Long-time self-diffusion in quasi-two-dimensional colloidal fluids of paramagnetic particles

Area of Science:

• Colloidal Science
• Soft Matter Physics
• Fluid Dynamics

Background:

• Investigating hydrodynamic interactions (HI) is crucial for understanding particle dynamics in fluids.
• Paramagnetic colloidal particles in quasi-two-dimensional systems offer a unique platform to study these interactions.
• Previous work suggests HI can influence diffusion, but their specific role in systems with long-ranged direct interactions needs clarification.

Purpose of the Study:

• To determine the effect of hydrodynamic interactions (HI) on the long-time self-diffusion of paramagnetic colloidal particles in quasi-two-dimensional fluids.
• To isolate and quantify the influence of HI by comparing experimental data with Brownian dynamics (BD) simulations that exclude HI.
• To identify the conditions under which HI become negligible for long-time diffusion.

Main Methods:

• Utilized a combination of experiments and Brownian dynamics (BD) simulations.
• BD simulations incorporated direct interactions (DI) with short-ranged repulsion and a 1/r³ long-ranged component.
• Analyzed the equation of state and radial distribution functions to validate simulation models against experimental data.

Main Results:

• Achieved quantitative agreement between simulation and experimental radial distribution functions, confirming identical direct interactions (DI).
• Identified a regime where fluid properties are solely controlled by the dimensionless interaction strength Γ.
• Demonstrated that the reduced long-time self-diffusion coefficient (D_L^★) collapses onto a master curve, independent of HI.

Conclusions:

• Hydrodynamic interactions (HI) significantly affect short-time self-diffusion but have a negligible impact on long-time self-diffusion in these quasi-two-dimensional systems.
• The 1/r³ long-ranged direct interactions (DI) dominate the long-time diffusion behavior.
• Findings are consistent with previous studies on quasi-two-dimensional colloidal hard spheres, suggesting a general principle for such systems.