Structures of Local Rearrangements in Soft Colloidal Glasses

We image local structural rearrangements in soft colloidal glasses under small periodic perturbations induced by thermal cycling. Local structural entropy $S_2$ positively correlates with observed rearrangements in colloidal glasses. The high $S_2$ values of the rearranging clusters in glasses indicate that fragile regions in glasses are structurally less correlated, similar to structural defects in crystalline solids. Slow-evolving high $S_2$ spots are capable of predicting local rearrangements long before the relaxations occur, while fluctuation-created high $S_2$ spots best correlate with local deformations right before the rearrangement events. Local free volumes are also found to correlate with particle rearrangements at extreme values, although the ability to identify relaxation sites is substantially lower than $S_2$. Our experiments provide an efficient structural identifier for the fragile regions in glasses and highlight the important role of structural correlations in the physics of glasses.

Figure 1

Spatial distribution of rearranging clusters during thermal cycling. Rearranging clusters from different thermal cycle numbers shown by large spheres in different colors. Small dots are background particles that did not experience neighbor changes during the experiment. Large blue dotted circles indicate regions with repeated rearranging events. Small red circles show the only four overlapping rearranging particles during the experiment.

Figure 2

Free volumes and local structural entropy in colloidal glasses. (a) Distribution of $V_f$ for all particles (black squares), initial $V_f$ for rearranging particles (red circles), and $V_f$ of rearranging particles after deformations (blue triangles). (b) Rearranging probability as a function of initial $V_f$ (subtracted by the average $V_f$). (c) $S_2$ distribution for all particles (black squares), for rearranging particles before deformations (red circles), and for rearranging particles after deformations (blue triangles). (d) Rearranging probability as a function of initial $S_2$. Dashed lines and arrows show the average value and standard deviation, respectively, of $V_f$ and $S_2$ for the distributions for all particles. All distributions are obtained from the combined data of ten cycles and are individually normalized.

Figure 3

Spatial distribution of particle $S_2$ and rearranging clusters. (a) Colored contour plot: $S_2$ distribution before thermal cycling and rearranging clusters observed in the following ten cycles (black circles). (b1)–(b3) Contour plots: local $S_2$ distributions before the first, second, and third thermal cycle in a subregion in the sample; black circles: rearranging clusters from first, second, and third in the same region. (c1)–(c6) Contour plots: local $S_2$ distributions from the third to eighth thermal cycle in another subregion in the sample; black circles: rearranging clusters from the fourth cycle (c2) and the seventh cycle (c5).

Figure 4

Stable regions in colloidal glasses. Spatial distributions of particles with $S_2$ values below $-1.6$ during the experiment (large filled gray circles), particles with $S_2$ values exceeding $-1.6$ for at least one cycle (small blue dots), and rearranging clusters (red empty circles).