A comment on the position dependent diffusion coefficient representation of structural heterogeneity

Area of Science:

• Colloid Science
• Statistical Mechanics
• Soft Matter Physics

Background:

• Mean square displacement (MSD) is crucial for understanding particle dynamics.
• In confined colloid suspensions with density variations, MSD predictions often conflict with experimental data.
• Previous models assumed particle forces are point functions of position.

Purpose of the Study:

• To investigate the discrepancy between predicted and experimental MSD variations in confined colloid suspensions.
• To re-evaluate the relationship between particle density, MSD, and force interactions.
• To propose an interpretation for local equilibrium in inhomogeneous systems.

Main Methods:

• Experimental studies of particle MSD in narrow ribbon channels and parallel plates.
• New experimental data on radial and azimuthal MSD in a circular cavity.
• Application of the Fisher-Methfessel interpretation for local equilibrium.

Main Results:

• Experimental MSD variations do not consistently mimic density variations in confined colloid systems.
• Density oscillations with particle diameter spacing were observed.
• Particle forces were found to be delocalized over a volume with a radius equal to the particle diameter.

Conclusions:

• The delocalized nature of particle forces explains the discrepancy between predicted and experimental MSD.
• This finding impacts the translation of particle MSD into position-dependent diffusion coefficients.
• The study provides insights into mass transport in heterogeneous systems.