Diffusion of rod-like particles in complex fluids.

Area of Science:

• Physics
• Physical Chemistry
• Materials Science

Background:

• Classical Stokes-Einstein relation fails for particle diffusion in complex fluids.
• Relating diffusivity to fluid's wave-vector-dependent shear viscosity is a key research area.
• Extending this framework to anisotropic particles like rods is needed.

Purpose of the Study:

• To extend the shear viscosity function framework to describe diffusion of rod-like particles.
• To derive explicit expressions for diffusion coefficients parallel and perpendicular to the rod axis.
• To provide a practical tool for interpreting diffusion experiments in complex fluids.

Main Methods:

• Utilized linear response theory.
• Applied a microscopic statistical-mechanical treatment of Smoluchowski dynamics.
• Used a dimer (two-bead particle) as a minimal rod-like probe.

Main Results:

• Derived explicit expressions for diffusion coefficients of rod-like particles in terms of the viscosity function.
• Demonstrated the framework captures isotropic to highly anisotropic (reptation-like) diffusion behaviors.
• The derived formulas are simple and practical for experimental interpretation.

Conclusions:

• The extended viscosity function framework successfully describes diffusion of rod-like particles in complex fluids.
• This approach offers a practical tool for analyzing diffusion experiments, particularly in polymer solutions.
• The formulation is indirectly applicable to gels, with primary suitability for complex liquids.