"Dense diffusion" in colloidal glasses: short-ranged long-time self-diffusion as a mechanistic model for relaxation...

Area of Science:

• Soft Matter Physics
• Materials Science
• Statistical Mechanics

Background:

• The mechanistic explanation of glassy relaxation processes in colloidal systems remains unclear, with theoretical models predicting activated processes and cooperative motion.
• Discrepancies exist between predicted and measured values of the colloidal glass transition volume fraction (φg), and the role of cooperative dynamics is ambiguous.
• Challenges in deep concentration quenches and accessing particle-scale dynamics hinder a clear understanding.

Purpose of the Study:

• To elucidate the mechanistic explanation of colloidal glass transition and relaxation dynamics.
• To address limitations in previous studies by employing dynamic simulations and experiments for deep concentration quenches.
• To investigate particle-scale dynamics and the role of cooperative motion during the glass transition.

Main Methods:

• Dynamic simulations coupled with experiments to perform deep concentration quenches by increasing particle size at constant particle number density.
• Quenches were conducted from liquid to final volume fractions ranging from 0.56 to 0.63.
• Aging of the glass for extended periods, monitoring relaxation dynamics, and analyzing mean-square displacement and self-intermediate scattering function.

Main Results:

• Correlated motion facilitates dynamics release from the glassy plateau over short length scales, allowing self-diffusion to re-emerge.
• Self-diffusion leads to relaxation into an intransient diffusive state, observed up to φ = 0.63.
• Long-time self-diffusion becomes short-ranged in the glass, with a shrinking glassy cage size relative to particle size, indicating a breakdown of the liquid's cage size equivalence.

Conclusions:

• The colloidal glass transition is mechanistically characterized by a shift from long-ranged to short-ranged self-diffusion.
• This transition occurs smoothly without diverging dynamics, as particle mobility decreases and local configuration space expands.
• Local particle motion allows sampling of numerous configurations, facilitating the transition to a glassy state.