Hindered Diffusion near Fluid-Solid Interfaces: Comparison of Molecular Dynamics to Continuum Hydrodynamics.

Area of Science:

• Fluid dynamics
• Colloid science
• Computational physics

Background:

• Near-wall hindered diffusion is vital for confined systems.
• Classic hydrodynamic theories by Brenner and Faxén provide foundational calculations.
• Extracting diffusion constants from mean squared displacements presents challenges.

Purpose of the Study:

• To review experimental and simulation efforts verifying classic hydrodynamic theories.
• To explore challenges in calculating hindered diffusion constants.
• To present an alternative method for reliable diffusion constant calculation.

Main Methods:

• Review of existing experimental and simulation studies.
• Molecular dynamics (MD) simulations.
• Calculation of diffusion constants using total force autocorrelation functions.

Main Results:

• Total force autocorrelation functions offer a reliable alternative to mean squared displacements for diffusion constant calculation in MD.
• MD-calculated perpendicular diffusion constants align with Brenner's hydrodynamic results when normalized.
• Discrepancies increase near the wall, indicating the importance of molecular details and differences in predicted interaction ranges.

Conclusions:

• Total force autocorrelation functions provide a robust method for determining diffusion constants in confined systems.
• While hydrodynamics offers a good approximation, molecular dynamics reveals crucial deviations near walls due to molecular interactions.
• Further investigation is needed to reconcile MD predictions with hydrodynamic theories, especially concerning wall interaction ranges.