Aging in attraction-driven colloidal glasses

TL;DR

This study uses computer simulations to analyze aging in attraction-driven colloidal glasses, revealing bond formation, slowed dynamics, and arrested states, with implications for understanding glass transition and phase separation.

Contribution

It demonstrates how bond formation and structural evolution influence aging and dynamics in attraction-driven colloidal glasses, especially near the glass transition.

Findings

Bond formation increases local density and slows dynamics.

Structural relaxation time grows super-linearly with waiting time.

Deep quenches lead to long-term arrested dynamics despite non-permanent bonds.

Abstract

Aging in an attraction-driven colloidal glass is studied by computer simulations. The system is equilibrated without attraction and instantaneously ``quenched'', at constant colloid volume fraction, to one of two states beyond the glass transition; one is close to the transition, and the other one deep in the glass. The evolution of structural properties shows that bonds form in the system, increasing the local density, creating density deficits (holes) elsewhere. This process slows down with the time elapsed since the quench. As a consequence of bond formation, there is a slowing down of the dynamics, as measured by the mean squared displacement and the density, bond, and environment correlation functions. The density correlations can be time-rescaled to collapse their long time (structural) decay. The time scale for structural relaxation shows for both quenches a super-linear dependence on waiting time; it grows faster than the bond lifetime, showing the collective origin of the transition. At long waiting times and high attraction strength, we observe {\rem completely} arrested dynamics for more than three decades in time, although individual bonds are not permanent on this time scale. The localization length decreases as the state moves deeper in the glass; the non-ergodicity parameter oscillates in phase with the structure factor. Our main results are obtained for systems with a barrier in the pair potential that inhibits phase separation. However, when this barrier is removed for the case of a deep quench, we find changes in the static structure but almost none in the dynamics. Hence our results for the aging behavior remain relevant to experiments in which the glass transition competes with phase separation.