Memory-induced motion reversal in Brownian liquids

Area of Science:

• Soft Matter Physics
• Non-equilibrium Statistical Mechanics
• Computational Fluid Dynamics

Background:

• Understanding the dynamic behavior of dense colloidal suspensions under external forces is critical.
• Spatially inhomogeneous shear flow presents complex challenges for theoretical and simulation-based studies.
• The role of viscoelasticity and memory effects in driven soft matter systems requires further investigation.

Purpose of the Study:

• To investigate the Brownian dynamics of hard spheres subjected to inhomogeneous shear.
• To analyze density and current profiles during steady states and transient dynamics.
• To explain the observed global motion reversal phenomenon using theoretical frameworks.

Main Methods:

• Event-driven Brownian dynamics simulations were employed to model hard sphere systems.
• Power functional theory with a spatially nonlocal memory kernel was utilized for theoretical analysis.
• Comparison of simulation data with theoretical predictions to validate the model.

Main Results:

• A dense hard sphere fluid (volume fraction ≈0.35) demonstrated global motion reversal after shear force removal.
• Power functional theory accurately predicted the superadiabatic force contributions leading to motion reversal.
• The study identified internal superadiabatic nonequilibrium forces as the cause of the opposing current.

Conclusions:

• The observed motion reversal is a genuinely viscoelastic phenomenon driven by memory effects.
• Internal nonequilibrium forces generated by memory oppose the externally driven current after shear cessation.
• The findings provide a theoretical explanation for transient dynamics in sheared colloidal systems.