Even hard-sphere colloidal suspensions display Fickian yet non-Gaussian diffusion

Area of Science:

• Soft Matter Physics
• Colloidal Science
• Statistical Mechanics

Background:

• Colloidal suspensions exhibit complex dynamics influenced by particle interactions and concentration.
• Tracer diffusion in such systems is often assumed to follow Gaussian statistics in the dilute limit.
• Understanding deviations from Gaussian diffusion is crucial for predicting material properties and behavior.

Purpose of the Study:

• To investigate the probability density displacement distribution of tracer nanoparticles in a colloidal suspension over three decades.
• To determine how increasing obstacle density affects tracer diffusion dynamics.
• To identify the onset and prevalence of non-Gaussian diffusion in hard-sphere colloidal systems.

Main Methods:

• Analysis of three decades of probability density displacement distribution data.
• Utilizing index-matched and density-matched solvents for clear observation.
• Experimentation with fluorescent tracer nanoparticles diffusing among larger matrix particles at varying concentrations (dilute to concentrated).
• Observing diffusion over timescales relevant to a few times the tracer's size.

Main Results:

• Displacement distributions, Gaussian in a pure solvent, systematically broaden with increased obstacle density.
• Non-Gaussian dynamics emerge even in modestly dilute suspensions.
• The observed diffusion is characterized as non-Gaussian yet Fickian.

Conclusions:

• The assumption of classic Gaussian diffusion is inadequate even for simple colloidal suspensions at low concentrations.
• Obstacle density is a key factor driving deviations from Gaussian dynamics.
• Non-Gaussian yet Fickian diffusion is a widespread phenomenon in soft matter systems.