. Solid lines guide the eye, and the dashed line has a slope of 1 on a log-log plot.

Figure 2

Irreversible rearrangements during aging. (a) $N_{\mathrm{IR}}$, the number of irreversible rearrangements, versus $t_w$. The line is a power law fit to guide the eye. Inset: Histogram of $N_C$, fast particle cluster sizes, for $0.03\le t_w\le 3\sec$. The line is a power law fit to guide the eye. (b) Average number of fast particles per cluster for all clusters (black circles) and the average number of fast particles per cluster for only clusters containing a particle undergoing an irreversible rearrangement (red circles), versus $t_w$. The line guides the eye. Inset: Snapshot of the 10% fastest particles at $t_w=0.62\sec$, featuring two large clusters. Particles that irreversibly rearrange are plotted in red.

Figure 3

Particle configuration snapshots. (a) Snapshot of colloidal glass immediately after the quench, at $t_w=0.03\mathrm{s}$. Particles with six nearest neighbors are plotted in blue. (b) Snapshot of aged colloidal glass long after the quench, at $t_w=10,000\mathrm{s}$. Particles with six nearest neighbors are plotted in blue. (c) Example of an irreversible rearrangement that finishes at $t_w=0.67\mathrm{s}$. The irreversibly rearranging particle is blue, its nearest neighbors are green, and nearest neighbors it loses are black. The circle represents the cutoff distance that defines nearest neighbors.

Figure 4

Growth of local order during aging. (a) $\frac{N_6}{N_{\mathrm{tot}}}$, the fraction of particles with a coordination number of 6 versus $t_w$. (b) $N_{6C}$, the number of particles in the largest cluster of particles with 6 nearest neighbors versus $t_w$. (c) Bond orientational order parameter, ${\psi }_6$ versus $t_w$. (d) Orientational order correlation decay length ${\xi }_6$, extracted from $g_6$, versus $t_w$.