How Local and Average Particle Diffusivities of Inhomogeneous Fluids Depend on Microscopic Dynamics

Area of Science:

• Physics
• Soft Matter Physics
• Computational Physics

Background:

• Understanding particle diffusion in complex environments is crucial for materials science and fluid dynamics.
• Hard-sphere fluids provide a fundamental model system for studying statistical mechanics and transport phenomena.
• Microscopic dynamics (Brownian vs. Newtonian) can influence macroscopic fluid properties.

Purpose of the Study:

• To compare single-particle diffusion coefficients in equilibrium hard-sphere fluids with identical inhomogeneous structures.
• To investigate the impact of varying density profiles (wavelength and amplitude) on particle mobility.
• To determine the influence of Brownian (overdamped Langevin) versus Newtonian microscopic dynamics on diffusion in inhomogeneous systems.

Main Methods:

• Utilizing computer simulations to model particle behavior.
• Employing a stochastic Fokker-Planck equation based approach.
• Imposing one-dimensional sinusoidal density profiles to create controlled inhomogeneity.

Main Results:

• For small density variations, bulk-like structure-mobility correlations are observed.
• Significant density variations lead to dynamics-sensitive, qualitatively different correlations.
• A previously proposed scaling for bulk fluids does not generalize to strongly inhomogeneous systems.
• Average diffusivities depend on inhomogeneity wavelength and are sensitive to microscopic dynamics.

Conclusions:

• The relationship between local structure and particle mobility in inhomogeneous fluids is highly dependent on the governing microscopic dynamics.
• Predictions for diffusion in inhomogeneous colloidal dispersions require careful consideration of dynamics and hydrodynamic interactions.
• Simple scaling laws applicable to bulk fluids are insufficient for accurately describing diffusion in complex, inhomogeneous environments.