The Stokes-Einstein relationship and the levitation effect: size-dependent diffusion maximum in dense fluids and...

Area of Science:

• Computational Physics
• Soft Matter Physics
• Chemical Engineering

Background:

• Diffusion anomalies and levitation effects have been observed in porous solids.
• Understanding anomalous diffusion in dense fluids is crucial for various applications.
• Previous studies focused on porous media, leaving dense fluid behavior less explored.

Purpose of the Study:

• To investigate the existence of diffusion anomalies in dense binary fluid mixtures.
• To characterize the role of dynamic void networks in anomalous diffusion.
• To compare diffusion behaviors in dense fluids with those in porous solids.

Main Methods:

• Molecular dynamics simulations of binary mixtures with varying guest particle sizes.
• Voronoi polyhedral analysis to characterize void and neck distributions.
• Analysis of diffusion coefficients, activation energies, and scattering functions.

Main Results:

• A diffusion anomaly and levitation effect are observed in dense fluids, mirroring porous solids.
• Two distinct diffusion regimes (linear and anomalous) are identified, governed by dynamic void networks.
• The Stokes-Einstein relationship breaks down, with a levitation parameter being key.

Conclusions:

• Dense fluids exhibit complex diffusion behaviors previously seen only in disordered solids.
• The dynamic nature of the void network significantly influences diffusion anomalies.
• The findings necessitate a re-evaluation of diffusion theories in condensed matter systems.